xref: /openbmc/qemu/block.c (revision be1a732c9a74da9b27884c20e14df608fd100401)

/*
 * QEMU System Emulator block driver
 *
 * Copyright (c) 2003 Fabrice Bellard
 * Copyright (c) 2020 Virtuozzo International GmbH.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include "qemu/osdep.h"
#include "block/trace.h"
#include "block/block_int.h"
#include "block/blockjob.h"
#include "block/dirty-bitmap.h"
#include "block/fuse.h"
#include "block/nbd.h"
#include "block/qdict.h"
#include "qemu/error-report.h"
#include "block/module_block.h"
#include "qemu/main-loop.h"
#include "qemu/module.h"
#include "qapi/error.h"
#include "qapi/qmp/qdict.h"
#include "qapi/qmp/qjson.h"
#include "qapi/qmp/qnull.h"
#include "qapi/qmp/qstring.h"
#include "qapi/qobject-output-visitor.h"
#include "qapi/qapi-visit-block-core.h"
#include "sysemu/block-backend.h"
#include "qemu/notify.h"
#include "qemu/option.h"
#include "qemu/coroutine.h"
#include "block/qapi.h"
#include "qemu/timer.h"
#include "qemu/cutils.h"
#include "qemu/id.h"
#include "qemu/range.h"
#include "qemu/rcu.h"
#include "block/coroutines.h"

#ifdef CONFIG_BSD
#include <sys/ioctl.h>
#include <sys/queue.h>
#if defined(HAVE_SYS_DISK_H)
#include <sys/disk.h>
#endif
#endif

#ifdef _WIN32
#include <windows.h>
#endif

#define NOT_DONE 0x7fffffff /* used while emulated sync operation in progress */

/* Protected by BQL */
static QTAILQ_HEAD(, BlockDriverState) graph_bdrv_states =
    QTAILQ_HEAD_INITIALIZER(graph_bdrv_states);

/* Protected by BQL */
static QTAILQ_HEAD(, BlockDriverState) all_bdrv_states =
    QTAILQ_HEAD_INITIALIZER(all_bdrv_states);

/* Protected by BQL */
static QLIST_HEAD(, BlockDriver) bdrv_drivers =
    QLIST_HEAD_INITIALIZER(bdrv_drivers);

static BlockDriverState *bdrv_open_inherit(const char *filename,
                                           const char *reference,
                                           QDict *options, int flags,
                                           BlockDriverState *parent,
                                           const BdrvChildClass *child_class,
                                           BdrvChildRole child_role,
                                           Error **errp);

static bool bdrv_recurse_has_child(BlockDriverState *bs,
                                   BlockDriverState *child);

static void bdrv_replace_child_noperm(BdrvChild *child,
                                      BlockDriverState *new_bs);
static void bdrv_remove_child(BdrvChild *child, Transaction *tran);

static int bdrv_reopen_prepare(BDRVReopenState *reopen_state,
                               BlockReopenQueue *queue,
                               Transaction *change_child_tran, Error **errp);
static void bdrv_reopen_commit(BDRVReopenState *reopen_state);
static void bdrv_reopen_abort(BDRVReopenState *reopen_state);

static bool bdrv_backing_overridden(BlockDriverState *bs);

static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
                                    GHashTable *visited, Transaction *tran,
                                    Error **errp);

/* If non-zero, use only whitelisted block drivers */
static int use_bdrv_whitelist;

#ifdef _WIN32
static int is_windows_drive_prefix(const char *filename)
{
    return (((filename[0] >= 'a' && filename[0] <= 'z') ||
             (filename[0] >= 'A' && filename[0] <= 'Z')) &&
            filename[1] == ':');
}

int is_windows_drive(const char *filename)
{
    if (is_windows_drive_prefix(filename) &&
        filename[2] == '\0')
        return 1;
    if (strstart(filename, "\\\\.\\", NULL) ||
        strstart(filename, "//./", NULL))
        return 1;
    return 0;
}
#endif

size_t bdrv_opt_mem_align(BlockDriverState *bs)
{
    if (!bs || !bs->drv) {
        /* page size or 4k (hdd sector size) should be on the safe side */
        return MAX(4096, qemu_real_host_page_size());
    }
    IO_CODE();

    return bs->bl.opt_mem_alignment;
}

size_t bdrv_min_mem_align(BlockDriverState *bs)
{
    if (!bs || !bs->drv) {
        /* page size or 4k (hdd sector size) should be on the safe side */
        return MAX(4096, qemu_real_host_page_size());
    }
    IO_CODE();

    return bs->bl.min_mem_alignment;
}

/* check if the path starts with "<protocol>:" */
int path_has_protocol(const char *path)
{
    const char *p;

#ifdef _WIN32
    if (is_windows_drive(path) ||
        is_windows_drive_prefix(path)) {
        return 0;
    }
    p = path + strcspn(path, ":/\\");
#else
    p = path + strcspn(path, ":/");
#endif

    return *p == ':';
}

int path_is_absolute(const char *path)
{
#ifdef _WIN32
    /* specific case for names like: "\\.\d:" */
    if (is_windows_drive(path) || is_windows_drive_prefix(path)) {
        return 1;
    }
    return (*path == '/' || *path == '\\');
#else
    return (*path == '/');
#endif
}

/* if filename is absolute, just return its duplicate. Otherwise, build a
   path to it by considering it is relative to base_path. URL are
   supported. */
char *path_combine(const char *base_path, const char *filename)
{
    const char *protocol_stripped = NULL;
    const char *p, *p1;
    char *result;
    int len;

    if (path_is_absolute(filename)) {
        return g_strdup(filename);
    }

    if (path_has_protocol(base_path)) {
        protocol_stripped = strchr(base_path, ':');
        if (protocol_stripped) {
            protocol_stripped++;
        }
    }
    p = protocol_stripped ?: base_path;

    p1 = strrchr(base_path, '/');
#ifdef _WIN32
    {
        const char *p2;
        p2 = strrchr(base_path, '\\');
        if (!p1 || p2 > p1) {
            p1 = p2;
        }
    }
#endif
    if (p1) {
        p1++;
    } else {
        p1 = base_path;
    }
    if (p1 > p) {
        p = p1;
    }
    len = p - base_path;

    result = g_malloc(len + strlen(filename) + 1);
    memcpy(result, base_path, len);
    strcpy(result + len, filename);

    return result;
}

/*
 * Helper function for bdrv_parse_filename() implementations to remove optional
 * protocol prefixes (especially "file:") from a filename and for putting the
 * stripped filename into the options QDict if there is such a prefix.
 */
void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
                                      QDict *options)
{
    if (strstart(filename, prefix, &filename)) {
        /* Stripping the explicit protocol prefix may result in a protocol
         * prefix being (wrongly) detected (if the filename contains a colon) */
        if (path_has_protocol(filename)) {
            GString *fat_filename;

            /* This means there is some colon before the first slash; therefore,
             * this cannot be an absolute path */
            assert(!path_is_absolute(filename));

            /* And we can thus fix the protocol detection issue by prefixing it
             * by "./" */
            fat_filename = g_string_new("./");
            g_string_append(fat_filename, filename);

            assert(!path_has_protocol(fat_filename->str));

            qdict_put(options, "filename",
                      qstring_from_gstring(fat_filename));
        } else {
            /* If no protocol prefix was detected, we can use the shortened
             * filename as-is */
            qdict_put_str(options, "filename", filename);
        }
    }
}


/* Returns whether the image file is opened as read-only. Note that this can
 * return false and writing to the image file is still not possible because the
 * image is inactivated. */
bool bdrv_is_read_only(BlockDriverState *bs)
{
    IO_CODE();
    return !(bs->open_flags & BDRV_O_RDWR);
}

int bdrv_can_set_read_only(BlockDriverState *bs, bool read_only,
                           bool ignore_allow_rdw, Error **errp)
{
    IO_CODE();

    /* Do not set read_only if copy_on_read is enabled */
    if (bs->copy_on_read && read_only) {
        error_setg(errp, "Can't set node '%s' to r/o with copy-on-read enabled",
                   bdrv_get_device_or_node_name(bs));
        return -EINVAL;
    }

    /* Do not clear read_only if it is prohibited */
    if (!read_only && !(bs->open_flags & BDRV_O_ALLOW_RDWR) &&
        !ignore_allow_rdw)
    {
        error_setg(errp, "Node '%s' is read only",
                   bdrv_get_device_or_node_name(bs));
        return -EPERM;
    }

    return 0;
}

/*
 * Called by a driver that can only provide a read-only image.
 *
 * Returns 0 if the node is already read-only or it could switch the node to
 * read-only because BDRV_O_AUTO_RDONLY is set.
 *
 * Returns -EACCES if the node is read-write and BDRV_O_AUTO_RDONLY is not set
 * or bdrv_can_set_read_only() forbids making the node read-only. If @errmsg
 * is not NULL, it is used as the error message for the Error object.
 */
int bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg,
                              Error **errp)
{
    int ret = 0;
    IO_CODE();

    if (!(bs->open_flags & BDRV_O_RDWR)) {
        return 0;
    }
    if (!(bs->open_flags & BDRV_O_AUTO_RDONLY)) {
        goto fail;
    }

    ret = bdrv_can_set_read_only(bs, true, false, NULL);
    if (ret < 0) {
        goto fail;
    }

    bs->open_flags &= ~BDRV_O_RDWR;

    return 0;

fail:
    error_setg(errp, "%s", errmsg ?: "Image is read-only");
    return -EACCES;
}

/*
 * If @backing is empty, this function returns NULL without setting
 * @errp.  In all other cases, NULL will only be returned with @errp
 * set.
 *
 * Therefore, a return value of NULL without @errp set means that
 * there is no backing file; if @errp is set, there is one but its
 * absolute filename cannot be generated.
 */
char *bdrv_get_full_backing_filename_from_filename(const char *backed,
                                                   const char *backing,
                                                   Error **errp)
{
    if (backing[0] == '\0') {
        return NULL;
    } else if (path_has_protocol(backing) || path_is_absolute(backing)) {
        return g_strdup(backing);
    } else if (backed[0] == '\0' || strstart(backed, "json:", NULL)) {
        error_setg(errp, "Cannot use relative backing file names for '%s'",
                   backed);
        return NULL;
    } else {
        return path_combine(backed, backing);
    }
}

/*
 * If @filename is empty or NULL, this function returns NULL without
 * setting @errp.  In all other cases, NULL will only be returned with
 * @errp set.
 */
static char *bdrv_make_absolute_filename(BlockDriverState *relative_to,
                                         const char *filename, Error **errp)
{
    char *dir, *full_name;

    if (!filename || filename[0] == '\0') {
        return NULL;
    } else if (path_has_protocol(filename) || path_is_absolute(filename)) {
        return g_strdup(filename);
    }

    dir = bdrv_dirname(relative_to, errp);
    if (!dir) {
        return NULL;
    }

    full_name = g_strconcat(dir, filename, NULL);
    g_free(dir);
    return full_name;
}

char *bdrv_get_full_backing_filename(BlockDriverState *bs, Error **errp)
{
    GLOBAL_STATE_CODE();
    return bdrv_make_absolute_filename(bs, bs->backing_file, errp);
}

void bdrv_register(BlockDriver *bdrv)
{
    assert(bdrv->format_name);
    GLOBAL_STATE_CODE();
    QLIST_INSERT_HEAD(&bdrv_drivers, bdrv, list);
}

BlockDriverState *bdrv_new(void)
{
    BlockDriverState *bs;
    int i;

    GLOBAL_STATE_CODE();

    bs = g_new0(BlockDriverState, 1);
    QLIST_INIT(&bs->dirty_bitmaps);
    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
        QLIST_INIT(&bs->op_blockers[i]);
    }
    qemu_co_mutex_init(&bs->reqs_lock);
    qemu_mutex_init(&bs->dirty_bitmap_mutex);
    bs->refcnt = 1;
    bs->aio_context = qemu_get_aio_context();

    qemu_co_queue_init(&bs->flush_queue);

    qemu_co_mutex_init(&bs->bsc_modify_lock);
    bs->block_status_cache = g_new0(BdrvBlockStatusCache, 1);

    for (i = 0; i < bdrv_drain_all_count; i++) {
        bdrv_drained_begin(bs);
    }

    QTAILQ_INSERT_TAIL(&all_bdrv_states, bs, bs_list);

    return bs;
}

static BlockDriver *bdrv_do_find_format(const char *format_name)
{
    BlockDriver *drv1;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH(drv1, &bdrv_drivers, list) {
        if (!strcmp(drv1->format_name, format_name)) {
            return drv1;
        }
    }

    return NULL;
}

BlockDriver *bdrv_find_format(const char *format_name)
{
    BlockDriver *drv1;
    int i;

    GLOBAL_STATE_CODE();

    drv1 = bdrv_do_find_format(format_name);
    if (drv1) {
        return drv1;
    }

    /* The driver isn't registered, maybe we need to load a module */
    for (i = 0; i < (int)ARRAY_SIZE(block_driver_modules); ++i) {
        if (!strcmp(block_driver_modules[i].format_name, format_name)) {
            Error *local_err = NULL;
            int rv = block_module_load(block_driver_modules[i].library_name,
                                       &local_err);
            if (rv > 0) {
                return bdrv_do_find_format(format_name);
            } else if (rv < 0) {
                error_report_err(local_err);
            }
            break;
        }
    }
    return NULL;
}

static int bdrv_format_is_whitelisted(const char *format_name, bool read_only)
{
    static const char *whitelist_rw[] = {
        CONFIG_BDRV_RW_WHITELIST
        NULL
    };
    static const char *whitelist_ro[] = {
        CONFIG_BDRV_RO_WHITELIST
        NULL
    };
    const char **p;

    if (!whitelist_rw[0] && !whitelist_ro[0]) {
        return 1;               /* no whitelist, anything goes */
    }

    for (p = whitelist_rw; *p; p++) {
        if (!strcmp(format_name, *p)) {
            return 1;
        }
    }
    if (read_only) {
        for (p = whitelist_ro; *p; p++) {
            if (!strcmp(format_name, *p)) {
                return 1;
            }
        }
    }
    return 0;
}

int bdrv_is_whitelisted(BlockDriver *drv, bool read_only)
{
    GLOBAL_STATE_CODE();
    return bdrv_format_is_whitelisted(drv->format_name, read_only);
}

bool bdrv_uses_whitelist(void)
{
    return use_bdrv_whitelist;
}

typedef struct CreateCo {
    BlockDriver *drv;
    char *filename;
    QemuOpts *opts;
    int ret;
    Error *err;
} CreateCo;

int coroutine_fn bdrv_co_create(BlockDriver *drv, const char *filename,
                                QemuOpts *opts, Error **errp)
{
    int ret;
    GLOBAL_STATE_CODE();
    ERRP_GUARD();

    if (!drv->bdrv_co_create_opts) {
        error_setg(errp, "Driver '%s' does not support image creation",
                   drv->format_name);
        return -ENOTSUP;
    }

    ret = drv->bdrv_co_create_opts(drv, filename, opts, errp);
    if (ret < 0 && !*errp) {
        error_setg_errno(errp, -ret, "Could not create image");
    }

    return ret;
}

/**
 * Helper function for bdrv_create_file_fallback(): Resize @blk to at
 * least the given @minimum_size.
 *
 * On success, return @blk's actual length.
 * Otherwise, return -errno.
 */
static int64_t create_file_fallback_truncate(BlockBackend *blk,
                                             int64_t minimum_size, Error **errp)
{
    Error *local_err = NULL;
    int64_t size;
    int ret;

    GLOBAL_STATE_CODE();

    ret = blk_truncate(blk, minimum_size, false, PREALLOC_MODE_OFF, 0,
                       &local_err);
    if (ret < 0 && ret != -ENOTSUP) {
        error_propagate(errp, local_err);
        return ret;
    }

    size = blk_getlength(blk);
    if (size < 0) {
        error_free(local_err);
        error_setg_errno(errp, -size,
                         "Failed to inquire the new image file's length");
        return size;
    }

    if (size < minimum_size) {
        /* Need to grow the image, but we failed to do that */
        error_propagate(errp, local_err);
        return -ENOTSUP;
    }

    error_free(local_err);
    local_err = NULL;

    return size;
}

/**
 * Helper function for bdrv_create_file_fallback(): Zero the first
 * sector to remove any potentially pre-existing image header.
 */
static int coroutine_fn
create_file_fallback_zero_first_sector(BlockBackend *blk,
                                       int64_t current_size,
                                       Error **errp)
{
    int64_t bytes_to_clear;
    int ret;

    GLOBAL_STATE_CODE();

    bytes_to_clear = MIN(current_size, BDRV_SECTOR_SIZE);
    if (bytes_to_clear) {
        ret = blk_co_pwrite_zeroes(blk, 0, bytes_to_clear, BDRV_REQ_MAY_UNMAP);
        if (ret < 0) {
            error_setg_errno(errp, -ret,
                             "Failed to clear the new image's first sector");
            return ret;
        }
    }

    return 0;
}

/**
 * Simple implementation of bdrv_co_create_opts for protocol drivers
 * which only support creation via opening a file
 * (usually existing raw storage device)
 */
int coroutine_fn bdrv_co_create_opts_simple(BlockDriver *drv,
                                            const char *filename,
                                            QemuOpts *opts,
                                            Error **errp)
{
    BlockBackend *blk;
    QDict *options;
    int64_t size = 0;
    char *buf = NULL;
    PreallocMode prealloc;
    Error *local_err = NULL;
    int ret;

    GLOBAL_STATE_CODE();

    size = qemu_opt_get_size_del(opts, BLOCK_OPT_SIZE, 0);
    buf = qemu_opt_get_del(opts, BLOCK_OPT_PREALLOC);
    prealloc = qapi_enum_parse(&PreallocMode_lookup, buf,
                               PREALLOC_MODE_OFF, &local_err);
    g_free(buf);
    if (local_err) {
        error_propagate(errp, local_err);
        return -EINVAL;
    }

    if (prealloc != PREALLOC_MODE_OFF) {
        error_setg(errp, "Unsupported preallocation mode '%s'",
                   PreallocMode_str(prealloc));
        return -ENOTSUP;
    }

    options = qdict_new();
    qdict_put_str(options, "driver", drv->format_name);

    blk = blk_co_new_open(filename, NULL, options,
                          BDRV_O_RDWR | BDRV_O_RESIZE, errp);
    if (!blk) {
        error_prepend(errp, "Protocol driver '%s' does not support image "
                      "creation, and opening the image failed: ",
                      drv->format_name);
        return -EINVAL;
    }

    size = create_file_fallback_truncate(blk, size, errp);
    if (size < 0) {
        ret = size;
        goto out;
    }

    ret = create_file_fallback_zero_first_sector(blk, size, errp);
    if (ret < 0) {
        goto out;
    }

    ret = 0;
out:
    blk_unref(blk);
    return ret;
}

int coroutine_fn bdrv_co_create_file(const char *filename, QemuOpts *opts,
                                     Error **errp)
{
    QemuOpts *protocol_opts;
    BlockDriver *drv;
    QDict *qdict;
    int ret;

    GLOBAL_STATE_CODE();

    drv = bdrv_find_protocol(filename, true, errp);
    if (drv == NULL) {
        return -ENOENT;
    }

    if (!drv->create_opts) {
        error_setg(errp, "Driver '%s' does not support image creation",
                   drv->format_name);
        return -ENOTSUP;
    }

    /*
     * 'opts' contains a QemuOptsList with a combination of format and protocol
     * default values.
     *
     * The format properly removes its options, but the default values remain
     * in 'opts->list'.  So if the protocol has options with the same name
     * (e.g. rbd has 'cluster_size' as qcow2), it will see the default values
     * of the format, since for overlapping options, the format wins.
     *
     * To avoid this issue, lets convert QemuOpts to QDict, in this way we take
     * only the set options, and then convert it back to QemuOpts, using the
     * create_opts of the protocol. So the new QemuOpts, will contain only the
     * protocol defaults.
     */
    qdict = qemu_opts_to_qdict(opts, NULL);
    protocol_opts = qemu_opts_from_qdict(drv->create_opts, qdict, errp);
    if (protocol_opts == NULL) {
        ret = -EINVAL;
        goto out;
    }

    ret = bdrv_co_create(drv, filename, protocol_opts, errp);
out:
    qemu_opts_del(protocol_opts);
    qobject_unref(qdict);
    return ret;
}

int coroutine_fn bdrv_co_delete_file(BlockDriverState *bs, Error **errp)
{
    Error *local_err = NULL;
    int ret;

    IO_CODE();
    assert(bs != NULL);

    if (!bs->drv) {
        error_setg(errp, "Block node '%s' is not opened", bs->filename);
        return -ENOMEDIUM;
    }

    if (!bs->drv->bdrv_co_delete_file) {
        error_setg(errp, "Driver '%s' does not support image deletion",
                   bs->drv->format_name);
        return -ENOTSUP;
    }

    ret = bs->drv->bdrv_co_delete_file(bs, &local_err);
    if (ret < 0) {
        error_propagate(errp, local_err);
    }

    return ret;
}

void coroutine_fn bdrv_co_delete_file_noerr(BlockDriverState *bs)
{
    Error *local_err = NULL;
    int ret;
    IO_CODE();

    if (!bs) {
        return;
    }

    ret = bdrv_co_delete_file(bs, &local_err);
    /*
     * ENOTSUP will happen if the block driver doesn't support
     * the 'bdrv_co_delete_file' interface. This is a predictable
     * scenario and shouldn't be reported back to the user.
     */
    if (ret == -ENOTSUP) {
        error_free(local_err);
    } else if (ret < 0) {
        error_report_err(local_err);
    }
}

/**
 * Try to get @bs's logical and physical block size.
 * On success, store them in @bsz struct and return 0.
 * On failure return -errno.
 * @bs must not be empty.
 */
int bdrv_probe_blocksizes(BlockDriverState *bs, BlockSizes *bsz)
{
    BlockDriver *drv = bs->drv;
    BlockDriverState *filtered = bdrv_filter_bs(bs);
    GLOBAL_STATE_CODE();

    if (drv && drv->bdrv_probe_blocksizes) {
        return drv->bdrv_probe_blocksizes(bs, bsz);
    } else if (filtered) {
        return bdrv_probe_blocksizes(filtered, bsz);
    }

    return -ENOTSUP;
}

/**
 * Try to get @bs's geometry (cyls, heads, sectors).
 * On success, store them in @geo struct and return 0.
 * On failure return -errno.
 * @bs must not be empty.
 */
int bdrv_probe_geometry(BlockDriverState *bs, HDGeometry *geo)
{
    BlockDriver *drv = bs->drv;
    BlockDriverState *filtered = bdrv_filter_bs(bs);
    GLOBAL_STATE_CODE();

    if (drv && drv->bdrv_probe_geometry) {
        return drv->bdrv_probe_geometry(bs, geo);
    } else if (filtered) {
        return bdrv_probe_geometry(filtered, geo);
    }

    return -ENOTSUP;
}

/*
 * Create a uniquely-named empty temporary file.
 * Return the actual file name used upon success, otherwise NULL.
 * This string should be freed with g_free() when not needed any longer.
 *
 * Note: creating a temporary file for the caller to (re)open is
 * inherently racy. Use g_file_open_tmp() instead whenever practical.
 */
char *create_tmp_file(Error **errp)
{
    int fd;
    const char *tmpdir;
    g_autofree char *filename = NULL;

    tmpdir = g_get_tmp_dir();
#ifndef _WIN32
    /*
     * See commit 69bef79 ("block: use /var/tmp instead of /tmp for -snapshot")
     *
     * This function is used to create temporary disk images (like -snapshot),
     * so the files can become very large. /tmp is often a tmpfs where as
     * /var/tmp is usually on a disk, so more appropriate for disk images.
     */
    if (!g_strcmp0(tmpdir, "/tmp")) {
        tmpdir = "/var/tmp";
    }
#endif

    filename = g_strdup_printf("%s/vl.XXXXXX", tmpdir);
    fd = g_mkstemp(filename);
    if (fd < 0) {
        error_setg_errno(errp, errno, "Could not open temporary file '%s'",
                         filename);
        return NULL;
    }
    close(fd);

    return g_steal_pointer(&filename);
}

/*
 * Detect host devices. By convention, /dev/cdrom[N] is always
 * recognized as a host CDROM.
 */
static BlockDriver *find_hdev_driver(const char *filename)
{
    int score_max = 0, score;
    BlockDriver *drv = NULL, *d;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH(d, &bdrv_drivers, list) {
        if (d->bdrv_probe_device) {
            score = d->bdrv_probe_device(filename);
            if (score > score_max) {
                score_max = score;
                drv = d;
            }
        }
    }

    return drv;
}

static BlockDriver *bdrv_do_find_protocol(const char *protocol)
{
    BlockDriver *drv1;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH(drv1, &bdrv_drivers, list) {
        if (drv1->protocol_name && !strcmp(drv1->protocol_name, protocol)) {
            return drv1;
        }
    }

    return NULL;
}

BlockDriver *bdrv_find_protocol(const char *filename,
                                bool allow_protocol_prefix,
                                Error **errp)
{
    BlockDriver *drv1;
    char protocol[128];
    int len;
    const char *p;
    int i;

    GLOBAL_STATE_CODE();
    /* TODO Drivers without bdrv_file_open must be specified explicitly */

    /*
     * XXX(hch): we really should not let host device detection
     * override an explicit protocol specification, but moving this
     * later breaks access to device names with colons in them.
     * Thanks to the brain-dead persistent naming schemes on udev-
     * based Linux systems those actually are quite common.
     */
    drv1 = find_hdev_driver(filename);
    if (drv1) {
        return drv1;
    }

    if (!path_has_protocol(filename) || !allow_protocol_prefix) {
        return &bdrv_file;
    }

    p = strchr(filename, ':');
    assert(p != NULL);
    len = p - filename;
    if (len > sizeof(protocol) - 1)
        len = sizeof(protocol) - 1;
    memcpy(protocol, filename, len);
    protocol[len] = '\0';

    drv1 = bdrv_do_find_protocol(protocol);
    if (drv1) {
        return drv1;
    }

    for (i = 0; i < (int)ARRAY_SIZE(block_driver_modules); ++i) {
        if (block_driver_modules[i].protocol_name &&
            !strcmp(block_driver_modules[i].protocol_name, protocol)) {
            int rv = block_module_load(block_driver_modules[i].library_name, errp);
            if (rv > 0) {
                drv1 = bdrv_do_find_protocol(protocol);
            } else if (rv < 0) {
                return NULL;
            }
            break;
        }
    }

    if (!drv1) {
        error_setg(errp, "Unknown protocol '%s'", protocol);
    }
    return drv1;
}

/*
 * Guess image format by probing its contents.
 * This is not a good idea when your image is raw (CVE-2008-2004), but
 * we do it anyway for backward compatibility.
 *
 * @buf         contains the image's first @buf_size bytes.
 * @buf_size    is the buffer size in bytes (generally BLOCK_PROBE_BUF_SIZE,
 *              but can be smaller if the image file is smaller)
 * @filename    is its filename.
 *
 * For all block drivers, call the bdrv_probe() method to get its
 * probing score.
 * Return the first block driver with the highest probing score.
 */
BlockDriver *bdrv_probe_all(const uint8_t *buf, int buf_size,
                            const char *filename)
{
    int score_max = 0, score;
    BlockDriver *drv = NULL, *d;
    IO_CODE();

    QLIST_FOREACH(d, &bdrv_drivers, list) {
        if (d->bdrv_probe) {
            score = d->bdrv_probe(buf, buf_size, filename);
            if (score > score_max) {
                score_max = score;
                drv = d;
            }
        }
    }

    return drv;
}

static int find_image_format(BlockBackend *file, const char *filename,
                             BlockDriver **pdrv, Error **errp)
{
    BlockDriver *drv;
    uint8_t buf[BLOCK_PROBE_BUF_SIZE];
    int ret = 0;

    GLOBAL_STATE_CODE();

    /* Return the raw BlockDriver * to scsi-generic devices or empty drives */
    if (blk_is_sg(file) || !blk_is_inserted(file) || blk_getlength(file) == 0) {
        *pdrv = &bdrv_raw;
        return ret;
    }

    ret = blk_pread(file, 0, sizeof(buf), buf, 0);
    if (ret < 0) {
        error_setg_errno(errp, -ret, "Could not read image for determining its "
                         "format");
        *pdrv = NULL;
        return ret;
    }

    drv = bdrv_probe_all(buf, sizeof(buf), filename);
    if (!drv) {
        error_setg(errp, "Could not determine image format: No compatible "
                   "driver found");
        *pdrv = NULL;
        return -ENOENT;
    }

    *pdrv = drv;
    return 0;
}

/**
 * Set the current 'total_sectors' value
 * Return 0 on success, -errno on error.
 */
int coroutine_fn bdrv_co_refresh_total_sectors(BlockDriverState *bs,
                                               int64_t hint)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();

    if (!drv) {
        return -ENOMEDIUM;
    }

    /* Do not attempt drv->bdrv_co_getlength() on scsi-generic devices */
    if (bdrv_is_sg(bs))
        return 0;

    /* query actual device if possible, otherwise just trust the hint */
    if (drv->bdrv_co_getlength) {
        int64_t length = drv->bdrv_co_getlength(bs);
        if (length < 0) {
            return length;
        }
        hint = DIV_ROUND_UP(length, BDRV_SECTOR_SIZE);
    }

    bs->total_sectors = hint;

    if (bs->total_sectors * BDRV_SECTOR_SIZE > BDRV_MAX_LENGTH) {
        return -EFBIG;
    }

    return 0;
}

/**
 * Combines a QDict of new block driver @options with any missing options taken
 * from @old_options, so that leaving out an option defaults to its old value.
 */
static void bdrv_join_options(BlockDriverState *bs, QDict *options,
                              QDict *old_options)
{
    GLOBAL_STATE_CODE();
    if (bs->drv && bs->drv->bdrv_join_options) {
        bs->drv->bdrv_join_options(options, old_options);
    } else {
        qdict_join(options, old_options, false);
    }
}

static BlockdevDetectZeroesOptions bdrv_parse_detect_zeroes(QemuOpts *opts,
                                                            int open_flags,
                                                            Error **errp)
{
    Error *local_err = NULL;
    char *value = qemu_opt_get_del(opts, "detect-zeroes");
    BlockdevDetectZeroesOptions detect_zeroes =
        qapi_enum_parse(&BlockdevDetectZeroesOptions_lookup, value,
                        BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF, &local_err);
    GLOBAL_STATE_CODE();
    g_free(value);
    if (local_err) {
        error_propagate(errp, local_err);
        return detect_zeroes;
    }

    if (detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP &&
        !(open_flags & BDRV_O_UNMAP))
    {
        error_setg(errp, "setting detect-zeroes to unmap is not allowed "
                   "without setting discard operation to unmap");
    }

    return detect_zeroes;
}

/**
 * Set open flags for aio engine
 *
 * Return 0 on success, -1 if the engine specified is invalid
 */
int bdrv_parse_aio(const char *mode, int *flags)
{
    if (!strcmp(mode, "threads")) {
        /* do nothing, default */
    } else if (!strcmp(mode, "native")) {
        *flags |= BDRV_O_NATIVE_AIO;
#ifdef CONFIG_LINUX_IO_URING
    } else if (!strcmp(mode, "io_uring")) {
        *flags |= BDRV_O_IO_URING;
#endif
    } else {
        return -1;
    }

    return 0;
}

/**
 * Set open flags for a given discard mode
 *
 * Return 0 on success, -1 if the discard mode was invalid.
 */
int bdrv_parse_discard_flags(const char *mode, int *flags)
{
    *flags &= ~BDRV_O_UNMAP;

    if (!strcmp(mode, "off") || !strcmp(mode, "ignore")) {
        /* do nothing */
    } else if (!strcmp(mode, "on") || !strcmp(mode, "unmap")) {
        *flags |= BDRV_O_UNMAP;
    } else {
        return -1;
    }

    return 0;
}

/**
 * Set open flags for a given cache mode
 *
 * Return 0 on success, -1 if the cache mode was invalid.
 */
int bdrv_parse_cache_mode(const char *mode, int *flags, bool *writethrough)
{
    *flags &= ~BDRV_O_CACHE_MASK;

    if (!strcmp(mode, "off") || !strcmp(mode, "none")) {
        *writethrough = false;
        *flags |= BDRV_O_NOCACHE;
    } else if (!strcmp(mode, "directsync")) {
        *writethrough = true;
        *flags |= BDRV_O_NOCACHE;
    } else if (!strcmp(mode, "writeback")) {
        *writethrough = false;
    } else if (!strcmp(mode, "unsafe")) {
        *writethrough = false;
        *flags |= BDRV_O_NO_FLUSH;
    } else if (!strcmp(mode, "writethrough")) {
        *writethrough = true;
    } else {
        return -1;
    }

    return 0;
}

static char *bdrv_child_get_parent_desc(BdrvChild *c)
{
    BlockDriverState *parent = c->opaque;
    return g_strdup_printf("node '%s'", bdrv_get_node_name(parent));
}

static void bdrv_child_cb_drained_begin(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    bdrv_do_drained_begin_quiesce(bs, NULL);
}

static bool bdrv_child_cb_drained_poll(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    return bdrv_drain_poll(bs, NULL, false);
}

static void bdrv_child_cb_drained_end(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    bdrv_drained_end(bs);
}

static int bdrv_child_cb_inactivate(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    GLOBAL_STATE_CODE();
    assert(bs->open_flags & BDRV_O_INACTIVE);
    return 0;
}

static bool bdrv_child_cb_change_aio_ctx(BdrvChild *child, AioContext *ctx,
                                         GHashTable *visited, Transaction *tran,
                                         Error **errp)
{
    BlockDriverState *bs = child->opaque;
    return bdrv_change_aio_context(bs, ctx, visited, tran, errp);
}

/*
 * Returns the options and flags that a temporary snapshot should get, based on
 * the originally requested flags (the originally requested image will have
 * flags like a backing file)
 */
static void bdrv_temp_snapshot_options(int *child_flags, QDict *child_options,
                                       int parent_flags, QDict *parent_options)
{
    GLOBAL_STATE_CODE();
    *child_flags = (parent_flags & ~BDRV_O_SNAPSHOT) | BDRV_O_TEMPORARY;

    /* For temporary files, unconditional cache=unsafe is fine */
    qdict_set_default_str(child_options, BDRV_OPT_CACHE_DIRECT, "off");
    qdict_set_default_str(child_options, BDRV_OPT_CACHE_NO_FLUSH, "on");

    /* Copy the read-only and discard options from the parent */
    qdict_copy_default(child_options, parent_options, BDRV_OPT_READ_ONLY);
    qdict_copy_default(child_options, parent_options, BDRV_OPT_DISCARD);

    /* aio=native doesn't work for cache.direct=off, so disable it for the
     * temporary snapshot */
    *child_flags &= ~BDRV_O_NATIVE_AIO;
}

static void bdrv_backing_attach(BdrvChild *c)
{
    BlockDriverState *parent = c->opaque;
    BlockDriverState *backing_hd = c->bs;

    GLOBAL_STATE_CODE();
    assert(!parent->backing_blocker);
    error_setg(&parent->backing_blocker,
               "node is used as backing hd of '%s'",
               bdrv_get_device_or_node_name(parent));

    bdrv_refresh_filename(backing_hd);

    parent->open_flags &= ~BDRV_O_NO_BACKING;

    bdrv_op_block_all(backing_hd, parent->backing_blocker);
    /* Otherwise we won't be able to commit or stream */
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_COMMIT_TARGET,
                    parent->backing_blocker);
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_STREAM,
                    parent->backing_blocker);
    /*
     * We do backup in 3 ways:
     * 1. drive backup
     *    The target bs is new opened, and the source is top BDS
     * 2. blockdev backup
     *    Both the source and the target are top BDSes.
     * 3. internal backup(used for block replication)
     *    Both the source and the target are backing file
     *
     * In case 1 and 2, neither the source nor the target is the backing file.
     * In case 3, we will block the top BDS, so there is only one block job
     * for the top BDS and its backing chain.
     */
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_BACKUP_SOURCE,
                    parent->backing_blocker);
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_BACKUP_TARGET,
                    parent->backing_blocker);
}

static void bdrv_backing_detach(BdrvChild *c)
{
    BlockDriverState *parent = c->opaque;

    GLOBAL_STATE_CODE();
    assert(parent->backing_blocker);
    bdrv_op_unblock_all(c->bs, parent->backing_blocker);
    error_free(parent->backing_blocker);
    parent->backing_blocker = NULL;
}

static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
                                        const char *filename, Error **errp)
{
    BlockDriverState *parent = c->opaque;
    bool read_only = bdrv_is_read_only(parent);
    int ret;
    GLOBAL_STATE_CODE();

    if (read_only) {
        ret = bdrv_reopen_set_read_only(parent, false, errp);
        if (ret < 0) {
            return ret;
        }
    }

    ret = bdrv_change_backing_file(parent, filename,
                                   base->drv ? base->drv->format_name : "",
                                   false);
    if (ret < 0) {
        error_setg_errno(errp, -ret, "Could not update backing file link");
    }

    if (read_only) {
        bdrv_reopen_set_read_only(parent, true, NULL);
    }

    return ret;
}

/*
 * Returns the options and flags that a generic child of a BDS should
 * get, based on the given options and flags for the parent BDS.
 */
static void bdrv_inherited_options(BdrvChildRole role, bool parent_is_format,
                                   int *child_flags, QDict *child_options,
                                   int parent_flags, QDict *parent_options)
{
    int flags = parent_flags;
    GLOBAL_STATE_CODE();

    /*
     * First, decide whether to set, clear, or leave BDRV_O_PROTOCOL.
     * Generally, the question to answer is: Should this child be
     * format-probed by default?
     */

    /*
     * Pure and non-filtered data children of non-format nodes should
     * be probed by default (even when the node itself has BDRV_O_PROTOCOL
     * set).  This only affects a very limited set of drivers (namely
     * quorum and blkverify when this comment was written).
     * Force-clear BDRV_O_PROTOCOL then.
     */
    if (!parent_is_format &&
        (role & BDRV_CHILD_DATA) &&
        !(role & (BDRV_CHILD_METADATA | BDRV_CHILD_FILTERED)))
    {
        flags &= ~BDRV_O_PROTOCOL;
    }

    /*
     * All children of format nodes (except for COW children) and all
     * metadata children in general should never be format-probed.
     * Force-set BDRV_O_PROTOCOL then.
     */
    if ((parent_is_format && !(role & BDRV_CHILD_COW)) ||
        (role & BDRV_CHILD_METADATA))
    {
        flags |= BDRV_O_PROTOCOL;
    }

    /*
     * If the cache mode isn't explicitly set, inherit direct and no-flush from
     * the parent.
     */
    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_DIRECT);
    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_NO_FLUSH);
    qdict_copy_default(child_options, parent_options, BDRV_OPT_FORCE_SHARE);

    if (role & BDRV_CHILD_COW) {
        /* backing files are opened read-only by default */
        qdict_set_default_str(child_options, BDRV_OPT_READ_ONLY, "on");
        qdict_set_default_str(child_options, BDRV_OPT_AUTO_READ_ONLY, "off");
    } else {
        /* Inherit the read-only option from the parent if it's not set */
        qdict_copy_default(child_options, parent_options, BDRV_OPT_READ_ONLY);
        qdict_copy_default(child_options, parent_options,
                           BDRV_OPT_AUTO_READ_ONLY);
    }

    /*
     * bdrv_co_pdiscard() respects unmap policy for the parent, so we
     * can default to enable it on lower layers regardless of the
     * parent option.
     */
    qdict_set_default_str(child_options, BDRV_OPT_DISCARD, "unmap");

    /* Clear flags that only apply to the top layer */
    flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ);

    if (role & BDRV_CHILD_METADATA) {
        flags &= ~BDRV_O_NO_IO;
    }
    if (role & BDRV_CHILD_COW) {
        flags &= ~BDRV_O_TEMPORARY;
    }

    *child_flags = flags;
}

static void GRAPH_WRLOCK bdrv_child_cb_attach(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;

    assert_bdrv_graph_writable();
    QLIST_INSERT_HEAD(&bs->children, child, next);
    if (bs->drv->is_filter || (child->role & BDRV_CHILD_FILTERED)) {
        /*
         * Here we handle filters and block/raw-format.c when it behave like
         * filter. They generally have a single PRIMARY child, which is also the
         * FILTERED child, and that they may have multiple more children, which
         * are neither PRIMARY nor FILTERED. And never we have a COW child here.
         * So bs->file will be the PRIMARY child, unless the PRIMARY child goes
         * into bs->backing on exceptional cases; and bs->backing will be
         * nothing else.
         */
        assert(!(child->role & BDRV_CHILD_COW));
        if (child->role & BDRV_CHILD_PRIMARY) {
            assert(child->role & BDRV_CHILD_FILTERED);
            assert(!bs->backing);
            assert(!bs->file);

            if (bs->drv->filtered_child_is_backing) {
                bs->backing = child;
            } else {
                bs->file = child;
            }
        } else {
            assert(!(child->role & BDRV_CHILD_FILTERED));
        }
    } else if (child->role & BDRV_CHILD_COW) {
        assert(bs->drv->supports_backing);
        assert(!(child->role & BDRV_CHILD_PRIMARY));
        assert(!bs->backing);
        bs->backing = child;
        bdrv_backing_attach(child);
    } else if (child->role & BDRV_CHILD_PRIMARY) {
        assert(!bs->file);
        bs->file = child;
    }
}

static void GRAPH_WRLOCK bdrv_child_cb_detach(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;

    if (child->role & BDRV_CHILD_COW) {
        bdrv_backing_detach(child);
    }

    assert_bdrv_graph_writable();
    QLIST_REMOVE(child, next);
    if (child == bs->backing) {
        assert(child != bs->file);
        bs->backing = NULL;
    } else if (child == bs->file) {
        bs->file = NULL;
    }
}

static int bdrv_child_cb_update_filename(BdrvChild *c, BlockDriverState *base,
                                         const char *filename, Error **errp)
{
    if (c->role & BDRV_CHILD_COW) {
        return bdrv_backing_update_filename(c, base, filename, errp);
    }
    return 0;
}

AioContext *child_of_bds_get_parent_aio_context(BdrvChild *c)
{
    BlockDriverState *bs = c->opaque;
    IO_CODE();

    return bdrv_get_aio_context(bs);
}

const BdrvChildClass child_of_bds = {
    .parent_is_bds   = true,
    .get_parent_desc = bdrv_child_get_parent_desc,
    .inherit_options = bdrv_inherited_options,
    .drained_begin   = bdrv_child_cb_drained_begin,
    .drained_poll    = bdrv_child_cb_drained_poll,
    .drained_end     = bdrv_child_cb_drained_end,
    .attach          = bdrv_child_cb_attach,
    .detach          = bdrv_child_cb_detach,
    .inactivate      = bdrv_child_cb_inactivate,
    .change_aio_ctx  = bdrv_child_cb_change_aio_ctx,
    .update_filename = bdrv_child_cb_update_filename,
    .get_parent_aio_context = child_of_bds_get_parent_aio_context,
};

AioContext *bdrv_child_get_parent_aio_context(BdrvChild *c)
{
    IO_CODE();
    return c->klass->get_parent_aio_context(c);
}

static int bdrv_open_flags(BlockDriverState *bs, int flags)
{
    int open_flags = flags;
    GLOBAL_STATE_CODE();

    /*
     * Clear flags that are internal to the block layer before opening the
     * image.
     */
    open_flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_PROTOCOL);

    return open_flags;
}

static void update_flags_from_options(int *flags, QemuOpts *opts)
{
    GLOBAL_STATE_CODE();

    *flags &= ~(BDRV_O_CACHE_MASK | BDRV_O_RDWR | BDRV_O_AUTO_RDONLY);

    if (qemu_opt_get_bool_del(opts, BDRV_OPT_CACHE_NO_FLUSH, false)) {
        *flags |= BDRV_O_NO_FLUSH;
    }

    if (qemu_opt_get_bool_del(opts, BDRV_OPT_CACHE_DIRECT, false)) {
        *flags |= BDRV_O_NOCACHE;
    }

    if (!qemu_opt_get_bool_del(opts, BDRV_OPT_READ_ONLY, false)) {
        *flags |= BDRV_O_RDWR;
    }

    if (qemu_opt_get_bool_del(opts, BDRV_OPT_AUTO_READ_ONLY, false)) {
        *flags |= BDRV_O_AUTO_RDONLY;
    }
}

static void update_options_from_flags(QDict *options, int flags)
{
    GLOBAL_STATE_CODE();
    if (!qdict_haskey(options, BDRV_OPT_CACHE_DIRECT)) {
        qdict_put_bool(options, BDRV_OPT_CACHE_DIRECT, flags & BDRV_O_NOCACHE);
    }
    if (!qdict_haskey(options, BDRV_OPT_CACHE_NO_FLUSH)) {
        qdict_put_bool(options, BDRV_OPT_CACHE_NO_FLUSH,
                       flags & BDRV_O_NO_FLUSH);
    }
    if (!qdict_haskey(options, BDRV_OPT_READ_ONLY)) {
        qdict_put_bool(options, BDRV_OPT_READ_ONLY, !(flags & BDRV_O_RDWR));
    }
    if (!qdict_haskey(options, BDRV_OPT_AUTO_READ_ONLY)) {
        qdict_put_bool(options, BDRV_OPT_AUTO_READ_ONLY,
                       flags & BDRV_O_AUTO_RDONLY);
    }
}

static void bdrv_assign_node_name(BlockDriverState *bs,
                                  const char *node_name,
                                  Error **errp)
{
    char *gen_node_name = NULL;
    GLOBAL_STATE_CODE();

    if (!node_name) {
        node_name = gen_node_name = id_generate(ID_BLOCK);
    } else if (!id_wellformed(node_name)) {
        /*
         * Check for empty string or invalid characters, but not if it is
         * generated (generated names use characters not available to the user)
         */
        error_setg(errp, "Invalid node-name: '%s'", node_name);
        return;
    }

    /* takes care of avoiding namespaces collisions */
    if (blk_by_name(node_name)) {
        error_setg(errp, "node-name=%s is conflicting with a device id",
                   node_name);
        goto out;
    }

    /* takes care of avoiding duplicates node names */
    if (bdrv_find_node(node_name)) {
        error_setg(errp, "Duplicate nodes with node-name='%s'", node_name);
        goto out;
    }

    /* Make sure that the node name isn't truncated */
    if (strlen(node_name) >= sizeof(bs->node_name)) {
        error_setg(errp, "Node name too long");
        goto out;
    }

    /* copy node name into the bs and insert it into the graph list */
    pstrcpy(bs->node_name, sizeof(bs->node_name), node_name);
    QTAILQ_INSERT_TAIL(&graph_bdrv_states, bs, node_list);
out:
    g_free(gen_node_name);
}

/*
 * The caller must always hold @bs AioContext lock, because this function calls
 * bdrv_refresh_total_sectors() which polls when called from non-coroutine
 * context.
 */
static int bdrv_open_driver(BlockDriverState *bs, BlockDriver *drv,
                            const char *node_name, QDict *options,
                            int open_flags, Error **errp)
{
    Error *local_err = NULL;
    int i, ret;
    GLOBAL_STATE_CODE();

    bdrv_assign_node_name(bs, node_name, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        return -EINVAL;
    }

    bs->drv = drv;
    bs->opaque = g_malloc0(drv->instance_size);

    if (drv->bdrv_file_open) {
        assert(!drv->bdrv_needs_filename || bs->filename[0]);
        ret = drv->bdrv_file_open(bs, options, open_flags, &local_err);
    } else if (drv->bdrv_open) {
        ret = drv->bdrv_open(bs, options, open_flags, &local_err);
    } else {
        ret = 0;
    }

    if (ret < 0) {
        if (local_err) {
            error_propagate(errp, local_err);
        } else if (bs->filename[0]) {
            error_setg_errno(errp, -ret, "Could not open '%s'", bs->filename);
        } else {
            error_setg_errno(errp, -ret, "Could not open image");
        }
        goto open_failed;
    }

    assert(!(bs->supported_read_flags & ~BDRV_REQ_MASK));
    assert(!(bs->supported_write_flags & ~BDRV_REQ_MASK));

    /*
     * Always allow the BDRV_REQ_REGISTERED_BUF optimization hint. This saves
     * drivers that pass read/write requests through to a child the trouble of
     * declaring support explicitly.
     *
     * Drivers must not propagate this flag accidentally when they initiate I/O
     * to a bounce buffer. That case should be rare though.
     */
    bs->supported_read_flags |= BDRV_REQ_REGISTERED_BUF;
    bs->supported_write_flags |= BDRV_REQ_REGISTERED_BUF;

    ret = bdrv_refresh_total_sectors(bs, bs->total_sectors);
    if (ret < 0) {
        error_setg_errno(errp, -ret, "Could not refresh total sector count");
        return ret;
    }

    bdrv_refresh_limits(bs, NULL, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        return -EINVAL;
    }

    assert(bdrv_opt_mem_align(bs) != 0);
    assert(bdrv_min_mem_align(bs) != 0);
    assert(is_power_of_2(bs->bl.request_alignment));

    for (i = 0; i < bs->quiesce_counter; i++) {
        if (drv->bdrv_drain_begin) {
            drv->bdrv_drain_begin(bs);
        }
    }

    return 0;
open_failed:
    bs->drv = NULL;
    if (bs->file != NULL) {
        bdrv_unref_child(bs, bs->file);
        assert(!bs->file);
    }
    g_free(bs->opaque);
    bs->opaque = NULL;
    return ret;
}

/*
 * Create and open a block node.
 *
 * @options is a QDict of options to pass to the block drivers, or NULL for an
 * empty set of options. The reference to the QDict belongs to the block layer
 * after the call (even on failure), so if the caller intends to reuse the
 * dictionary, it needs to use qobject_ref() before calling bdrv_open.
 */
BlockDriverState *bdrv_new_open_driver_opts(BlockDriver *drv,
                                            const char *node_name,
                                            QDict *options, int flags,
                                            Error **errp)
{
    BlockDriverState *bs;
    int ret;

    GLOBAL_STATE_CODE();

    bs = bdrv_new();
    bs->open_flags = flags;
    bs->options = options ?: qdict_new();
    bs->explicit_options = qdict_clone_shallow(bs->options);
    bs->opaque = NULL;

    update_options_from_flags(bs->options, flags);

    ret = bdrv_open_driver(bs, drv, node_name, bs->options, flags, errp);
    if (ret < 0) {
        qobject_unref(bs->explicit_options);
        bs->explicit_options = NULL;
        qobject_unref(bs->options);
        bs->options = NULL;
        bdrv_unref(bs);
        return NULL;
    }

    return bs;
}

/* Create and open a block node. */
BlockDriverState *bdrv_new_open_driver(BlockDriver *drv, const char *node_name,
                                       int flags, Error **errp)
{
    GLOBAL_STATE_CODE();
    return bdrv_new_open_driver_opts(drv, node_name, NULL, flags, errp);
}

QemuOptsList bdrv_runtime_opts = {
    .name = "bdrv_common",
    .head = QTAILQ_HEAD_INITIALIZER(bdrv_runtime_opts.head),
    .desc = {
        {
            .name = "node-name",
            .type = QEMU_OPT_STRING,
            .help = "Node name of the block device node",
        },
        {
            .name = "driver",
            .type = QEMU_OPT_STRING,
            .help = "Block driver to use for the node",
        },
        {
            .name = BDRV_OPT_CACHE_DIRECT,
            .type = QEMU_OPT_BOOL,
            .help = "Bypass software writeback cache on the host",
        },
        {
            .name = BDRV_OPT_CACHE_NO_FLUSH,
            .type = QEMU_OPT_BOOL,
            .help = "Ignore flush requests",
        },
        {
            .name = BDRV_OPT_READ_ONLY,
            .type = QEMU_OPT_BOOL,
            .help = "Node is opened in read-only mode",
        },
        {
            .name = BDRV_OPT_AUTO_READ_ONLY,
            .type = QEMU_OPT_BOOL,
            .help = "Node can become read-only if opening read-write fails",
        },
        {
            .name = "detect-zeroes",
            .type = QEMU_OPT_STRING,
            .help = "try to optimize zero writes (off, on, unmap)",
        },
        {
            .name = BDRV_OPT_DISCARD,
            .type = QEMU_OPT_STRING,
            .help = "discard operation (ignore/off, unmap/on)",
        },
        {
            .name = BDRV_OPT_FORCE_SHARE,
            .type = QEMU_OPT_BOOL,
            .help = "always accept other writers (default: off)",
        },
        { /* end of list */ }
    },
};

QemuOptsList bdrv_create_opts_simple = {
    .name = "simple-create-opts",
    .head = QTAILQ_HEAD_INITIALIZER(bdrv_create_opts_simple.head),
    .desc = {
        {
            .name = BLOCK_OPT_SIZE,
            .type = QEMU_OPT_SIZE,
            .help = "Virtual disk size"
        },
        {
            .name = BLOCK_OPT_PREALLOC,
            .type = QEMU_OPT_STRING,
            .help = "Preallocation mode (allowed values: off)"
        },
        { /* end of list */ }
    }
};

/*
 * Common part for opening disk images and files
 *
 * Removes all processed options from *options.
 */
static int bdrv_open_common(BlockDriverState *bs, BlockBackend *file,
                            QDict *options, Error **errp)
{
    int ret, open_flags;
    const char *filename;
    const char *driver_name = NULL;
    const char *node_name = NULL;
    const char *discard;
    QemuOpts *opts;
    BlockDriver *drv;
    Error *local_err = NULL;
    bool ro;

    assert(bs->file == NULL);
    assert(options != NULL && bs->options != options);
    GLOBAL_STATE_CODE();

    opts = qemu_opts_create(&bdrv_runtime_opts, NULL, 0, &error_abort);
    if (!qemu_opts_absorb_qdict(opts, options, errp)) {
        ret = -EINVAL;
        goto fail_opts;
    }

    update_flags_from_options(&bs->open_flags, opts);

    driver_name = qemu_opt_get(opts, "driver");
    drv = bdrv_find_format(driver_name);
    assert(drv != NULL);

    bs->force_share = qemu_opt_get_bool(opts, BDRV_OPT_FORCE_SHARE, false);

    if (bs->force_share && (bs->open_flags & BDRV_O_RDWR)) {
        error_setg(errp,
                   BDRV_OPT_FORCE_SHARE
                   "=on can only be used with read-only images");
        ret = -EINVAL;
        goto fail_opts;
    }

    if (file != NULL) {
        bdrv_refresh_filename(blk_bs(file));
        filename = blk_bs(file)->filename;
    } else {
        /*
         * Caution: while qdict_get_try_str() is fine, getting
         * non-string types would require more care.  When @options
         * come from -blockdev or blockdev_add, its members are typed
         * according to the QAPI schema, but when they come from
         * -drive, they're all QString.
         */
        filename = qdict_get_try_str(options, "filename");
    }

    if (drv->bdrv_needs_filename && (!filename || !filename[0])) {
        error_setg(errp, "The '%s' block driver requires a file name",
                   drv->format_name);
        ret = -EINVAL;
        goto fail_opts;
    }

    trace_bdrv_open_common(bs, filename ?: "", bs->open_flags,
                           drv->format_name);

    ro = bdrv_is_read_only(bs);

    if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, ro)) {
        if (!ro && bdrv_is_whitelisted(drv, true)) {
            ret = bdrv_apply_auto_read_only(bs, NULL, NULL);
        } else {
            ret = -ENOTSUP;
        }
        if (ret < 0) {
            error_setg(errp,
                       !ro && bdrv_is_whitelisted(drv, true)
                       ? "Driver '%s' can only be used for read-only devices"
                       : "Driver '%s' is not whitelisted",
                       drv->format_name);
            goto fail_opts;
        }
    }

    /* bdrv_new() and bdrv_close() make it so */
    assert(qatomic_read(&bs->copy_on_read) == 0);

    if (bs->open_flags & BDRV_O_COPY_ON_READ) {
        if (!ro) {
            bdrv_enable_copy_on_read(bs);
        } else {
            error_setg(errp, "Can't use copy-on-read on read-only device");
            ret = -EINVAL;
            goto fail_opts;
        }
    }

    discard = qemu_opt_get(opts, BDRV_OPT_DISCARD);
    if (discard != NULL) {
        if (bdrv_parse_discard_flags(discard, &bs->open_flags) != 0) {
            error_setg(errp, "Invalid discard option");
            ret = -EINVAL;
            goto fail_opts;
        }
    }

    bs->detect_zeroes =
        bdrv_parse_detect_zeroes(opts, bs->open_flags, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        ret = -EINVAL;
        goto fail_opts;
    }

    if (filename != NULL) {
        pstrcpy(bs->filename, sizeof(bs->filename), filename);
    } else {
        bs->filename[0] = '\0';
    }
    pstrcpy(bs->exact_filename, sizeof(bs->exact_filename), bs->filename);

    /* Open the image, either directly or using a protocol */
    open_flags = bdrv_open_flags(bs, bs->open_flags);
    node_name = qemu_opt_get(opts, "node-name");

    assert(!drv->bdrv_file_open || file == NULL);
    ret = bdrv_open_driver(bs, drv, node_name, options, open_flags, errp);
    if (ret < 0) {
        goto fail_opts;
    }

    qemu_opts_del(opts);
    return 0;

fail_opts:
    qemu_opts_del(opts);
    return ret;
}

static QDict *parse_json_filename(const char *filename, Error **errp)
{
    QObject *options_obj;
    QDict *options;
    int ret;
    GLOBAL_STATE_CODE();

    ret = strstart(filename, "json:", &filename);
    assert(ret);

    options_obj = qobject_from_json(filename, errp);
    if (!options_obj) {
        error_prepend(errp, "Could not parse the JSON options: ");
        return NULL;
    }

    options = qobject_to(QDict, options_obj);
    if (!options) {
        qobject_unref(options_obj);
        error_setg(errp, "Invalid JSON object given");
        return NULL;
    }

    qdict_flatten(options);

    return options;
}

static void parse_json_protocol(QDict *options, const char **pfilename,
                                Error **errp)
{
    QDict *json_options;
    Error *local_err = NULL;
    GLOBAL_STATE_CODE();

    /* Parse json: pseudo-protocol */
    if (!*pfilename || !g_str_has_prefix(*pfilename, "json:")) {
        return;
    }

    json_options = parse_json_filename(*pfilename, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        return;
    }

    /* Options given in the filename have lower priority than options
     * specified directly */
    qdict_join(options, json_options, false);
    qobject_unref(json_options);
    *pfilename = NULL;
}

/*
 * Fills in default options for opening images and converts the legacy
 * filename/flags pair to option QDict entries.
 * The BDRV_O_PROTOCOL flag in *flags will be set or cleared accordingly if a
 * block driver has been specified explicitly.
 */
static int bdrv_fill_options(QDict **options, const char *filename,
                             int *flags, Error **errp)
{
    const char *drvname;
    bool protocol = *flags & BDRV_O_PROTOCOL;
    bool parse_filename = false;
    BlockDriver *drv = NULL;
    Error *local_err = NULL;

    GLOBAL_STATE_CODE();

    /*
     * Caution: while qdict_get_try_str() is fine, getting non-string
     * types would require more care.  When @options come from
     * -blockdev or blockdev_add, its members are typed according to
     * the QAPI schema, but when they come from -drive, they're all
     * QString.
     */
    drvname = qdict_get_try_str(*options, "driver");
    if (drvname) {
        drv = bdrv_find_format(drvname);
        if (!drv) {
            error_setg(errp, "Unknown driver '%s'", drvname);
            return -ENOENT;
        }
        /* If the user has explicitly specified the driver, this choice should
         * override the BDRV_O_PROTOCOL flag */
        protocol = drv->bdrv_file_open;
    }

    if (protocol) {
        *flags |= BDRV_O_PROTOCOL;
    } else {
        *flags &= ~BDRV_O_PROTOCOL;
    }

    /* Translate cache options from flags into options */
    update_options_from_flags(*options, *flags);

    /* Fetch the file name from the options QDict if necessary */
    if (protocol && filename) {
        if (!qdict_haskey(*options, "filename")) {
            qdict_put_str(*options, "filename", filename);
            parse_filename = true;
        } else {
            error_setg(errp, "Can't specify 'file' and 'filename' options at "
                             "the same time");
            return -EINVAL;
        }
    }

    /* Find the right block driver */
    /* See cautionary note on accessing @options above */
    filename = qdict_get_try_str(*options, "filename");

    if (!drvname && protocol) {
        if (filename) {
            drv = bdrv_find_protocol(filename, parse_filename, errp);
            if (!drv) {
                return -EINVAL;
            }

            drvname = drv->format_name;
            qdict_put_str(*options, "driver", drvname);
        } else {
            error_setg(errp, "Must specify either driver or file");
            return -EINVAL;
        }
    }

    assert(drv || !protocol);

    /* Driver-specific filename parsing */
    if (drv && drv->bdrv_parse_filename && parse_filename) {
        drv->bdrv_parse_filename(filename, *options, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
            return -EINVAL;
        }

        if (!drv->bdrv_needs_filename) {
            qdict_del(*options, "filename");
        }
    }

    return 0;
}

typedef struct BlockReopenQueueEntry {
     bool prepared;
     bool perms_checked;
     BDRVReopenState state;
     QTAILQ_ENTRY(BlockReopenQueueEntry) entry;
} BlockReopenQueueEntry;

/*
 * Return the flags that @bs will have after the reopens in @q have
 * successfully completed. If @q is NULL (or @bs is not contained in @q),
 * return the current flags.
 */
static int bdrv_reopen_get_flags(BlockReopenQueue *q, BlockDriverState *bs)
{
    BlockReopenQueueEntry *entry;

    if (q != NULL) {
        QTAILQ_FOREACH(entry, q, entry) {
            if (entry->state.bs == bs) {
                return entry->state.flags;
            }
        }
    }

    return bs->open_flags;
}

/* Returns whether the image file can be written to after the reopen queue @q
 * has been successfully applied, or right now if @q is NULL. */
static bool bdrv_is_writable_after_reopen(BlockDriverState *bs,
                                          BlockReopenQueue *q)
{
    int flags = bdrv_reopen_get_flags(q, bs);

    return (flags & (BDRV_O_RDWR | BDRV_O_INACTIVE)) == BDRV_O_RDWR;
}

/*
 * Return whether the BDS can be written to.  This is not necessarily
 * the same as !bdrv_is_read_only(bs), as inactivated images may not
 * be written to but do not count as read-only images.
 */
bool bdrv_is_writable(BlockDriverState *bs)
{
    IO_CODE();
    return bdrv_is_writable_after_reopen(bs, NULL);
}

static char *bdrv_child_user_desc(BdrvChild *c)
{
    GLOBAL_STATE_CODE();
    return c->klass->get_parent_desc(c);
}

/*
 * Check that @a allows everything that @b needs. @a and @b must reference same
 * child node.
 */
static bool bdrv_a_allow_b(BdrvChild *a, BdrvChild *b, Error **errp)
{
    const char *child_bs_name;
    g_autofree char *a_user = NULL;
    g_autofree char *b_user = NULL;
    g_autofree char *perms = NULL;

    assert(a->bs);
    assert(a->bs == b->bs);
    GLOBAL_STATE_CODE();

    if ((b->perm & a->shared_perm) == b->perm) {
        return true;
    }

    child_bs_name = bdrv_get_node_name(b->bs);
    a_user = bdrv_child_user_desc(a);
    b_user = bdrv_child_user_desc(b);
    perms = bdrv_perm_names(b->perm & ~a->shared_perm);

    error_setg(errp, "Permission conflict on node '%s': permissions '%s' are "
               "both required by %s (uses node '%s' as '%s' child) and "
               "unshared by %s (uses node '%s' as '%s' child).",
               child_bs_name, perms,
               b_user, child_bs_name, b->name,
               a_user, child_bs_name, a->name);

    return false;
}

static bool bdrv_parent_perms_conflict(BlockDriverState *bs, Error **errp)
{
    BdrvChild *a, *b;
    GLOBAL_STATE_CODE();

    /*
     * During the loop we'll look at each pair twice. That's correct because
     * bdrv_a_allow_b() is asymmetric and we should check each pair in both
     * directions.
     */
    QLIST_FOREACH(a, &bs->parents, next_parent) {
        QLIST_FOREACH(b, &bs->parents, next_parent) {
            if (a == b) {
                continue;
            }

            if (!bdrv_a_allow_b(a, b, errp)) {
                return true;
            }
        }
    }

    return false;
}

static void bdrv_child_perm(BlockDriverState *bs, BlockDriverState *child_bs,
                            BdrvChild *c, BdrvChildRole role,
                            BlockReopenQueue *reopen_queue,
                            uint64_t parent_perm, uint64_t parent_shared,
                            uint64_t *nperm, uint64_t *nshared)
{
    assert(bs->drv && bs->drv->bdrv_child_perm);
    GLOBAL_STATE_CODE();
    bs->drv->bdrv_child_perm(bs, c, role, reopen_queue,
                             parent_perm, parent_shared,
                             nperm, nshared);
    /* TODO Take force_share from reopen_queue */
    if (child_bs && child_bs->force_share) {
        *nshared = BLK_PERM_ALL;
    }
}

/*
 * Adds the whole subtree of @bs (including @bs itself) to the @list (except for
 * nodes that are already in the @list, of course) so that final list is
 * topologically sorted. Return the result (GSList @list object is updated, so
 * don't use old reference after function call).
 *
 * On function start @list must be already topologically sorted and for any node
 * in the @list the whole subtree of the node must be in the @list as well. The
 * simplest way to satisfy this criteria: use only result of
 * bdrv_topological_dfs() or NULL as @list parameter.
 */
static GSList *bdrv_topological_dfs(GSList *list, GHashTable *found,
                                    BlockDriverState *bs)
{
    BdrvChild *child;
    g_autoptr(GHashTable) local_found = NULL;

    GLOBAL_STATE_CODE();

    if (!found) {
        assert(!list);
        found = local_found = g_hash_table_new(NULL, NULL);
    }

    if (g_hash_table_contains(found, bs)) {
        return list;
    }
    g_hash_table_add(found, bs);

    QLIST_FOREACH(child, &bs->children, next) {
        list = bdrv_topological_dfs(list, found, child->bs);
    }

    return g_slist_prepend(list, bs);
}

typedef struct BdrvChildSetPermState {
    BdrvChild *child;
    uint64_t old_perm;
    uint64_t old_shared_perm;
} BdrvChildSetPermState;

static void bdrv_child_set_perm_abort(void *opaque)
{
    BdrvChildSetPermState *s = opaque;

    GLOBAL_STATE_CODE();

    s->child->perm = s->old_perm;
    s->child->shared_perm = s->old_shared_perm;
}

static TransactionActionDrv bdrv_child_set_pem_drv = {
    .abort = bdrv_child_set_perm_abort,
    .clean = g_free,
};

static void bdrv_child_set_perm(BdrvChild *c, uint64_t perm,
                                uint64_t shared, Transaction *tran)
{
    BdrvChildSetPermState *s = g_new(BdrvChildSetPermState, 1);
    GLOBAL_STATE_CODE();

    *s = (BdrvChildSetPermState) {
        .child = c,
        .old_perm = c->perm,
        .old_shared_perm = c->shared_perm,
    };

    c->perm = perm;
    c->shared_perm = shared;

    tran_add(tran, &bdrv_child_set_pem_drv, s);
}

static void bdrv_drv_set_perm_commit(void *opaque)
{
    BlockDriverState *bs = opaque;
    uint64_t cumulative_perms, cumulative_shared_perms;
    GLOBAL_STATE_CODE();

    if (bs->drv->bdrv_set_perm) {
        bdrv_get_cumulative_perm(bs, &cumulative_perms,
                                 &cumulative_shared_perms);
        bs->drv->bdrv_set_perm(bs, cumulative_perms, cumulative_shared_perms);
    }
}

static void bdrv_drv_set_perm_abort(void *opaque)
{
    BlockDriverState *bs = opaque;
    GLOBAL_STATE_CODE();

    if (bs->drv->bdrv_abort_perm_update) {
        bs->drv->bdrv_abort_perm_update(bs);
    }
}

TransactionActionDrv bdrv_drv_set_perm_drv = {
    .abort = bdrv_drv_set_perm_abort,
    .commit = bdrv_drv_set_perm_commit,
};

static int bdrv_drv_set_perm(BlockDriverState *bs, uint64_t perm,
                             uint64_t shared_perm, Transaction *tran,
                             Error **errp)
{
    GLOBAL_STATE_CODE();
    if (!bs->drv) {
        return 0;
    }

    if (bs->drv->bdrv_check_perm) {
        int ret = bs->drv->bdrv_check_perm(bs, perm, shared_perm, errp);
        if (ret < 0) {
            return ret;
        }
    }

    if (tran) {
        tran_add(tran, &bdrv_drv_set_perm_drv, bs);
    }

    return 0;
}

typedef struct BdrvReplaceChildState {
    BdrvChild *child;
    BlockDriverState *old_bs;
} BdrvReplaceChildState;

static void bdrv_replace_child_commit(void *opaque)
{
    BdrvReplaceChildState *s = opaque;
    GLOBAL_STATE_CODE();

    bdrv_unref(s->old_bs);
}

static void bdrv_replace_child_abort(void *opaque)
{
    BdrvReplaceChildState *s = opaque;
    BlockDriverState *new_bs = s->child->bs;

    GLOBAL_STATE_CODE();
    /* old_bs reference is transparently moved from @s to @s->child */
    if (!s->child->bs) {
        /*
         * The parents were undrained when removing old_bs from the child. New
         * requests can't have been made, though, because the child was empty.
         *
         * TODO Make bdrv_replace_child_noperm() transactionable to avoid
         * undraining the parent in the first place. Once this is done, having
         * new_bs drained when calling bdrv_replace_child_tran() is not a
         * requirement any more.
         */
        bdrv_parent_drained_begin_single(s->child);
        assert(!bdrv_parent_drained_poll_single(s->child));
    }
    assert(s->child->quiesced_parent);
    bdrv_replace_child_noperm(s->child, s->old_bs);
    bdrv_unref(new_bs);
}

static TransactionActionDrv bdrv_replace_child_drv = {
    .commit = bdrv_replace_child_commit,
    .abort = bdrv_replace_child_abort,
    .clean = g_free,
};

/*
 * bdrv_replace_child_tran
 *
 * Note: real unref of old_bs is done only on commit.
 *
 * Both @child->bs and @new_bs (if non-NULL) must be drained. @new_bs must be
 * kept drained until the transaction is completed.
 *
 * The function doesn't update permissions, caller is responsible for this.
 */
static void bdrv_replace_child_tran(BdrvChild *child, BlockDriverState *new_bs,
                                    Transaction *tran)
{
    BdrvReplaceChildState *s = g_new(BdrvReplaceChildState, 1);

    assert(child->quiesced_parent);
    assert(!new_bs || new_bs->quiesce_counter);

    *s = (BdrvReplaceChildState) {
        .child = child,
        .old_bs = child->bs,
    };
    tran_add(tran, &bdrv_replace_child_drv, s);

    if (new_bs) {
        bdrv_ref(new_bs);
    }
    bdrv_replace_child_noperm(child, new_bs);
    /* old_bs reference is transparently moved from @child to @s */
}

/*
 * Refresh permissions in @bs subtree. The function is intended to be called
 * after some graph modification that was done without permission update.
 */
static int bdrv_node_refresh_perm(BlockDriverState *bs, BlockReopenQueue *q,
                                  Transaction *tran, Error **errp)
{
    BlockDriver *drv = bs->drv;
    BdrvChild *c;
    int ret;
    uint64_t cumulative_perms, cumulative_shared_perms;
    GLOBAL_STATE_CODE();

    bdrv_get_cumulative_perm(bs, &cumulative_perms, &cumulative_shared_perms);

    /* Write permissions never work with read-only images */
    if ((cumulative_perms & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED)) &&
        !bdrv_is_writable_after_reopen(bs, q))
    {
        if (!bdrv_is_writable_after_reopen(bs, NULL)) {
            error_setg(errp, "Block node is read-only");
        } else {
            error_setg(errp, "Read-only block node '%s' cannot support "
                       "read-write users", bdrv_get_node_name(bs));
        }

        return -EPERM;
    }

    /*
     * Unaligned requests will automatically be aligned to bl.request_alignment
     * and without RESIZE we can't extend requests to write to space beyond the
     * end of the image, so it's required that the image size is aligned.
     */
    if ((cumulative_perms & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED)) &&
        !(cumulative_perms & BLK_PERM_RESIZE))
    {
        if ((bs->total_sectors * BDRV_SECTOR_SIZE) % bs->bl.request_alignment) {
            error_setg(errp, "Cannot get 'write' permission without 'resize': "
                             "Image size is not a multiple of request "
                             "alignment");
            return -EPERM;
        }
    }

    /* Check this node */
    if (!drv) {
        return 0;
    }

    ret = bdrv_drv_set_perm(bs, cumulative_perms, cumulative_shared_perms, tran,
                            errp);
    if (ret < 0) {
        return ret;
    }

    /* Drivers that never have children can omit .bdrv_child_perm() */
    if (!drv->bdrv_child_perm) {
        assert(QLIST_EMPTY(&bs->children));
        return 0;
    }

    /* Check all children */
    QLIST_FOREACH(c, &bs->children, next) {
        uint64_t cur_perm, cur_shared;

        bdrv_child_perm(bs, c->bs, c, c->role, q,
                        cumulative_perms, cumulative_shared_perms,
                        &cur_perm, &cur_shared);
        bdrv_child_set_perm(c, cur_perm, cur_shared, tran);
    }

    return 0;
}

/*
 * @list is a product of bdrv_topological_dfs() (may be called several times) -
 * a topologically sorted subgraph.
 */
static int bdrv_do_refresh_perms(GSList *list, BlockReopenQueue *q,
                                 Transaction *tran, Error **errp)
{
    int ret;
    BlockDriverState *bs;
    GLOBAL_STATE_CODE();

    for ( ; list; list = list->next) {
        bs = list->data;

        if (bdrv_parent_perms_conflict(bs, errp)) {
            return -EINVAL;
        }

        ret = bdrv_node_refresh_perm(bs, q, tran, errp);
        if (ret < 0) {
            return ret;
        }
    }

    return 0;
}

/*
 * @list is any list of nodes. List is completed by all subtrees and
 * topologically sorted. It's not a problem if some node occurs in the @list
 * several times.
 */
static int bdrv_list_refresh_perms(GSList *list, BlockReopenQueue *q,
                                   Transaction *tran, Error **errp)
{
    g_autoptr(GHashTable) found = g_hash_table_new(NULL, NULL);
    g_autoptr(GSList) refresh_list = NULL;

    for ( ; list; list = list->next) {
        refresh_list = bdrv_topological_dfs(refresh_list, found, list->data);
    }

    return bdrv_do_refresh_perms(refresh_list, q, tran, errp);
}

void bdrv_get_cumulative_perm(BlockDriverState *bs, uint64_t *perm,
                              uint64_t *shared_perm)
{
    BdrvChild *c;
    uint64_t cumulative_perms = 0;
    uint64_t cumulative_shared_perms = BLK_PERM_ALL;

    GLOBAL_STATE_CODE();

    QLIST_FOREACH(c, &bs->parents, next_parent) {
        cumulative_perms |= c->perm;
        cumulative_shared_perms &= c->shared_perm;
    }

    *perm = cumulative_perms;
    *shared_perm = cumulative_shared_perms;
}

char *bdrv_perm_names(uint64_t perm)
{
    struct perm_name {
        uint64_t perm;
        const char *name;
    } permissions[] = {
        { BLK_PERM_CONSISTENT_READ, "consistent read" },
        { BLK_PERM_WRITE,           "write" },
        { BLK_PERM_WRITE_UNCHANGED, "write unchanged" },
        { BLK_PERM_RESIZE,          "resize" },
        { 0, NULL }
    };

    GString *result = g_string_sized_new(30);
    struct perm_name *p;

    for (p = permissions; p->name; p++) {
        if (perm & p->perm) {
            if (result->len > 0) {
                g_string_append(result, ", ");
            }
            g_string_append(result, p->name);
        }
    }

    return g_string_free(result, FALSE);
}


/* @tran is allowed to be NULL. In this case no rollback is possible */
static int bdrv_refresh_perms(BlockDriverState *bs, Transaction *tran,
                              Error **errp)
{
    int ret;
    Transaction *local_tran = NULL;
    g_autoptr(GSList) list = bdrv_topological_dfs(NULL, NULL, bs);
    GLOBAL_STATE_CODE();

    if (!tran) {
        tran = local_tran = tran_new();
    }

    ret = bdrv_do_refresh_perms(list, NULL, tran, errp);

    if (local_tran) {
        tran_finalize(local_tran, ret);
    }

    return ret;
}

int bdrv_child_try_set_perm(BdrvChild *c, uint64_t perm, uint64_t shared,
                            Error **errp)
{
    Error *local_err = NULL;
    Transaction *tran = tran_new();
    int ret;

    GLOBAL_STATE_CODE();

    bdrv_child_set_perm(c, perm, shared, tran);

    ret = bdrv_refresh_perms(c->bs, tran, &local_err);

    tran_finalize(tran, ret);

    if (ret < 0) {
        if ((perm & ~c->perm) || (c->shared_perm & ~shared)) {
            /* tighten permissions */
            error_propagate(errp, local_err);
        } else {
            /*
             * Our caller may intend to only loosen restrictions and
             * does not expect this function to fail.  Errors are not
             * fatal in such a case, so we can just hide them from our
             * caller.
             */
            error_free(local_err);
            ret = 0;
        }
    }

    return ret;
}

int bdrv_child_refresh_perms(BlockDriverState *bs, BdrvChild *c, Error **errp)
{
    uint64_t parent_perms, parent_shared;
    uint64_t perms, shared;

    GLOBAL_STATE_CODE();

    bdrv_get_cumulative_perm(bs, &parent_perms, &parent_shared);
    bdrv_child_perm(bs, c->bs, c, c->role, NULL,
                    parent_perms, parent_shared, &perms, &shared);

    return bdrv_child_try_set_perm(c, perms, shared, errp);
}

/*
 * Default implementation for .bdrv_child_perm() for block filters:
 * Forward CONSISTENT_READ, WRITE, WRITE_UNCHANGED, and RESIZE to the
 * filtered child.
 */
static void bdrv_filter_default_perms(BlockDriverState *bs, BdrvChild *c,
                                      BdrvChildRole role,
                                      BlockReopenQueue *reopen_queue,
                                      uint64_t perm, uint64_t shared,
                                      uint64_t *nperm, uint64_t *nshared)
{
    GLOBAL_STATE_CODE();
    *nperm = perm & DEFAULT_PERM_PASSTHROUGH;
    *nshared = (shared & DEFAULT_PERM_PASSTHROUGH) | DEFAULT_PERM_UNCHANGED;
}

static void bdrv_default_perms_for_cow(BlockDriverState *bs, BdrvChild *c,
                                       BdrvChildRole role,
                                       BlockReopenQueue *reopen_queue,
                                       uint64_t perm, uint64_t shared,
                                       uint64_t *nperm, uint64_t *nshared)
{
    assert(role & BDRV_CHILD_COW);
    GLOBAL_STATE_CODE();

    /*
     * We want consistent read from backing files if the parent needs it.
     * No other operations are performed on backing files.
     */
    perm &= BLK_PERM_CONSISTENT_READ;

    /*
     * If the parent can deal with changing data, we're okay with a
     * writable and resizable backing file.
     * TODO Require !(perm & BLK_PERM_CONSISTENT_READ), too?
     */
    if (shared & BLK_PERM_WRITE) {
        shared = BLK_PERM_WRITE | BLK_PERM_RESIZE;
    } else {
        shared = 0;
    }

    shared |= BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;

    if (bs->open_flags & BDRV_O_INACTIVE) {
        shared |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
    }

    *nperm = perm;
    *nshared = shared;
}

static void bdrv_default_perms_for_storage(BlockDriverState *bs, BdrvChild *c,
                                           BdrvChildRole role,
                                           BlockReopenQueue *reopen_queue,
                                           uint64_t perm, uint64_t shared,
                                           uint64_t *nperm, uint64_t *nshared)
{
    int flags;

    GLOBAL_STATE_CODE();
    assert(role & (BDRV_CHILD_METADATA | BDRV_CHILD_DATA));

    flags = bdrv_reopen_get_flags(reopen_queue, bs);

    /*
     * Apart from the modifications below, the same permissions are
     * forwarded and left alone as for filters
     */
    bdrv_filter_default_perms(bs, c, role, reopen_queue,
                              perm, shared, &perm, &shared);

    if (role & BDRV_CHILD_METADATA) {
        /* Format drivers may touch metadata even if the guest doesn't write */
        if (bdrv_is_writable_after_reopen(bs, reopen_queue)) {
            perm |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
        }

        /*
         * bs->file always needs to be consistent because of the
         * metadata. We can never allow other users to resize or write
         * to it.
         */
        if (!(flags & BDRV_O_NO_IO)) {
            perm |= BLK_PERM_CONSISTENT_READ;
        }
        shared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
    }

    if (role & BDRV_CHILD_DATA) {
        /*
         * Technically, everything in this block is a subset of the
         * BDRV_CHILD_METADATA path taken above, and so this could
         * be an "else if" branch.  However, that is not obvious, and
         * this function is not performance critical, therefore we let
         * this be an independent "if".
         */

        /*
         * We cannot allow other users to resize the file because the
         * format driver might have some assumptions about the size
         * (e.g. because it is stored in metadata, or because the file
         * is split into fixed-size data files).
         */
        shared &= ~BLK_PERM_RESIZE;

        /*
         * WRITE_UNCHANGED often cannot be performed as such on the
         * data file.  For example, the qcow2 driver may still need to
         * write copied clusters on copy-on-read.
         */
        if (perm & BLK_PERM_WRITE_UNCHANGED) {
            perm |= BLK_PERM_WRITE;
        }

        /*
         * If the data file is written to, the format driver may
         * expect to be able to resize it by writing beyond the EOF.
         */
        if (perm & BLK_PERM_WRITE) {
            perm |= BLK_PERM_RESIZE;
        }
    }

    if (bs->open_flags & BDRV_O_INACTIVE) {
        shared |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
    }

    *nperm = perm;
    *nshared = shared;
}

void bdrv_default_perms(BlockDriverState *bs, BdrvChild *c,
                        BdrvChildRole role, BlockReopenQueue *reopen_queue,
                        uint64_t perm, uint64_t shared,
                        uint64_t *nperm, uint64_t *nshared)
{
    GLOBAL_STATE_CODE();
    if (role & BDRV_CHILD_FILTERED) {
        assert(!(role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
                         BDRV_CHILD_COW)));
        bdrv_filter_default_perms(bs, c, role, reopen_queue,
                                  perm, shared, nperm, nshared);
    } else if (role & BDRV_CHILD_COW) {
        assert(!(role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA)));
        bdrv_default_perms_for_cow(bs, c, role, reopen_queue,
                                   perm, shared, nperm, nshared);
    } else if (role & (BDRV_CHILD_METADATA | BDRV_CHILD_DATA)) {
        bdrv_default_perms_for_storage(bs, c, role, reopen_queue,
                                       perm, shared, nperm, nshared);
    } else {
        g_assert_not_reached();
    }
}

uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm)
{
    static const uint64_t permissions[] = {
        [BLOCK_PERMISSION_CONSISTENT_READ]  = BLK_PERM_CONSISTENT_READ,
        [BLOCK_PERMISSION_WRITE]            = BLK_PERM_WRITE,
        [BLOCK_PERMISSION_WRITE_UNCHANGED]  = BLK_PERM_WRITE_UNCHANGED,
        [BLOCK_PERMISSION_RESIZE]           = BLK_PERM_RESIZE,
    };

    QEMU_BUILD_BUG_ON(ARRAY_SIZE(permissions) != BLOCK_PERMISSION__MAX);
    QEMU_BUILD_BUG_ON(1UL << ARRAY_SIZE(permissions) != BLK_PERM_ALL + 1);

    assert(qapi_perm < BLOCK_PERMISSION__MAX);

    return permissions[qapi_perm];
}

/*
 * Replaces the node that a BdrvChild points to without updating permissions.
 *
 * If @new_bs is non-NULL, the parent of @child must already be drained through
 * @child.
 */
static void bdrv_replace_child_noperm(BdrvChild *child,
                                      BlockDriverState *new_bs)
{
    BlockDriverState *old_bs = child->bs;
    int new_bs_quiesce_counter;

    assert(!child->frozen);

    /*
     * If we want to change the BdrvChild to point to a drained node as its new
     * child->bs, we need to make sure that its new parent is drained, too. In
     * other words, either child->quiesce_parent must already be true or we must
     * be able to set it and keep the parent's quiesce_counter consistent with
     * that, but without polling or starting new requests (this function
     * guarantees that it doesn't poll, and starting new requests would be
     * against the invariants of drain sections).
     *
     * To keep things simple, we pick the first option (child->quiesce_parent
     * must already be true). We also generalise the rule a bit to make it
     * easier to verify in callers and more likely to be covered in test cases:
     * The parent must be quiesced through this child even if new_bs isn't
     * currently drained.
     *
     * The only exception is for callers that always pass new_bs == NULL. In
     * this case, we obviously never need to consider the case of a drained
     * new_bs, so we can keep the callers simpler by allowing them not to drain
     * the parent.
     */
    assert(!new_bs || child->quiesced_parent);
    assert(old_bs != new_bs);
    GLOBAL_STATE_CODE();

    if (old_bs && new_bs) {
        assert(bdrv_get_aio_context(old_bs) == bdrv_get_aio_context(new_bs));
    }

    /* TODO Pull this up into the callers to avoid polling here */
    bdrv_graph_wrlock();
    if (old_bs) {
        if (child->klass->detach) {
            child->klass->detach(child);
        }
        QLIST_REMOVE(child, next_parent);
    }

    child->bs = new_bs;

    if (new_bs) {
        QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
        if (child->klass->attach) {
            child->klass->attach(child);
        }
    }
    bdrv_graph_wrunlock();

    /*
     * If the parent was drained through this BdrvChild previously, but new_bs
     * is not drained, allow requests to come in only after the new node has
     * been attached.
     */
    new_bs_quiesce_counter = (new_bs ? new_bs->quiesce_counter : 0);
    if (!new_bs_quiesce_counter && child->quiesced_parent) {
        bdrv_parent_drained_end_single(child);
    }
}

/**
 * Free the given @child.
 *
 * The child must be empty (i.e. `child->bs == NULL`) and it must be
 * unused (i.e. not in a children list).
 */
static void bdrv_child_free(BdrvChild *child)
{
    assert(!child->bs);
    GLOBAL_STATE_CODE();
    assert(!child->next.le_prev); /* not in children list */

    g_free(child->name);
    g_free(child);
}

typedef struct BdrvAttachChildCommonState {
    BdrvChild *child;
    AioContext *old_parent_ctx;
    AioContext *old_child_ctx;
} BdrvAttachChildCommonState;

static void bdrv_attach_child_common_abort(void *opaque)
{
    BdrvAttachChildCommonState *s = opaque;
    BlockDriverState *bs = s->child->bs;

    GLOBAL_STATE_CODE();
    bdrv_replace_child_noperm(s->child, NULL);

    if (bdrv_get_aio_context(bs) != s->old_child_ctx) {
        bdrv_try_change_aio_context(bs, s->old_child_ctx, NULL, &error_abort);
    }

    if (bdrv_child_get_parent_aio_context(s->child) != s->old_parent_ctx) {
        Transaction *tran;
        GHashTable *visited;
        bool ret;

        tran = tran_new();

        /* No need to visit `child`, because it has been detached already */
        visited = g_hash_table_new(NULL, NULL);
        ret = s->child->klass->change_aio_ctx(s->child, s->old_parent_ctx,
                                              visited, tran, &error_abort);
        g_hash_table_destroy(visited);

        /* transaction is supposed to always succeed */
        assert(ret == true);
        tran_commit(tran);
    }

    bdrv_unref(bs);
    bdrv_child_free(s->child);
}

static TransactionActionDrv bdrv_attach_child_common_drv = {
    .abort = bdrv_attach_child_common_abort,
    .clean = g_free,
};

/*
 * Common part of attaching bdrv child to bs or to blk or to job
 *
 * Function doesn't update permissions, caller is responsible for this.
 *
 * Returns new created child.
 */
static BdrvChild *bdrv_attach_child_common(BlockDriverState *child_bs,
                                           const char *child_name,
                                           const BdrvChildClass *child_class,
                                           BdrvChildRole child_role,
                                           uint64_t perm, uint64_t shared_perm,
                                           void *opaque,
                                           Transaction *tran, Error **errp)
{
    BdrvChild *new_child;
    AioContext *parent_ctx;
    AioContext *child_ctx = bdrv_get_aio_context(child_bs);

    assert(child_class->get_parent_desc);
    GLOBAL_STATE_CODE();

    new_child = g_new(BdrvChild, 1);
    *new_child = (BdrvChild) {
        .bs             = NULL,
        .name           = g_strdup(child_name),
        .klass          = child_class,
        .role           = child_role,
        .perm           = perm,
        .shared_perm    = shared_perm,
        .opaque         = opaque,
    };

    /*
     * If the AioContexts don't match, first try to move the subtree of
     * child_bs into the AioContext of the new parent. If this doesn't work,
     * try moving the parent into the AioContext of child_bs instead.
     */
    parent_ctx = bdrv_child_get_parent_aio_context(new_child);
    if (child_ctx != parent_ctx) {
        Error *local_err = NULL;
        int ret = bdrv_try_change_aio_context(child_bs, parent_ctx, NULL,
                                              &local_err);

        if (ret < 0 && child_class->change_aio_ctx) {
            Transaction *tran = tran_new();
            GHashTable *visited = g_hash_table_new(NULL, NULL);
            bool ret_child;

            g_hash_table_add(visited, new_child);
            ret_child = child_class->change_aio_ctx(new_child, child_ctx,
                                                    visited, tran, NULL);
            if (ret_child == true) {
                error_free(local_err);
                ret = 0;
            }
            tran_finalize(tran, ret_child == true ? 0 : -1);
            g_hash_table_destroy(visited);
        }

        if (ret < 0) {
            error_propagate(errp, local_err);
            bdrv_child_free(new_child);
            return NULL;
        }
    }

    bdrv_ref(child_bs);
    /*
     * Let every new BdrvChild start with a drained parent. Inserting the child
     * in the graph with bdrv_replace_child_noperm() will undrain it if
     * @child_bs is not drained.
     *
     * The child was only just created and is not yet visible in global state
     * until bdrv_replace_child_noperm() inserts it into the graph, so nobody
     * could have sent requests and polling is not necessary.
     *
     * Note that this means that the parent isn't fully drained yet, we only
     * stop new requests from coming in. This is fine, we don't care about the
     * old requests here, they are not for this child. If another place enters a
     * drain section for the same parent, but wants it to be fully quiesced, it
     * will not run most of the the code in .drained_begin() again (which is not
     * a problem, we already did this), but it will still poll until the parent
     * is fully quiesced, so it will not be negatively affected either.
     */
    bdrv_parent_drained_begin_single(new_child);
    bdrv_replace_child_noperm(new_child, child_bs);

    BdrvAttachChildCommonState *s = g_new(BdrvAttachChildCommonState, 1);
    *s = (BdrvAttachChildCommonState) {
        .child = new_child,
        .old_parent_ctx = parent_ctx,
        .old_child_ctx = child_ctx,
    };
    tran_add(tran, &bdrv_attach_child_common_drv, s);

    return new_child;
}

/*
 * Function doesn't update permissions, caller is responsible for this.
 */
static BdrvChild *bdrv_attach_child_noperm(BlockDriverState *parent_bs,
                                           BlockDriverState *child_bs,
                                           const char *child_name,
                                           const BdrvChildClass *child_class,
                                           BdrvChildRole child_role,
                                           Transaction *tran,
                                           Error **errp)
{
    uint64_t perm, shared_perm;

    assert(parent_bs->drv);
    GLOBAL_STATE_CODE();

    if (bdrv_recurse_has_child(child_bs, parent_bs)) {
        error_setg(errp, "Making '%s' a %s child of '%s' would create a cycle",
                   child_bs->node_name, child_name, parent_bs->node_name);
        return NULL;
    }

    bdrv_get_cumulative_perm(parent_bs, &perm, &shared_perm);
    bdrv_child_perm(parent_bs, child_bs, NULL, child_role, NULL,
                    perm, shared_perm, &perm, &shared_perm);

    return bdrv_attach_child_common(child_bs, child_name, child_class,
                                    child_role, perm, shared_perm, parent_bs,
                                    tran, errp);
}

/*
 * This function steals the reference to child_bs from the caller.
 * That reference is later dropped by bdrv_root_unref_child().
 *
 * On failure NULL is returned, errp is set and the reference to
 * child_bs is also dropped.
 *
 * The caller must hold the AioContext lock @child_bs, but not that of @ctx
 * (unless @child_bs is already in @ctx).
 */
BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                  const char *child_name,
                                  const BdrvChildClass *child_class,
                                  BdrvChildRole child_role,
                                  uint64_t perm, uint64_t shared_perm,
                                  void *opaque, Error **errp)
{
    int ret;
    BdrvChild *child;
    Transaction *tran = tran_new();

    GLOBAL_STATE_CODE();

    child = bdrv_attach_child_common(child_bs, child_name, child_class,
                                   child_role, perm, shared_perm, opaque,
                                   tran, errp);
    if (!child) {
        ret = -EINVAL;
        goto out;
    }

    ret = bdrv_refresh_perms(child_bs, tran, errp);

out:
    tran_finalize(tran, ret);

    bdrv_unref(child_bs);

    return ret < 0 ? NULL : child;
}

/*
 * This function transfers the reference to child_bs from the caller
 * to parent_bs. That reference is later dropped by parent_bs on
 * bdrv_close() or if someone calls bdrv_unref_child().
 *
 * On failure NULL is returned, errp is set and the reference to
 * child_bs is also dropped.
 *
 * If @parent_bs and @child_bs are in different AioContexts, the caller must
 * hold the AioContext lock for @child_bs, but not for @parent_bs.
 */
BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
                             BlockDriverState *child_bs,
                             const char *child_name,
                             const BdrvChildClass *child_class,
                             BdrvChildRole child_role,
                             Error **errp)
{
    int ret;
    BdrvChild *child;
    Transaction *tran = tran_new();

    GLOBAL_STATE_CODE();

    child = bdrv_attach_child_noperm(parent_bs, child_bs, child_name,
                                     child_class, child_role, tran, errp);
    if (!child) {
        ret = -EINVAL;
        goto out;
    }

    ret = bdrv_refresh_perms(parent_bs, tran, errp);
    if (ret < 0) {
        goto out;
    }

out:
    tran_finalize(tran, ret);

    bdrv_unref(child_bs);

    return ret < 0 ? NULL : child;
}

/* Callers must ensure that child->frozen is false. */
void bdrv_root_unref_child(BdrvChild *child)
{
    BlockDriverState *child_bs = child->bs;

    GLOBAL_STATE_CODE();
    bdrv_replace_child_noperm(child, NULL);
    bdrv_child_free(child);

    if (child_bs) {
        /*
         * Update permissions for old node. We're just taking a parent away, so
         * we're loosening restrictions. Errors of permission update are not
         * fatal in this case, ignore them.
         */
        bdrv_refresh_perms(child_bs, NULL, NULL);

        /*
         * When the parent requiring a non-default AioContext is removed, the
         * node moves back to the main AioContext
         */
        bdrv_try_change_aio_context(child_bs, qemu_get_aio_context(), NULL,
                                    NULL);
    }

    bdrv_unref(child_bs);
}

typedef struct BdrvSetInheritsFrom {
    BlockDriverState *bs;
    BlockDriverState *old_inherits_from;
} BdrvSetInheritsFrom;

static void bdrv_set_inherits_from_abort(void *opaque)
{
    BdrvSetInheritsFrom *s = opaque;

    s->bs->inherits_from = s->old_inherits_from;
}

static TransactionActionDrv bdrv_set_inherits_from_drv = {
    .abort = bdrv_set_inherits_from_abort,
    .clean = g_free,
};

/* @tran is allowed to be NULL. In this case no rollback is possible */
static void bdrv_set_inherits_from(BlockDriverState *bs,
                                   BlockDriverState *new_inherits_from,
                                   Transaction *tran)
{
    if (tran) {
        BdrvSetInheritsFrom *s = g_new(BdrvSetInheritsFrom, 1);

        *s = (BdrvSetInheritsFrom) {
            .bs = bs,
            .old_inherits_from = bs->inherits_from,
        };

        tran_add(tran, &bdrv_set_inherits_from_drv, s);
    }

    bs->inherits_from = new_inherits_from;
}

/**
 * Clear all inherits_from pointers from children and grandchildren of
 * @root that point to @root, where necessary.
 * @tran is allowed to be NULL. In this case no rollback is possible
 */
static void bdrv_unset_inherits_from(BlockDriverState *root, BdrvChild *child,
                                     Transaction *tran)
{
    BdrvChild *c;

    if (child->bs->inherits_from == root) {
        /*
         * Remove inherits_from only when the last reference between root and
         * child->bs goes away.
         */
        QLIST_FOREACH(c, &root->children, next) {
            if (c != child && c->bs == child->bs) {
                break;
            }
        }
        if (c == NULL) {
            bdrv_set_inherits_from(child->bs, NULL, tran);
        }
    }

    QLIST_FOREACH(c, &child->bs->children, next) {
        bdrv_unset_inherits_from(root, c, tran);
    }
}

/* Callers must ensure that child->frozen is false. */
void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child)
{
    GLOBAL_STATE_CODE();
    if (child == NULL) {
        return;
    }

    bdrv_unset_inherits_from(parent, child, NULL);
    bdrv_root_unref_child(child);
}


static void bdrv_parent_cb_change_media(BlockDriverState *bs, bool load)
{
    BdrvChild *c;
    GLOBAL_STATE_CODE();
    QLIST_FOREACH(c, &bs->parents, next_parent) {
        if (c->klass->change_media) {
            c->klass->change_media(c, load);
        }
    }
}

/* Return true if you can reach parent going through child->inherits_from
 * recursively. If parent or child are NULL, return false */
static bool bdrv_inherits_from_recursive(BlockDriverState *child,
                                         BlockDriverState *parent)
{
    while (child && child != parent) {
        child = child->inherits_from;
    }

    return child != NULL;
}

/*
 * Return the BdrvChildRole for @bs's backing child.  bs->backing is
 * mostly used for COW backing children (role = COW), but also for
 * filtered children (role = FILTERED | PRIMARY).
 */
static BdrvChildRole bdrv_backing_role(BlockDriverState *bs)
{
    if (bs->drv && bs->drv->is_filter) {
        return BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY;
    } else {
        return BDRV_CHILD_COW;
    }
}

/*
 * Sets the bs->backing or bs->file link of a BDS. A new reference is created;
 * callers which don't need their own reference any more must call bdrv_unref().
 *
 * Function doesn't update permissions, caller is responsible for this.
 */
static int bdrv_set_file_or_backing_noperm(BlockDriverState *parent_bs,
                                           BlockDriverState *child_bs,
                                           bool is_backing,
                                           Transaction *tran, Error **errp)
{
    bool update_inherits_from =
        bdrv_inherits_from_recursive(child_bs, parent_bs);
    BdrvChild *child = is_backing ? parent_bs->backing : parent_bs->file;
    BdrvChildRole role;

    GLOBAL_STATE_CODE();

    if (!parent_bs->drv) {
        /*
         * Node without drv is an object without a class :/. TODO: finally fix
         * qcow2 driver to never clear bs->drv and implement format corruption
         * handling in other way.
         */
        error_setg(errp, "Node corrupted");
        return -EINVAL;
    }

    if (child && child->frozen) {
        error_setg(errp, "Cannot change frozen '%s' link from '%s' to '%s'",
                   child->name, parent_bs->node_name, child->bs->node_name);
        return -EPERM;
    }

    if (is_backing && !parent_bs->drv->is_filter &&
        !parent_bs->drv->supports_backing)
    {
        error_setg(errp, "Driver '%s' of node '%s' does not support backing "
                   "files", parent_bs->drv->format_name, parent_bs->node_name);
        return -EINVAL;
    }

    if (parent_bs->drv->is_filter) {
        role = BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY;
    } else if (is_backing) {
        role = BDRV_CHILD_COW;
    } else {
        /*
         * We only can use same role as it is in existing child. We don't have
         * infrastructure to determine role of file child in generic way
         */
        if (!child) {
            error_setg(errp, "Cannot set file child to format node without "
                       "file child");
            return -EINVAL;
        }
        role = child->role;
    }

    if (child) {
        bdrv_unset_inherits_from(parent_bs, child, tran);
        bdrv_remove_child(child, tran);
    }

    if (!child_bs) {
        goto out;
    }

    child = bdrv_attach_child_noperm(parent_bs, child_bs,
                                     is_backing ? "backing" : "file",
                                     &child_of_bds, role,
                                     tran, errp);
    if (!child) {
        return -EINVAL;
    }


    /*
     * If inherits_from pointed recursively to bs then let's update it to
     * point directly to bs (else it will become NULL).
     */
    if (update_inherits_from) {
        bdrv_set_inherits_from(child_bs, parent_bs, tran);
    }

out:
    bdrv_refresh_limits(parent_bs, tran, NULL);

    return 0;
}

static int bdrv_set_backing_noperm(BlockDriverState *bs,
                                   BlockDriverState *backing_hd,
                                   Transaction *tran, Error **errp)
{
    GLOBAL_STATE_CODE();
    return bdrv_set_file_or_backing_noperm(bs, backing_hd, true, tran, errp);
}

int bdrv_set_backing_hd_drained(BlockDriverState *bs,
                                BlockDriverState *backing_hd,
                                Error **errp)
{
    int ret;
    Transaction *tran = tran_new();

    GLOBAL_STATE_CODE();
    assert(bs->quiesce_counter > 0);

    ret = bdrv_set_backing_noperm(bs, backing_hd, tran, errp);
    if (ret < 0) {
        goto out;
    }

    ret = bdrv_refresh_perms(bs, tran, errp);
out:
    tran_finalize(tran, ret);
    return ret;
}

int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
                        Error **errp)
{
    int ret;
    GLOBAL_STATE_CODE();

    bdrv_drained_begin(bs);
    ret = bdrv_set_backing_hd_drained(bs, backing_hd, errp);
    bdrv_drained_end(bs);

    return ret;
}

/*
 * Opens the backing file for a BlockDriverState if not yet open
 *
 * bdref_key specifies the key for the image's BlockdevRef in the options QDict.
 * That QDict has to be flattened; therefore, if the BlockdevRef is a QDict
 * itself, all options starting with "${bdref_key}." are considered part of the
 * BlockdevRef.
 *
 * TODO Can this be unified with bdrv_open_image()?
 */
int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
                           const char *bdref_key, Error **errp)
{
    char *backing_filename = NULL;
    char *bdref_key_dot;
    const char *reference = NULL;
    int ret = 0;
    bool implicit_backing = false;
    BlockDriverState *backing_hd;
    QDict *options;
    QDict *tmp_parent_options = NULL;
    Error *local_err = NULL;

    GLOBAL_STATE_CODE();

    if (bs->backing != NULL) {
        goto free_exit;
    }

    /* NULL means an empty set of options */
    if (parent_options == NULL) {
        tmp_parent_options = qdict_new();
        parent_options = tmp_parent_options;
    }

    bs->open_flags &= ~BDRV_O_NO_BACKING;

    bdref_key_dot = g_strdup_printf("%s.", bdref_key);
    qdict_extract_subqdict(parent_options, &options, bdref_key_dot);
    g_free(bdref_key_dot);

    /*
     * Caution: while qdict_get_try_str() is fine, getting non-string
     * types would require more care.  When @parent_options come from
     * -blockdev or blockdev_add, its members are typed according to
     * the QAPI schema, but when they come from -drive, they're all
     * QString.
     */
    reference = qdict_get_try_str(parent_options, bdref_key);
    if (reference || qdict_haskey(options, "file.filename")) {
        /* keep backing_filename NULL */
    } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
        qobject_unref(options);
        goto free_exit;
    } else {
        if (qdict_size(options) == 0) {
            /* If the user specifies options that do not modify the
             * backing file's behavior, we might still consider it the
             * implicit backing file.  But it's easier this way, and
             * just specifying some of the backing BDS's options is
             * only possible with -drive anyway (otherwise the QAPI
             * schema forces the user to specify everything). */
            implicit_backing = !strcmp(bs->auto_backing_file, bs->backing_file);
        }

        backing_filename = bdrv_get_full_backing_filename(bs, &local_err);
        if (local_err) {
            ret = -EINVAL;
            error_propagate(errp, local_err);
            qobject_unref(options);
            goto free_exit;
        }
    }

    if (!bs->drv || !bs->drv->supports_backing) {
        ret = -EINVAL;
        error_setg(errp, "Driver doesn't support backing files");
        qobject_unref(options);
        goto free_exit;
    }

    if (!reference &&
        bs->backing_format[0] != '\0' && !qdict_haskey(options, "driver")) {
        qdict_put_str(options, "driver", bs->backing_format);
    }

    backing_hd = bdrv_open_inherit(backing_filename, reference, options, 0, bs,
                                   &child_of_bds, bdrv_backing_role(bs), errp);
    if (!backing_hd) {
        bs->open_flags |= BDRV_O_NO_BACKING;
        error_prepend(errp, "Could not open backing file: ");
        ret = -EINVAL;
        goto free_exit;
    }

    if (implicit_backing) {
        bdrv_refresh_filename(backing_hd);
        pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
                backing_hd->filename);
    }

    /* Hook up the backing file link; drop our reference, bs owns the
     * backing_hd reference now */
    ret = bdrv_set_backing_hd(bs, backing_hd, errp);
    bdrv_unref(backing_hd);
    if (ret < 0) {
        goto free_exit;
    }

    qdict_del(parent_options, bdref_key);

free_exit:
    g_free(backing_filename);
    qobject_unref(tmp_parent_options);
    return ret;
}

static BlockDriverState *
bdrv_open_child_bs(const char *filename, QDict *options, const char *bdref_key,
                   BlockDriverState *parent, const BdrvChildClass *child_class,
                   BdrvChildRole child_role, bool allow_none, Error **errp)
{
    BlockDriverState *bs = NULL;
    QDict *image_options;
    char *bdref_key_dot;
    const char *reference;

    assert(child_class != NULL);

    bdref_key_dot = g_strdup_printf("%s.", bdref_key);
    qdict_extract_subqdict(options, &image_options, bdref_key_dot);
    g_free(bdref_key_dot);

    /*
     * Caution: while qdict_get_try_str() is fine, getting non-string
     * types would require more care.  When @options come from
     * -blockdev or blockdev_add, its members are typed according to
     * the QAPI schema, but when they come from -drive, they're all
     * QString.
     */
    reference = qdict_get_try_str(options, bdref_key);
    if (!filename && !reference && !qdict_size(image_options)) {
        if (!allow_none) {
            error_setg(errp, "A block device must be specified for \"%s\"",
                       bdref_key);
        }
        qobject_unref(image_options);
        goto done;
    }

    bs = bdrv_open_inherit(filename, reference, image_options, 0,
                           parent, child_class, child_role, errp);
    if (!bs) {
        goto done;
    }

done:
    qdict_del(options, bdref_key);
    return bs;
}

/*
 * Opens a disk image whose options are given as BlockdevRef in another block
 * device's options.
 *
 * If allow_none is true, no image will be opened if filename is false and no
 * BlockdevRef is given. NULL will be returned, but errp remains unset.
 *
 * bdrev_key specifies the key for the image's BlockdevRef in the options QDict.
 * That QDict has to be flattened; therefore, if the BlockdevRef is a QDict
 * itself, all options starting with "${bdref_key}." are considered part of the
 * BlockdevRef.
 *
 * The BlockdevRef will be removed from the options QDict.
 */
BdrvChild *bdrv_open_child(const char *filename,
                           QDict *options, const char *bdref_key,
                           BlockDriverState *parent,
                           const BdrvChildClass *child_class,
                           BdrvChildRole child_role,
                           bool allow_none, Error **errp)
{
    BlockDriverState *bs;

    GLOBAL_STATE_CODE();

    bs = bdrv_open_child_bs(filename, options, bdref_key, parent, child_class,
                            child_role, allow_none, errp);
    if (bs == NULL) {
        return NULL;
    }

    return bdrv_attach_child(parent, bs, bdref_key, child_class, child_role,
                             errp);
}

/*
 * Wrapper on bdrv_open_child() for most popular case: open primary child of bs.
 */
int bdrv_open_file_child(const char *filename,
                         QDict *options, const char *bdref_key,
                         BlockDriverState *parent, Error **errp)
{
    BdrvChildRole role;

    /* commit_top and mirror_top don't use this function */
    assert(!parent->drv->filtered_child_is_backing);
    role = parent->drv->is_filter ?
        (BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY) : BDRV_CHILD_IMAGE;

    if (!bdrv_open_child(filename, options, bdref_key, parent,
                         &child_of_bds, role, false, errp))
    {
        return -EINVAL;
    }

    return 0;
}

/*
 * TODO Future callers may need to specify parent/child_class in order for
 * option inheritance to work. Existing callers use it for the root node.
 */
BlockDriverState *bdrv_open_blockdev_ref(BlockdevRef *ref, Error **errp)
{
    BlockDriverState *bs = NULL;
    QObject *obj = NULL;
    QDict *qdict = NULL;
    const char *reference = NULL;
    Visitor *v = NULL;

    GLOBAL_STATE_CODE();

    if (ref->type == QTYPE_QSTRING) {
        reference = ref->u.reference;
    } else {
        BlockdevOptions *options = &ref->u.definition;
        assert(ref->type == QTYPE_QDICT);

        v = qobject_output_visitor_new(&obj);
        visit_type_BlockdevOptions(v, NULL, &options, &error_abort);
        visit_complete(v, &obj);

        qdict = qobject_to(QDict, obj);
        qdict_flatten(qdict);

        /* bdrv_open_inherit() defaults to the values in bdrv_flags (for
         * compatibility with other callers) rather than what we want as the
         * real defaults. Apply the defaults here instead. */
        qdict_set_default_str(qdict, BDRV_OPT_CACHE_DIRECT, "off");
        qdict_set_default_str(qdict, BDRV_OPT_CACHE_NO_FLUSH, "off");
        qdict_set_default_str(qdict, BDRV_OPT_READ_ONLY, "off");
        qdict_set_default_str(qdict, BDRV_OPT_AUTO_READ_ONLY, "off");

    }

    bs = bdrv_open_inherit(NULL, reference, qdict, 0, NULL, NULL, 0, errp);
    obj = NULL;
    qobject_unref(obj);
    visit_free(v);
    return bs;
}

static BlockDriverState *bdrv_append_temp_snapshot(BlockDriverState *bs,
                                                   int flags,
                                                   QDict *snapshot_options,
                                                   Error **errp)
{
    g_autofree char *tmp_filename = NULL;
    int64_t total_size;
    QemuOpts *opts = NULL;
    BlockDriverState *bs_snapshot = NULL;
    int ret;

    GLOBAL_STATE_CODE();

    /* if snapshot, we create a temporary backing file and open it
       instead of opening 'filename' directly */

    /* Get the required size from the image */
    total_size = bdrv_getlength(bs);
    if (total_size < 0) {
        error_setg_errno(errp, -total_size, "Could not get image size");
        goto out;
    }

    /* Create the temporary image */
    tmp_filename = create_tmp_file(errp);
    if (!tmp_filename) {
        goto out;
    }

    opts = qemu_opts_create(bdrv_qcow2.create_opts, NULL, 0,
                            &error_abort);
    qemu_opt_set_number(opts, BLOCK_OPT_SIZE, total_size, &error_abort);
    ret = bdrv_create(&bdrv_qcow2, tmp_filename, opts, errp);
    qemu_opts_del(opts);
    if (ret < 0) {
        error_prepend(errp, "Could not create temporary overlay '%s': ",
                      tmp_filename);
        goto out;
    }

    /* Prepare options QDict for the temporary file */
    qdict_put_str(snapshot_options, "file.driver", "file");
    qdict_put_str(snapshot_options, "file.filename", tmp_filename);
    qdict_put_str(snapshot_options, "driver", "qcow2");

    bs_snapshot = bdrv_open(NULL, NULL, snapshot_options, flags, errp);
    snapshot_options = NULL;
    if (!bs_snapshot) {
        goto out;
    }

    ret = bdrv_append(bs_snapshot, bs, errp);
    if (ret < 0) {
        bs_snapshot = NULL;
        goto out;
    }

out:
    qobject_unref(snapshot_options);
    return bs_snapshot;
}

/*
 * Opens a disk image (raw, qcow2, vmdk, ...)
 *
 * options is a QDict of options to pass to the block drivers, or NULL for an
 * empty set of options. The reference to the QDict belongs to the block layer
 * after the call (even on failure), so if the caller intends to reuse the
 * dictionary, it needs to use qobject_ref() before calling bdrv_open.
 *
 * If *pbs is NULL, a new BDS will be created with a pointer to it stored there.
 * If it is not NULL, the referenced BDS will be reused.
 *
 * The reference parameter may be used to specify an existing block device which
 * should be opened. If specified, neither options nor a filename may be given,
 * nor can an existing BDS be reused (that is, *pbs has to be NULL).
 *
 * The caller must always hold @filename AioContext lock, because this
 * function eventually calls bdrv_refresh_total_sectors() which polls
 * when called from non-coroutine context.
 */
static BlockDriverState *bdrv_open_inherit(const char *filename,
                                           const char *reference,
                                           QDict *options, int flags,
                                           BlockDriverState *parent,
                                           const BdrvChildClass *child_class,
                                           BdrvChildRole child_role,
                                           Error **errp)
{
    int ret;
    BlockBackend *file = NULL;
    BlockDriverState *bs;
    BlockDriver *drv = NULL;
    BdrvChild *child;
    const char *drvname;
    const char *backing;
    Error *local_err = NULL;
    QDict *snapshot_options = NULL;
    int snapshot_flags = 0;

    assert(!child_class || !flags);
    assert(!child_class == !parent);
    GLOBAL_STATE_CODE();

    if (reference) {
        bool options_non_empty = options ? qdict_size(options) : false;
        qobject_unref(options);

        if (filename || options_non_empty) {
            error_setg(errp, "Cannot reference an existing block device with "
                       "additional options or a new filename");
            return NULL;
        }

        bs = bdrv_lookup_bs(reference, reference, errp);
        if (!bs) {
            return NULL;
        }

        bdrv_ref(bs);
        return bs;
    }

    bs = bdrv_new();

    /* NULL means an empty set of options */
    if (options == NULL) {
        options = qdict_new();
    }

    /* json: syntax counts as explicit options, as if in the QDict */
    parse_json_protocol(options, &filename, &local_err);
    if (local_err) {
        goto fail;
    }

    bs->explicit_options = qdict_clone_shallow(options);

    if (child_class) {
        bool parent_is_format;

        if (parent->drv) {
            parent_is_format = parent->drv->is_format;
        } else {
            /*
             * parent->drv is not set yet because this node is opened for
             * (potential) format probing.  That means that @parent is going
             * to be a format node.
             */
            parent_is_format = true;
        }

        bs->inherits_from = parent;
        child_class->inherit_options(child_role, parent_is_format,
                                     &flags, options,
                                     parent->open_flags, parent->options);
    }

    ret = bdrv_fill_options(&options, filename, &flags, &local_err);
    if (ret < 0) {
        goto fail;
    }

    /*
     * Set the BDRV_O_RDWR and BDRV_O_ALLOW_RDWR flags.
     * Caution: getting a boolean member of @options requires care.
     * When @options come from -blockdev or blockdev_add, members are
     * typed according to the QAPI schema, but when they come from
     * -drive, they're all QString.
     */
    if (g_strcmp0(qdict_get_try_str(options, BDRV_OPT_READ_ONLY), "on") &&
        !qdict_get_try_bool(options, BDRV_OPT_READ_ONLY, false)) {
        flags |= (BDRV_O_RDWR | BDRV_O_ALLOW_RDWR);
    } else {
        flags &= ~BDRV_O_RDWR;
    }

    if (flags & BDRV_O_SNAPSHOT) {
        snapshot_options = qdict_new();
        bdrv_temp_snapshot_options(&snapshot_flags, snapshot_options,
                                   flags, options);
        /* Let bdrv_backing_options() override "read-only" */
        qdict_del(options, BDRV_OPT_READ_ONLY);
        bdrv_inherited_options(BDRV_CHILD_COW, true,
                               &flags, options, flags, options);
    }

    bs->open_flags = flags;
    bs->options = options;
    options = qdict_clone_shallow(options);

    /* Find the right image format driver */
    /* See cautionary note on accessing @options above */
    drvname = qdict_get_try_str(options, "driver");
    if (drvname) {
        drv = bdrv_find_format(drvname);
        if (!drv) {
            error_setg(errp, "Unknown driver: '%s'", drvname);
            goto fail;
        }
    }

    assert(drvname || !(flags & BDRV_O_PROTOCOL));

    /* See cautionary note on accessing @options above */
    backing = qdict_get_try_str(options, "backing");
    if (qobject_to(QNull, qdict_get(options, "backing")) != NULL ||
        (backing && *backing == '\0'))
    {
        if (backing) {
            warn_report("Use of \"backing\": \"\" is deprecated; "
                        "use \"backing\": null instead");
        }
        flags |= BDRV_O_NO_BACKING;
        qdict_del(bs->explicit_options, "backing");
        qdict_del(bs->options, "backing");
        qdict_del(options, "backing");
    }

    /* Open image file without format layer. This BlockBackend is only used for
     * probing, the block drivers will do their own bdrv_open_child() for the
     * same BDS, which is why we put the node name back into options. */
    if ((flags & BDRV_O_PROTOCOL) == 0) {
        BlockDriverState *file_bs;

        file_bs = bdrv_open_child_bs(filename, options, "file", bs,
                                     &child_of_bds, BDRV_CHILD_IMAGE,
                                     true, &local_err);
        if (local_err) {
            goto fail;
        }
        if (file_bs != NULL) {
            /* Not requesting BLK_PERM_CONSISTENT_READ because we're only
             * looking at the header to guess the image format. This works even
             * in cases where a guest would not see a consistent state. */
            file = blk_new(bdrv_get_aio_context(file_bs), 0, BLK_PERM_ALL);
            blk_insert_bs(file, file_bs, &local_err);
            bdrv_unref(file_bs);
            if (local_err) {
                goto fail;
            }

            qdict_put_str(options, "file", bdrv_get_node_name(file_bs));
        }
    }

    /* Image format probing */
    bs->probed = !drv;
    if (!drv && file) {
        ret = find_image_format(file, filename, &drv, &local_err);
        if (ret < 0) {
            goto fail;
        }
        /*
         * This option update would logically belong in bdrv_fill_options(),
         * but we first need to open bs->file for the probing to work, while
         * opening bs->file already requires the (mostly) final set of options
         * so that cache mode etc. can be inherited.
         *
         * Adding the driver later is somewhat ugly, but it's not an option
         * that would ever be inherited, so it's correct. We just need to make
         * sure to update both bs->options (which has the full effective
         * options for bs) and options (which has file.* already removed).
         */
        qdict_put_str(bs->options, "driver", drv->format_name);
        qdict_put_str(options, "driver", drv->format_name);
    } else if (!drv) {
        error_setg(errp, "Must specify either driver or file");
        goto fail;
    }

    /* BDRV_O_PROTOCOL must be set iff a protocol BDS is about to be created */
    assert(!!(flags & BDRV_O_PROTOCOL) == !!drv->bdrv_file_open);
    /* file must be NULL if a protocol BDS is about to be created
     * (the inverse results in an error message from bdrv_open_common()) */
    assert(!(flags & BDRV_O_PROTOCOL) || !file);

    /* Open the image */
    ret = bdrv_open_common(bs, file, options, &local_err);
    if (ret < 0) {
        goto fail;
    }

    if (file) {
        blk_unref(file);
        file = NULL;
    }

    /* If there is a backing file, use it */
    if ((flags & BDRV_O_NO_BACKING) == 0) {
        ret = bdrv_open_backing_file(bs, options, "backing", &local_err);
        if (ret < 0) {
            goto close_and_fail;
        }
    }

    /* Remove all children options and references
     * from bs->options and bs->explicit_options */
    QLIST_FOREACH(child, &bs->children, next) {
        char *child_key_dot;
        child_key_dot = g_strdup_printf("%s.", child->name);
        qdict_extract_subqdict(bs->explicit_options, NULL, child_key_dot);
        qdict_extract_subqdict(bs->options, NULL, child_key_dot);
        qdict_del(bs->explicit_options, child->name);
        qdict_del(bs->options, child->name);
        g_free(child_key_dot);
    }

    /* Check if any unknown options were used */
    if (qdict_size(options) != 0) {
        const QDictEntry *entry = qdict_first(options);
        if (flags & BDRV_O_PROTOCOL) {
            error_setg(errp, "Block protocol '%s' doesn't support the option "
                       "'%s'", drv->format_name, entry->key);
        } else {
            error_setg(errp,
                       "Block format '%s' does not support the option '%s'",
                       drv->format_name, entry->key);
        }

        goto close_and_fail;
    }

    bdrv_parent_cb_change_media(bs, true);

    qobject_unref(options);
    options = NULL;

    /* For snapshot=on, create a temporary qcow2 overlay. bs points to the
     * temporary snapshot afterwards. */
    if (snapshot_flags) {
        BlockDriverState *snapshot_bs;
        snapshot_bs = bdrv_append_temp_snapshot(bs, snapshot_flags,
                                                snapshot_options, &local_err);
        snapshot_options = NULL;
        if (local_err) {
            goto close_and_fail;
        }
        /* We are not going to return bs but the overlay on top of it
         * (snapshot_bs); thus, we have to drop the strong reference to bs
         * (which we obtained by calling bdrv_new()). bs will not be deleted,
         * though, because the overlay still has a reference to it. */
        bdrv_unref(bs);
        bs = snapshot_bs;
    }

    return bs;

fail:
    blk_unref(file);
    qobject_unref(snapshot_options);
    qobject_unref(bs->explicit_options);
    qobject_unref(bs->options);
    qobject_unref(options);
    bs->options = NULL;
    bs->explicit_options = NULL;
    bdrv_unref(bs);
    error_propagate(errp, local_err);
    return NULL;

close_and_fail:
    bdrv_unref(bs);
    qobject_unref(snapshot_options);
    qobject_unref(options);
    error_propagate(errp, local_err);
    return NULL;
}

/*
 * The caller must always hold @filename AioContext lock, because this
 * function eventually calls bdrv_refresh_total_sectors() which polls
 * when called from non-coroutine context.
 */
BlockDriverState *bdrv_open(const char *filename, const char *reference,
                            QDict *options, int flags, Error **errp)
{
    GLOBAL_STATE_CODE();

    return bdrv_open_inherit(filename, reference, options, flags, NULL,
                             NULL, 0, errp);
}

/* Return true if the NULL-terminated @list contains @str */
static bool is_str_in_list(const char *str, const char *const *list)
{
    if (str && list) {
        int i;
        for (i = 0; list[i] != NULL; i++) {
            if (!strcmp(str, list[i])) {
                return true;
            }
        }
    }
    return false;
}

/*
 * Check that every option set in @bs->options is also set in
 * @new_opts.
 *
 * Options listed in the common_options list and in
 * @bs->drv->mutable_opts are skipped.
 *
 * Return 0 on success, otherwise return -EINVAL and set @errp.
 */
static int bdrv_reset_options_allowed(BlockDriverState *bs,
                                      const QDict *new_opts, Error **errp)
{
    const QDictEntry *e;
    /* These options are common to all block drivers and are handled
     * in bdrv_reopen_prepare() so they can be left out of @new_opts */
    const char *const common_options[] = {
        "node-name", "discard", "cache.direct", "cache.no-flush",
        "read-only", "auto-read-only", "detect-zeroes", NULL
    };

    for (e = qdict_first(bs->options); e; e = qdict_next(bs->options, e)) {
        if (!qdict_haskey(new_opts, e->key) &&
            !is_str_in_list(e->key, common_options) &&
            !is_str_in_list(e->key, bs->drv->mutable_opts)) {
            error_setg(errp, "Option '%s' cannot be reset "
                       "to its default value", e->key);
            return -EINVAL;
        }
    }

    return 0;
}

/*
 * Returns true if @child can be reached recursively from @bs
 */
static bool bdrv_recurse_has_child(BlockDriverState *bs,
                                   BlockDriverState *child)
{
    BdrvChild *c;

    if (bs == child) {
        return true;
    }

    QLIST_FOREACH(c, &bs->children, next) {
        if (bdrv_recurse_has_child(c->bs, child)) {
            return true;
        }
    }

    return false;
}

/*
 * Adds a BlockDriverState to a simple queue for an atomic, transactional
 * reopen of multiple devices.
 *
 * bs_queue can either be an existing BlockReopenQueue that has had QTAILQ_INIT
 * already performed, or alternatively may be NULL a new BlockReopenQueue will
 * be created and initialized. This newly created BlockReopenQueue should be
 * passed back in for subsequent calls that are intended to be of the same
 * atomic 'set'.
 *
 * bs is the BlockDriverState to add to the reopen queue.
 *
 * options contains the changed options for the associated bs
 * (the BlockReopenQueue takes ownership)
 *
 * flags contains the open flags for the associated bs
 *
 * returns a pointer to bs_queue, which is either the newly allocated
 * bs_queue, or the existing bs_queue being used.
 *
 * bs is drained here and undrained by bdrv_reopen_queue_free().
 *
 * To be called with bs->aio_context locked.
 */
static BlockReopenQueue *bdrv_reopen_queue_child(BlockReopenQueue *bs_queue,
                                                 BlockDriverState *bs,
                                                 QDict *options,
                                                 const BdrvChildClass *klass,
                                                 BdrvChildRole role,
                                                 bool parent_is_format,
                                                 QDict *parent_options,
                                                 int parent_flags,
                                                 bool keep_old_opts)
{
    assert(bs != NULL);

    BlockReopenQueueEntry *bs_entry;
    BdrvChild *child;
    QDict *old_options, *explicit_options, *options_copy;
    int flags;
    QemuOpts *opts;

    GLOBAL_STATE_CODE();

    bdrv_drained_begin(bs);

    if (bs_queue == NULL) {
        bs_queue = g_new0(BlockReopenQueue, 1);
        QTAILQ_INIT(bs_queue);
    }

    if (!options) {
        options = qdict_new();
    }

    /* Check if this BlockDriverState is already in the queue */
    QTAILQ_FOREACH(bs_entry, bs_queue, entry) {
        if (bs == bs_entry->state.bs) {
            break;
        }
    }

    /*
     * Precedence of options:
     * 1. Explicitly passed in options (highest)
     * 2. Retained from explicitly set options of bs
     * 3. Inherited from parent node
     * 4. Retained from effective options of bs
     */

    /* Old explicitly set values (don't overwrite by inherited value) */
    if (bs_entry || keep_old_opts) {
        old_options = qdict_clone_shallow(bs_entry ?
                                          bs_entry->state.explicit_options :
                                          bs->explicit_options);
        bdrv_join_options(bs, options, old_options);
        qobject_unref(old_options);
    }

    explicit_options = qdict_clone_shallow(options);

    /* Inherit from parent node */
    if (parent_options) {
        flags = 0;
        klass->inherit_options(role, parent_is_format, &flags, options,
                               parent_flags, parent_options);
    } else {
        flags = bdrv_get_flags(bs);
    }

    if (keep_old_opts) {
        /* Old values are used for options that aren't set yet */
        old_options = qdict_clone_shallow(bs->options);
        bdrv_join_options(bs, options, old_options);
        qobject_unref(old_options);
    }

    /* We have the final set of options so let's update the flags */
    options_copy = qdict_clone_shallow(options);
    opts = qemu_opts_create(&bdrv_runtime_opts, NULL, 0, &error_abort);
    qemu_opts_absorb_qdict(opts, options_copy, NULL);
    update_flags_from_options(&flags, opts);
    qemu_opts_del(opts);
    qobject_unref(options_copy);

    /* bdrv_open_inherit() sets and clears some additional flags internally */
    flags &= ~BDRV_O_PROTOCOL;
    if (flags & BDRV_O_RDWR) {
        flags |= BDRV_O_ALLOW_RDWR;
    }

    if (!bs_entry) {
        bs_entry = g_new0(BlockReopenQueueEntry, 1);
        QTAILQ_INSERT_TAIL(bs_queue, bs_entry, entry);
    } else {
        qobject_unref(bs_entry->state.options);
        qobject_unref(bs_entry->state.explicit_options);
    }

    bs_entry->state.bs = bs;
    bs_entry->state.options = options;
    bs_entry->state.explicit_options = explicit_options;
    bs_entry->state.flags = flags;

    /*
     * If keep_old_opts is false then it means that unspecified
     * options must be reset to their original value. We don't allow
     * resetting 'backing' but we need to know if the option is
     * missing in order to decide if we have to return an error.
     */
    if (!keep_old_opts) {
        bs_entry->state.backing_missing =
            !qdict_haskey(options, "backing") &&
            !qdict_haskey(options, "backing.driver");
    }

    QLIST_FOREACH(child, &bs->children, next) {
        QDict *new_child_options = NULL;
        bool child_keep_old = keep_old_opts;

        /* reopen can only change the options of block devices that were
         * implicitly created and inherited options. For other (referenced)
         * block devices, a syntax like "backing.foo" results in an error. */
        if (child->bs->inherits_from != bs) {
            continue;
        }

        /* Check if the options contain a child reference */
        if (qdict_haskey(options, child->name)) {
            const char *childref = qdict_get_try_str(options, child->name);
            /*
             * The current child must not be reopened if the child
             * reference is null or points to a different node.
             */
            if (g_strcmp0(childref, child->bs->node_name)) {
                continue;
            }
            /*
             * If the child reference points to the current child then
             * reopen it with its existing set of options (note that
             * it can still inherit new options from the parent).
             */
            child_keep_old = true;
        } else {
            /* Extract child options ("child-name.*") */
            char *child_key_dot = g_strdup_printf("%s.", child->name);
            qdict_extract_subqdict(explicit_options, NULL, child_key_dot);
            qdict_extract_subqdict(options, &new_child_options, child_key_dot);
            g_free(child_key_dot);
        }

        bdrv_reopen_queue_child(bs_queue, child->bs, new_child_options,
                                child->klass, child->role, bs->drv->is_format,
                                options, flags, child_keep_old);
    }

    return bs_queue;
}

/* To be called with bs->aio_context locked */
BlockReopenQueue *bdrv_reopen_queue(BlockReopenQueue *bs_queue,
                                    BlockDriverState *bs,
                                    QDict *options, bool keep_old_opts)
{
    GLOBAL_STATE_CODE();

    return bdrv_reopen_queue_child(bs_queue, bs, options, NULL, 0, false,
                                   NULL, 0, keep_old_opts);
}

void bdrv_reopen_queue_free(BlockReopenQueue *bs_queue)
{
    GLOBAL_STATE_CODE();
    if (bs_queue) {
        BlockReopenQueueEntry *bs_entry, *next;
        QTAILQ_FOREACH_SAFE(bs_entry, bs_queue, entry, next) {
            AioContext *ctx = bdrv_get_aio_context(bs_entry->state.bs);

            aio_context_acquire(ctx);
            bdrv_drained_end(bs_entry->state.bs);
            aio_context_release(ctx);

            qobject_unref(bs_entry->state.explicit_options);
            qobject_unref(bs_entry->state.options);
            g_free(bs_entry);
        }
        g_free(bs_queue);
    }
}

/*
 * Reopen multiple BlockDriverStates atomically & transactionally.
 *
 * The queue passed in (bs_queue) must have been built up previous
 * via bdrv_reopen_queue().
 *
 * Reopens all BDS specified in the queue, with the appropriate
 * flags.  All devices are prepared for reopen, and failure of any
 * device will cause all device changes to be abandoned, and intermediate
 * data cleaned up.
 *
 * If all devices prepare successfully, then the changes are committed
 * to all devices.
 *
 * All affected nodes must be drained between bdrv_reopen_queue() and
 * bdrv_reopen_multiple().
 *
 * To be called from the main thread, with all other AioContexts unlocked.
 */
int bdrv_reopen_multiple(BlockReopenQueue *bs_queue, Error **errp)
{
    int ret = -1;
    BlockReopenQueueEntry *bs_entry, *next;
    AioContext *ctx;
    Transaction *tran = tran_new();
    g_autoptr(GSList) refresh_list = NULL;

    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
    assert(bs_queue != NULL);
    GLOBAL_STATE_CODE();

    QTAILQ_FOREACH(bs_entry, bs_queue, entry) {
        ctx = bdrv_get_aio_context(bs_entry->state.bs);
        aio_context_acquire(ctx);
        ret = bdrv_flush(bs_entry->state.bs);
        aio_context_release(ctx);
        if (ret < 0) {
            error_setg_errno(errp, -ret, "Error flushing drive");
            goto abort;
        }
    }

    QTAILQ_FOREACH(bs_entry, bs_queue, entry) {
        assert(bs_entry->state.bs->quiesce_counter > 0);
        ctx = bdrv_get_aio_context(bs_entry->state.bs);
        aio_context_acquire(ctx);
        ret = bdrv_reopen_prepare(&bs_entry->state, bs_queue, tran, errp);
        aio_context_release(ctx);
        if (ret < 0) {
            goto abort;
        }
        bs_entry->prepared = true;
    }

    QTAILQ_FOREACH(bs_entry, bs_queue, entry) {
        BDRVReopenState *state = &bs_entry->state;

        refresh_list = g_slist_prepend(refresh_list, state->bs);
        if (state->old_backing_bs) {
            refresh_list = g_slist_prepend(refresh_list, state->old_backing_bs);
        }
        if (state->old_file_bs) {
            refresh_list = g_slist_prepend(refresh_list, state->old_file_bs);
        }
    }

    /*
     * Note that file-posix driver rely on permission update done during reopen
     * (even if no permission changed), because it wants "new" permissions for
     * reconfiguring the fd and that's why it does it in raw_check_perm(), not
     * in raw_reopen_prepare() which is called with "old" permissions.
     */
    ret = bdrv_list_refresh_perms(refresh_list, bs_queue, tran, errp);
    if (ret < 0) {
        goto abort;
    }

    /*
     * If we reach this point, we have success and just need to apply the
     * changes.
     *
     * Reverse order is used to comfort qcow2 driver: on commit it need to write
     * IN_USE flag to the image, to mark bitmaps in the image as invalid. But
     * children are usually goes after parents in reopen-queue, so go from last
     * to first element.
     */
    QTAILQ_FOREACH_REVERSE(bs_entry, bs_queue, entry) {
        ctx = bdrv_get_aio_context(bs_entry->state.bs);
        aio_context_acquire(ctx);
        bdrv_reopen_commit(&bs_entry->state);
        aio_context_release(ctx);
    }

    tran_commit(tran);

    QTAILQ_FOREACH_REVERSE(bs_entry, bs_queue, entry) {
        BlockDriverState *bs = bs_entry->state.bs;

        if (bs->drv->bdrv_reopen_commit_post) {
            ctx = bdrv_get_aio_context(bs);
            aio_context_acquire(ctx);
            bs->drv->bdrv_reopen_commit_post(&bs_entry->state);
            aio_context_release(ctx);
        }
    }

    ret = 0;
    goto cleanup;

abort:
    tran_abort(tran);
    QTAILQ_FOREACH_SAFE(bs_entry, bs_queue, entry, next) {
        if (bs_entry->prepared) {
            ctx = bdrv_get_aio_context(bs_entry->state.bs);
            aio_context_acquire(ctx);
            bdrv_reopen_abort(&bs_entry->state);
            aio_context_release(ctx);
        }
    }

cleanup:
    bdrv_reopen_queue_free(bs_queue);

    return ret;
}

int bdrv_reopen(BlockDriverState *bs, QDict *opts, bool keep_old_opts,
                Error **errp)
{
    AioContext *ctx = bdrv_get_aio_context(bs);
    BlockReopenQueue *queue;
    int ret;

    GLOBAL_STATE_CODE();

    queue = bdrv_reopen_queue(NULL, bs, opts, keep_old_opts);

    if (ctx != qemu_get_aio_context()) {
        aio_context_release(ctx);
    }
    ret = bdrv_reopen_multiple(queue, errp);

    if (ctx != qemu_get_aio_context()) {
        aio_context_acquire(ctx);
    }

    return ret;
}

int bdrv_reopen_set_read_only(BlockDriverState *bs, bool read_only,
                              Error **errp)
{
    QDict *opts = qdict_new();

    GLOBAL_STATE_CODE();

    qdict_put_bool(opts, BDRV_OPT_READ_ONLY, read_only);

    return bdrv_reopen(bs, opts, true, errp);
}

/*
 * Take a BDRVReopenState and check if the value of 'backing' in the
 * reopen_state->options QDict is valid or not.
 *
 * If 'backing' is missing from the QDict then return 0.
 *
 * If 'backing' contains the node name of the backing file of
 * reopen_state->bs then return 0.
 *
 * If 'backing' contains a different node name (or is null) then check
 * whether the current backing file can be replaced with the new one.
 * If that's the case then reopen_state->replace_backing_bs is set to
 * true and reopen_state->new_backing_bs contains a pointer to the new
 * backing BlockDriverState (or NULL).
 *
 * Return 0 on success, otherwise return < 0 and set @errp.
 */
static int bdrv_reopen_parse_file_or_backing(BDRVReopenState *reopen_state,
                                             bool is_backing, Transaction *tran,
                                             Error **errp)
{
    BlockDriverState *bs = reopen_state->bs;
    BlockDriverState *new_child_bs;
    BlockDriverState *old_child_bs = is_backing ? child_bs(bs->backing) :
                                                  child_bs(bs->file);
    const char *child_name = is_backing ? "backing" : "file";
    QObject *value;
    const char *str;

    GLOBAL_STATE_CODE();

    value = qdict_get(reopen_state->options, child_name);
    if (value == NULL) {
        return 0;
    }

    switch (qobject_type(value)) {
    case QTYPE_QNULL:
        assert(is_backing); /* The 'file' option does not allow a null value */
        new_child_bs = NULL;
        break;
    case QTYPE_QSTRING:
        str = qstring_get_str(qobject_to(QString, value));
        new_child_bs = bdrv_lookup_bs(NULL, str, errp);
        if (new_child_bs == NULL) {
            return -EINVAL;
        } else if (bdrv_recurse_has_child(new_child_bs, bs)) {
            error_setg(errp, "Making '%s' a %s child of '%s' would create a "
                       "cycle", str, child_name, bs->node_name);
            return -EINVAL;
        }
        break;
    default:
        /*
         * The options QDict has been flattened, so 'backing' and 'file'
         * do not allow any other data type here.
         */
        g_assert_not_reached();
    }

    if (old_child_bs == new_child_bs) {
        return 0;
    }

    if (old_child_bs) {
        if (bdrv_skip_implicit_filters(old_child_bs) == new_child_bs) {
            return 0;
        }

        if (old_child_bs->implicit) {
            error_setg(errp, "Cannot replace implicit %s child of %s",
                       child_name, bs->node_name);
            return -EPERM;
        }
    }

    if (bs->drv->is_filter && !old_child_bs) {
        /*
         * Filters always have a file or a backing child, so we are trying to
         * change wrong child
         */
        error_setg(errp, "'%s' is a %s filter node that does not support a "
                   "%s child", bs->node_name, bs->drv->format_name, child_name);
        return -EINVAL;
    }

    if (is_backing) {
        reopen_state->old_backing_bs = old_child_bs;
    } else {
        reopen_state->old_file_bs = old_child_bs;
    }

    return bdrv_set_file_or_backing_noperm(bs, new_child_bs, is_backing,
                                           tran, errp);
}

/*
 * Prepares a BlockDriverState for reopen. All changes are staged in the
 * 'opaque' field of the BDRVReopenState, which is used and allocated by
 * the block driver layer .bdrv_reopen_prepare()
 *
 * bs is the BlockDriverState to reopen
 * flags are the new open flags
 * queue is the reopen queue
 *
 * Returns 0 on success, non-zero on error.  On error errp will be set
 * as well.
 *
 * On failure, bdrv_reopen_abort() will be called to clean up any data.
 * It is the responsibility of the caller to then call the abort() or
 * commit() for any other BDS that have been left in a prepare() state
 *
 */
static int bdrv_reopen_prepare(BDRVReopenState *reopen_state,
                               BlockReopenQueue *queue,
                               Transaction *change_child_tran, Error **errp)
{
    int ret = -1;
    int old_flags;
    Error *local_err = NULL;
    BlockDriver *drv;
    QemuOpts *opts;
    QDict *orig_reopen_opts;
    char *discard = NULL;
    bool read_only;
    bool drv_prepared = false;

    assert(reopen_state != NULL);
    assert(reopen_state->bs->drv != NULL);
    GLOBAL_STATE_CODE();
    drv = reopen_state->bs->drv;

    /* This function and each driver's bdrv_reopen_prepare() remove
     * entries from reopen_state->options as they are processed, so
     * we need to make a copy of the original QDict. */
    orig_reopen_opts = qdict_clone_shallow(reopen_state->options);

    /* Process generic block layer options */
    opts = qemu_opts_create(&bdrv_runtime_opts, NULL, 0, &error_abort);
    if (!qemu_opts_absorb_qdict(opts, reopen_state->options, errp)) {
        ret = -EINVAL;
        goto error;
    }

    /* This was already called in bdrv_reopen_queue_child() so the flags
     * are up-to-date. This time we simply want to remove the options from
     * QemuOpts in order to indicate that they have been processed. */
    old_flags = reopen_state->flags;
    update_flags_from_options(&reopen_state->flags, opts);
    assert(old_flags == reopen_state->flags);

    discard = qemu_opt_get_del(opts, BDRV_OPT_DISCARD);
    if (discard != NULL) {
        if (bdrv_parse_discard_flags(discard, &reopen_state->flags) != 0) {
            error_setg(errp, "Invalid discard option");
            ret = -EINVAL;
            goto error;
        }
    }

    reopen_state->detect_zeroes =
        bdrv_parse_detect_zeroes(opts, reopen_state->flags, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        ret = -EINVAL;
        goto error;
    }

    /* All other options (including node-name and driver) must be unchanged.
     * Put them back into the QDict, so that they are checked at the end
     * of this function. */
    qemu_opts_to_qdict(opts, reopen_state->options);

    /* If we are to stay read-only, do not allow permission change
     * to r/w. Attempting to set to r/w may fail if either BDRV_O_ALLOW_RDWR is
     * not set, or if the BDS still has copy_on_read enabled */
    read_only = !(reopen_state->flags & BDRV_O_RDWR);
    ret = bdrv_can_set_read_only(reopen_state->bs, read_only, true, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        goto error;
    }

    if (drv->bdrv_reopen_prepare) {
        /*
         * If a driver-specific option is missing, it means that we
         * should reset it to its default value.
         * But not all options allow that, so we need to check it first.
         */
        ret = bdrv_reset_options_allowed(reopen_state->bs,
                                         reopen_state->options, errp);
        if (ret) {
            goto error;
        }

        ret = drv->bdrv_reopen_prepare(reopen_state, queue, &local_err);
        if (ret) {
            if (local_err != NULL) {
                error_propagate(errp, local_err);
            } else {
                bdrv_refresh_filename(reopen_state->bs);
                error_setg(errp, "failed while preparing to reopen image '%s'",
                           reopen_state->bs->filename);
            }
            goto error;
        }
    } else {
        /* It is currently mandatory to have a bdrv_reopen_prepare()
         * handler for each supported drv. */
        error_setg(errp, "Block format '%s' used by node '%s' "
                   "does not support reopening files", drv->format_name,
                   bdrv_get_device_or_node_name(reopen_state->bs));
        ret = -1;
        goto error;
    }

    drv_prepared = true;

    /*
     * We must provide the 'backing' option if the BDS has a backing
     * file or if the image file has a backing file name as part of
     * its metadata. Otherwise the 'backing' option can be omitted.
     */
    if (drv->supports_backing && reopen_state->backing_missing &&
        (reopen_state->bs->backing || reopen_state->bs->backing_file[0])) {
        error_setg(errp, "backing is missing for '%s'",
                   reopen_state->bs->node_name);
        ret = -EINVAL;
        goto error;
    }

    /*
     * Allow changing the 'backing' option. The new value can be
     * either a reference to an existing node (using its node name)
     * or NULL to simply detach the current backing file.
     */
    ret = bdrv_reopen_parse_file_or_backing(reopen_state, true,
                                            change_child_tran, errp);
    if (ret < 0) {
        goto error;
    }
    qdict_del(reopen_state->options, "backing");

    /* Allow changing the 'file' option. In this case NULL is not allowed */
    ret = bdrv_reopen_parse_file_or_backing(reopen_state, false,
                                            change_child_tran, errp);
    if (ret < 0) {
        goto error;
    }
    qdict_del(reopen_state->options, "file");

    /* Options that are not handled are only okay if they are unchanged
     * compared to the old state. It is expected that some options are only
     * used for the initial open, but not reopen (e.g. filename) */
    if (qdict_size(reopen_state->options)) {
        const QDictEntry *entry = qdict_first(reopen_state->options);

        do {
            QObject *new = entry->value;
            QObject *old = qdict_get(reopen_state->bs->options, entry->key);

            /* Allow child references (child_name=node_name) as long as they
             * point to the current child (i.e. everything stays the same). */
            if (qobject_type(new) == QTYPE_QSTRING) {
                BdrvChild *child;
                QLIST_FOREACH(child, &reopen_state->bs->children, next) {
                    if (!strcmp(child->name, entry->key)) {
                        break;
                    }
                }

                if (child) {
                    if (!strcmp(child->bs->node_name,
                                qstring_get_str(qobject_to(QString, new)))) {
                        continue; /* Found child with this name, skip option */
                    }
                }
            }

            /*
             * TODO: When using -drive to specify blockdev options, all values
             * will be strings; however, when using -blockdev, blockdev-add or
             * filenames using the json:{} pseudo-protocol, they will be
             * correctly typed.
             * In contrast, reopening options are (currently) always strings
             * (because you can only specify them through qemu-io; all other
             * callers do not specify any options).
             * Therefore, when using anything other than -drive to create a BDS,
             * this cannot detect non-string options as unchanged, because
             * qobject_is_equal() always returns false for objects of different
             * type.  In the future, this should be remedied by correctly typing
             * all options.  For now, this is not too big of an issue because
             * the user can simply omit options which cannot be changed anyway,
             * so they will stay unchanged.
             */
            if (!qobject_is_equal(new, old)) {
                error_setg(errp, "Cannot change the option '%s'", entry->key);
                ret = -EINVAL;
                goto error;
            }
        } while ((entry = qdict_next(reopen_state->options, entry)));
    }

    ret = 0;

    /* Restore the original reopen_state->options QDict */
    qobject_unref(reopen_state->options);
    reopen_state->options = qobject_ref(orig_reopen_opts);

error:
    if (ret < 0 && drv_prepared) {
        /* drv->bdrv_reopen_prepare() has succeeded, so we need to
         * call drv->bdrv_reopen_abort() before signaling an error
         * (bdrv_reopen_multiple() will not call bdrv_reopen_abort()
         * when the respective bdrv_reopen_prepare() has failed) */
        if (drv->bdrv_reopen_abort) {
            drv->bdrv_reopen_abort(reopen_state);
        }
    }
    qemu_opts_del(opts);
    qobject_unref(orig_reopen_opts);
    g_free(discard);
    return ret;
}

/*
 * Takes the staged changes for the reopen from bdrv_reopen_prepare(), and
 * makes them final by swapping the staging BlockDriverState contents into
 * the active BlockDriverState contents.
 */
static void bdrv_reopen_commit(BDRVReopenState *reopen_state)
{
    BlockDriver *drv;
    BlockDriverState *bs;
    BdrvChild *child;

    assert(reopen_state != NULL);
    bs = reopen_state->bs;
    drv = bs->drv;
    assert(drv != NULL);
    GLOBAL_STATE_CODE();

    /* If there are any driver level actions to take */
    if (drv->bdrv_reopen_commit) {
        drv->bdrv_reopen_commit(reopen_state);
    }

    /* set BDS specific flags now */
    qobject_unref(bs->explicit_options);
    qobject_unref(bs->options);
    qobject_ref(reopen_state->explicit_options);
    qobject_ref(reopen_state->options);

    bs->explicit_options   = reopen_state->explicit_options;
    bs->options            = reopen_state->options;
    bs->open_flags         = reopen_state->flags;
    bs->detect_zeroes      = reopen_state->detect_zeroes;

    /* Remove child references from bs->options and bs->explicit_options.
     * Child options were already removed in bdrv_reopen_queue_child() */
    QLIST_FOREACH(child, &bs->children, next) {
        qdict_del(bs->explicit_options, child->name);
        qdict_del(bs->options, child->name);
    }
    /* backing is probably removed, so it's not handled by previous loop */
    qdict_del(bs->explicit_options, "backing");
    qdict_del(bs->options, "backing");

    bdrv_refresh_limits(bs, NULL, NULL);
}

/*
 * Abort the reopen, and delete and free the staged changes in
 * reopen_state
 */
static void bdrv_reopen_abort(BDRVReopenState *reopen_state)
{
    BlockDriver *drv;

    assert(reopen_state != NULL);
    drv = reopen_state->bs->drv;
    assert(drv != NULL);
    GLOBAL_STATE_CODE();

    if (drv->bdrv_reopen_abort) {
        drv->bdrv_reopen_abort(reopen_state);
    }
}


static void bdrv_close(BlockDriverState *bs)
{
    BdrvAioNotifier *ban, *ban_next;
    BdrvChild *child, *next;

    GLOBAL_STATE_CODE();
    assert(!bs->refcnt);

    bdrv_drained_begin(bs); /* complete I/O */
    bdrv_flush(bs);
    bdrv_drain(bs); /* in case flush left pending I/O */

    if (bs->drv) {
        if (bs->drv->bdrv_close) {
            /* Must unfreeze all children, so bdrv_unref_child() works */
            bs->drv->bdrv_close(bs);
        }
        bs->drv = NULL;
    }

    QLIST_FOREACH_SAFE(child, &bs->children, next, next) {
        bdrv_unref_child(bs, child);
    }

    assert(!bs->backing);
    assert(!bs->file);
    g_free(bs->opaque);
    bs->opaque = NULL;
    qatomic_set(&bs->copy_on_read, 0);
    bs->backing_file[0] = '\0';
    bs->backing_format[0] = '\0';
    bs->total_sectors = 0;
    bs->encrypted = false;
    bs->sg = false;
    qobject_unref(bs->options);
    qobject_unref(bs->explicit_options);
    bs->options = NULL;
    bs->explicit_options = NULL;
    qobject_unref(bs->full_open_options);
    bs->full_open_options = NULL;
    g_free(bs->block_status_cache);
    bs->block_status_cache = NULL;

    bdrv_release_named_dirty_bitmaps(bs);
    assert(QLIST_EMPTY(&bs->dirty_bitmaps));

    QLIST_FOREACH_SAFE(ban, &bs->aio_notifiers, list, ban_next) {
        g_free(ban);
    }
    QLIST_INIT(&bs->aio_notifiers);
    bdrv_drained_end(bs);

    /*
     * If we're still inside some bdrv_drain_all_begin()/end() sections, end
     * them now since this BDS won't exist anymore when bdrv_drain_all_end()
     * gets called.
     */
    if (bs->quiesce_counter) {
        bdrv_drain_all_end_quiesce(bs);
    }
}

void bdrv_close_all(void)
{
    GLOBAL_STATE_CODE();
    assert(job_next(NULL) == NULL);

    /* Drop references from requests still in flight, such as canceled block
     * jobs whose AIO context has not been polled yet */
    bdrv_drain_all();

    blk_remove_all_bs();
    blockdev_close_all_bdrv_states();

    assert(QTAILQ_EMPTY(&all_bdrv_states));
}

static bool should_update_child(BdrvChild *c, BlockDriverState *to)
{
    GQueue *queue;
    GHashTable *found;
    bool ret;

    if (c->klass->stay_at_node) {
        return false;
    }

    /* If the child @c belongs to the BDS @to, replacing the current
     * c->bs by @to would mean to create a loop.
     *
     * Such a case occurs when appending a BDS to a backing chain.
     * For instance, imagine the following chain:
     *
     *   guest device -> node A -> further backing chain...
     *
     * Now we create a new BDS B which we want to put on top of this
     * chain, so we first attach A as its backing node:
     *
     *                   node B
     *                     |
     *                     v
     *   guest device -> node A -> further backing chain...
     *
     * Finally we want to replace A by B.  When doing that, we want to
     * replace all pointers to A by pointers to B -- except for the
     * pointer from B because (1) that would create a loop, and (2)
     * that pointer should simply stay intact:
     *
     *   guest device -> node B
     *                     |
     *                     v
     *                   node A -> further backing chain...
     *
     * In general, when replacing a node A (c->bs) by a node B (@to),
     * if A is a child of B, that means we cannot replace A by B there
     * because that would create a loop.  Silently detaching A from B
     * is also not really an option.  So overall just leaving A in
     * place there is the most sensible choice.
     *
     * We would also create a loop in any cases where @c is only
     * indirectly referenced by @to. Prevent this by returning false
     * if @c is found (by breadth-first search) anywhere in the whole
     * subtree of @to.
     */

    ret = true;
    found = g_hash_table_new(NULL, NULL);
    g_hash_table_add(found, to);
    queue = g_queue_new();
    g_queue_push_tail(queue, to);

    while (!g_queue_is_empty(queue)) {
        BlockDriverState *v = g_queue_pop_head(queue);
        BdrvChild *c2;

        QLIST_FOREACH(c2, &v->children, next) {
            if (c2 == c) {
                ret = false;
                break;
            }

            if (g_hash_table_contains(found, c2->bs)) {
                continue;
            }

            g_queue_push_tail(queue, c2->bs);
            g_hash_table_add(found, c2->bs);
        }
    }

    g_queue_free(queue);
    g_hash_table_destroy(found);

    return ret;
}

static void bdrv_remove_child_commit(void *opaque)
{
    GLOBAL_STATE_CODE();
    bdrv_child_free(opaque);
}

static TransactionActionDrv bdrv_remove_child_drv = {
    .commit = bdrv_remove_child_commit,
};

/* Function doesn't update permissions, caller is responsible for this. */
static void bdrv_remove_child(BdrvChild *child, Transaction *tran)
{
    if (!child) {
        return;
    }

    if (child->bs) {
        BlockDriverState *bs = child->bs;
        bdrv_drained_begin(bs);
        bdrv_replace_child_tran(child, NULL, tran);
        bdrv_drained_end(bs);
    }

    tran_add(tran, &bdrv_remove_child_drv, child);
}

static void undrain_on_clean_cb(void *opaque)
{
    bdrv_drained_end(opaque);
}

static TransactionActionDrv undrain_on_clean = {
    .clean = undrain_on_clean_cb,
};

static int bdrv_replace_node_noperm(BlockDriverState *from,
                                    BlockDriverState *to,
                                    bool auto_skip, Transaction *tran,
                                    Error **errp)
{
    BdrvChild *c, *next;

    GLOBAL_STATE_CODE();

    bdrv_drained_begin(from);
    bdrv_drained_begin(to);
    tran_add(tran, &undrain_on_clean, from);
    tran_add(tran, &undrain_on_clean, to);

    QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
        assert(c->bs == from);
        if (!should_update_child(c, to)) {
            if (auto_skip) {
                continue;
            }
            error_setg(errp, "Should not change '%s' link to '%s'",
                       c->name, from->node_name);
            return -EINVAL;
        }
        if (c->frozen) {
            error_setg(errp, "Cannot change '%s' link to '%s'",
                       c->name, from->node_name);
            return -EPERM;
        }
        bdrv_replace_child_tran(c, to, tran);
    }

    return 0;
}

/*
 * With auto_skip=true bdrv_replace_node_common skips updating from parents
 * if it creates a parent-child relation loop or if parent is block-job.
 *
 * With auto_skip=false the error is returned if from has a parent which should
 * not be updated.
 *
 * With @detach_subchain=true @to must be in a backing chain of @from. In this
 * case backing link of the cow-parent of @to is removed.
 */
static int bdrv_replace_node_common(BlockDriverState *from,
                                    BlockDriverState *to,
                                    bool auto_skip, bool detach_subchain,
                                    Error **errp)
{
    Transaction *tran = tran_new();
    g_autoptr(GSList) refresh_list = NULL;
    BlockDriverState *to_cow_parent = NULL;
    int ret;

    GLOBAL_STATE_CODE();

    if (detach_subchain) {
        assert(bdrv_chain_contains(from, to));
        assert(from != to);
        for (to_cow_parent = from;
             bdrv_filter_or_cow_bs(to_cow_parent) != to;
             to_cow_parent = bdrv_filter_or_cow_bs(to_cow_parent))
        {
            ;
        }
    }

    /* Make sure that @from doesn't go away until we have successfully attached
     * all of its parents to @to. */
    bdrv_ref(from);

    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
    assert(bdrv_get_aio_context(from) == bdrv_get_aio_context(to));
    bdrv_drained_begin(from);

    /*
     * Do the replacement without permission update.
     * Replacement may influence the permissions, we should calculate new
     * permissions based on new graph. If we fail, we'll roll-back the
     * replacement.
     */
    ret = bdrv_replace_node_noperm(from, to, auto_skip, tran, errp);
    if (ret < 0) {
        goto out;
    }

    if (detach_subchain) {
        bdrv_remove_child(bdrv_filter_or_cow_child(to_cow_parent), tran);
    }

    refresh_list = g_slist_prepend(refresh_list, to);
    refresh_list = g_slist_prepend(refresh_list, from);

    ret = bdrv_list_refresh_perms(refresh_list, NULL, tran, errp);
    if (ret < 0) {
        goto out;
    }

    ret = 0;

out:
    tran_finalize(tran, ret);

    bdrv_drained_end(from);
    bdrv_unref(from);

    return ret;
}

int bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
                      Error **errp)
{
    GLOBAL_STATE_CODE();

    return bdrv_replace_node_common(from, to, true, false, errp);
}

int bdrv_drop_filter(BlockDriverState *bs, Error **errp)
{
    GLOBAL_STATE_CODE();

    return bdrv_replace_node_common(bs, bdrv_filter_or_cow_bs(bs), true, true,
                                    errp);
}

/*
 * Add new bs contents at the top of an image chain while the chain is
 * live, while keeping required fields on the top layer.
 *
 * This will modify the BlockDriverState fields, and swap contents
 * between bs_new and bs_top. Both bs_new and bs_top are modified.
 *
 * bs_new must not be attached to a BlockBackend and must not have backing
 * child.
 *
 * This function does not create any image files.
 */
int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
                Error **errp)
{
    int ret;
    BdrvChild *child;
    Transaction *tran = tran_new();

    GLOBAL_STATE_CODE();

    assert(!bs_new->backing);

    child = bdrv_attach_child_noperm(bs_new, bs_top, "backing",
                                     &child_of_bds, bdrv_backing_role(bs_new),
                                     tran, errp);
    if (!child) {
        ret = -EINVAL;
        goto out;
    }

    ret = bdrv_replace_node_noperm(bs_top, bs_new, true, tran, errp);
    if (ret < 0) {
        goto out;
    }

    ret = bdrv_refresh_perms(bs_new, tran, errp);
out:
    tran_finalize(tran, ret);

    bdrv_refresh_limits(bs_top, NULL, NULL);

    return ret;
}

/* Not for empty child */
int bdrv_replace_child_bs(BdrvChild *child, BlockDriverState *new_bs,
                          Error **errp)
{
    int ret;
    Transaction *tran = tran_new();
    g_autoptr(GSList) refresh_list = NULL;
    BlockDriverState *old_bs = child->bs;

    GLOBAL_STATE_CODE();

    bdrv_ref(old_bs);
    bdrv_drained_begin(old_bs);
    bdrv_drained_begin(new_bs);

    bdrv_replace_child_tran(child, new_bs, tran);

    refresh_list = g_slist_prepend(refresh_list, old_bs);
    refresh_list = g_slist_prepend(refresh_list, new_bs);

    ret = bdrv_list_refresh_perms(refresh_list, NULL, tran, errp);

    tran_finalize(tran, ret);

    bdrv_drained_end(old_bs);
    bdrv_drained_end(new_bs);
    bdrv_unref(old_bs);

    return ret;
}

static void bdrv_delete(BlockDriverState *bs)
{
    assert(bdrv_op_blocker_is_empty(bs));
    assert(!bs->refcnt);
    GLOBAL_STATE_CODE();

    /* remove from list, if necessary */
    if (bs->node_name[0] != '\0') {
        QTAILQ_REMOVE(&graph_bdrv_states, bs, node_list);
    }
    QTAILQ_REMOVE(&all_bdrv_states, bs, bs_list);

    bdrv_close(bs);

    g_free(bs);
}


/*
 * Replace @bs by newly created block node.
 *
 * @options is a QDict of options to pass to the block drivers, or NULL for an
 * empty set of options. The reference to the QDict belongs to the block layer
 * after the call (even on failure), so if the caller intends to reuse the
 * dictionary, it needs to use qobject_ref() before calling bdrv_open.
 */
BlockDriverState *bdrv_insert_node(BlockDriverState *bs, QDict *options,
                                   int flags, Error **errp)
{
    ERRP_GUARD();
    int ret;
    BlockDriverState *new_node_bs = NULL;
    const char *drvname, *node_name;
    BlockDriver *drv;

    drvname = qdict_get_try_str(options, "driver");
    if (!drvname) {
        error_setg(errp, "driver is not specified");
        goto fail;
    }

    drv = bdrv_find_format(drvname);
    if (!drv) {
        error_setg(errp, "Unknown driver: '%s'", drvname);
        goto fail;
    }

    node_name = qdict_get_try_str(options, "node-name");

    GLOBAL_STATE_CODE();

    new_node_bs = bdrv_new_open_driver_opts(drv, node_name, options, flags,
                                            errp);
    options = NULL; /* bdrv_new_open_driver() eats options */
    if (!new_node_bs) {
        error_prepend(errp, "Could not create node: ");
        goto fail;
    }

    bdrv_drained_begin(bs);
    ret = bdrv_replace_node(bs, new_node_bs, errp);
    bdrv_drained_end(bs);

    if (ret < 0) {
        error_prepend(errp, "Could not replace node: ");
        goto fail;
    }

    return new_node_bs;

fail:
    qobject_unref(options);
    bdrv_unref(new_node_bs);
    return NULL;
}

/*
 * Run consistency checks on an image
 *
 * Returns 0 if the check could be completed (it doesn't mean that the image is
 * free of errors) or -errno when an internal error occurred. The results of the
 * check are stored in res.
 */
int coroutine_fn bdrv_co_check(BlockDriverState *bs,
                               BdrvCheckResult *res, BdrvCheckMode fix)
{
    IO_CODE();
    assert_bdrv_graph_readable();
    if (bs->drv == NULL) {
        return -ENOMEDIUM;
    }
    if (bs->drv->bdrv_co_check == NULL) {
        return -ENOTSUP;
    }

    memset(res, 0, sizeof(*res));
    return bs->drv->bdrv_co_check(bs, res, fix);
}

/*
 * Return values:
 * 0        - success
 * -EINVAL  - backing format specified, but no file
 * -ENOSPC  - can't update the backing file because no space is left in the
 *            image file header
 * -ENOTSUP - format driver doesn't support changing the backing file
 */
int bdrv_change_backing_file(BlockDriverState *bs, const char *backing_file,
                             const char *backing_fmt, bool require)
{
    BlockDriver *drv = bs->drv;
    int ret;

    GLOBAL_STATE_CODE();

    if (!drv) {
        return -ENOMEDIUM;
    }

    /* Backing file format doesn't make sense without a backing file */
    if (backing_fmt && !backing_file) {
        return -EINVAL;
    }

    if (require && backing_file && !backing_fmt) {
        return -EINVAL;
    }

    if (drv->bdrv_change_backing_file != NULL) {
        ret = drv->bdrv_change_backing_file(bs, backing_file, backing_fmt);
    } else {
        ret = -ENOTSUP;
    }

    if (ret == 0) {
        pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
        pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");
        pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
                backing_file ?: "");
    }
    return ret;
}

/*
 * Finds the first non-filter node above bs in the chain between
 * active and bs.  The returned node is either an immediate parent of
 * bs, or there are only filter nodes between the two.
 *
 * Returns NULL if bs is not found in active's image chain,
 * or if active == bs.
 *
 * Returns the bottommost base image if bs == NULL.
 */
BlockDriverState *bdrv_find_overlay(BlockDriverState *active,
                                    BlockDriverState *bs)
{

    GLOBAL_STATE_CODE();

    bs = bdrv_skip_filters(bs);
    active = bdrv_skip_filters(active);

    while (active) {
        BlockDriverState *next = bdrv_backing_chain_next(active);
        if (bs == next) {
            return active;
        }
        active = next;
    }

    return NULL;
}

/* Given a BDS, searches for the base layer. */
BlockDriverState *bdrv_find_base(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();

    return bdrv_find_overlay(bs, NULL);
}

/*
 * Return true if at least one of the COW (backing) and filter links
 * between @bs and @base is frozen. @errp is set if that's the case.
 * @base must be reachable from @bs, or NULL.
 */
bool bdrv_is_backing_chain_frozen(BlockDriverState *bs, BlockDriverState *base,
                                  Error **errp)
{
    BlockDriverState *i;
    BdrvChild *child;

    GLOBAL_STATE_CODE();

    for (i = bs; i != base; i = child_bs(child)) {
        child = bdrv_filter_or_cow_child(i);

        if (child && child->frozen) {
            error_setg(errp, "Cannot change '%s' link from '%s' to '%s'",
                       child->name, i->node_name, child->bs->node_name);
            return true;
        }
    }

    return false;
}

/*
 * Freeze all COW (backing) and filter links between @bs and @base.
 * If any of the links is already frozen the operation is aborted and
 * none of the links are modified.
 * @base must be reachable from @bs, or NULL.
 * Returns 0 on success. On failure returns < 0 and sets @errp.
 */
int bdrv_freeze_backing_chain(BlockDriverState *bs, BlockDriverState *base,
                              Error **errp)
{
    BlockDriverState *i;
    BdrvChild *child;

    GLOBAL_STATE_CODE();

    if (bdrv_is_backing_chain_frozen(bs, base, errp)) {
        return -EPERM;
    }

    for (i = bs; i != base; i = child_bs(child)) {
        child = bdrv_filter_or_cow_child(i);
        if (child && child->bs->never_freeze) {
            error_setg(errp, "Cannot freeze '%s' link to '%s'",
                       child->name, child->bs->node_name);
            return -EPERM;
        }
    }

    for (i = bs; i != base; i = child_bs(child)) {
        child = bdrv_filter_or_cow_child(i);
        if (child) {
            child->frozen = true;
        }
    }

    return 0;
}

/*
 * Unfreeze all COW (backing) and filter links between @bs and @base.
 * The caller must ensure that all links are frozen before using this
 * function.
 * @base must be reachable from @bs, or NULL.
 */
void bdrv_unfreeze_backing_chain(BlockDriverState *bs, BlockDriverState *base)
{
    BlockDriverState *i;
    BdrvChild *child;

    GLOBAL_STATE_CODE();

    for (i = bs; i != base; i = child_bs(child)) {
        child = bdrv_filter_or_cow_child(i);
        if (child) {
            assert(child->frozen);
            child->frozen = false;
        }
    }
}

/*
 * Drops images above 'base' up to and including 'top', and sets the image
 * above 'top' to have base as its backing file.
 *
 * Requires that the overlay to 'top' is opened r/w, so that the backing file
 * information in 'bs' can be properly updated.
 *
 * E.g., this will convert the following chain:
 * bottom <- base <- intermediate <- top <- active
 *
 * to
 *
 * bottom <- base <- active
 *
 * It is allowed for bottom==base, in which case it converts:
 *
 * base <- intermediate <- top <- active
 *
 * to
 *
 * base <- active
 *
 * If backing_file_str is non-NULL, it will be used when modifying top's
 * overlay image metadata.
 *
 * Error conditions:
 *  if active == top, that is considered an error
 *
 */
int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
                           const char *backing_file_str)
{
    BlockDriverState *explicit_top = top;
    bool update_inherits_from;
    BdrvChild *c;
    Error *local_err = NULL;
    int ret = -EIO;
    g_autoptr(GSList) updated_children = NULL;
    GSList *p;

    GLOBAL_STATE_CODE();

    bdrv_ref(top);
    bdrv_drained_begin(base);

    if (!top->drv || !base->drv) {
        goto exit;
    }

    /* Make sure that base is in the backing chain of top */
    if (!bdrv_chain_contains(top, base)) {
        goto exit;
    }

    /* If 'base' recursively inherits from 'top' then we should set
     * base->inherits_from to top->inherits_from after 'top' and all
     * other intermediate nodes have been dropped.
     * If 'top' is an implicit node (e.g. "commit_top") we should skip
     * it because no one inherits from it. We use explicit_top for that. */
    explicit_top = bdrv_skip_implicit_filters(explicit_top);
    update_inherits_from = bdrv_inherits_from_recursive(base, explicit_top);

    /* success - we can delete the intermediate states, and link top->base */
    if (!backing_file_str) {
        bdrv_refresh_filename(base);
        backing_file_str = base->filename;
    }

    QLIST_FOREACH(c, &top->parents, next_parent) {
        updated_children = g_slist_prepend(updated_children, c);
    }

    /*
     * It seems correct to pass detach_subchain=true here, but it triggers
     * one more yet not fixed bug, when due to nested aio_poll loop we switch to
     * another drained section, which modify the graph (for example, removing
     * the child, which we keep in updated_children list). So, it's a TODO.
     *
     * Note, bug triggered if pass detach_subchain=true here and run
     * test-bdrv-drain. test_drop_intermediate_poll() test-case will crash.
     * That's a FIXME.
     */
    bdrv_replace_node_common(top, base, false, false, &local_err);
    if (local_err) {
        error_report_err(local_err);
        goto exit;
    }

    for (p = updated_children; p; p = p->next) {
        c = p->data;

        if (c->klass->update_filename) {
            ret = c->klass->update_filename(c, base, backing_file_str,
                                            &local_err);
            if (ret < 0) {
                /*
                 * TODO: Actually, we want to rollback all previous iterations
                 * of this loop, and (which is almost impossible) previous
                 * bdrv_replace_node()...
                 *
                 * Note, that c->klass->update_filename may lead to permission
                 * update, so it's a bad idea to call it inside permission
                 * update transaction of bdrv_replace_node.
                 */
                error_report_err(local_err);
                goto exit;
            }
        }
    }

    if (update_inherits_from) {
        base->inherits_from = explicit_top->inherits_from;
    }

    ret = 0;
exit:
    bdrv_drained_end(base);
    bdrv_unref(top);
    return ret;
}

/**
 * Implementation of BlockDriver.bdrv_co_get_allocated_file_size() that
 * sums the size of all data-bearing children.  (This excludes backing
 * children.)
 */
static int64_t bdrv_sum_allocated_file_size(BlockDriverState *bs)
{
    BdrvChild *child;
    int64_t child_size, sum = 0;

    QLIST_FOREACH(child, &bs->children, next) {
        if (child->role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
                           BDRV_CHILD_FILTERED))
        {
            child_size = bdrv_co_get_allocated_file_size(child->bs);
            if (child_size < 0) {
                return child_size;
            }
            sum += child_size;
        }
    }

    return sum;
}

/**
 * Length of a allocated file in bytes. Sparse files are counted by actual
 * allocated space. Return < 0 if error or unknown.
 */
int64_t coroutine_fn bdrv_co_get_allocated_file_size(BlockDriverState *bs)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();

    if (!drv) {
        return -ENOMEDIUM;
    }
    if (drv->bdrv_co_get_allocated_file_size) {
        return drv->bdrv_co_get_allocated_file_size(bs);
    }

    if (drv->bdrv_file_open) {
        /*
         * Protocol drivers default to -ENOTSUP (most of their data is
         * not stored in any of their children (if they even have any),
         * so there is no generic way to figure it out).
         */
        return -ENOTSUP;
    } else if (drv->is_filter) {
        /* Filter drivers default to the size of their filtered child */
        return bdrv_co_get_allocated_file_size(bdrv_filter_bs(bs));
    } else {
        /* Other drivers default to summing their children's sizes */
        return bdrv_sum_allocated_file_size(bs);
    }
}

/*
 * bdrv_measure:
 * @drv: Format driver
 * @opts: Creation options for new image
 * @in_bs: Existing image containing data for new image (may be NULL)
 * @errp: Error object
 * Returns: A #BlockMeasureInfo (free using qapi_free_BlockMeasureInfo())
 *          or NULL on error
 *
 * Calculate file size required to create a new image.
 *
 * If @in_bs is given then space for allocated clusters and zero clusters
 * from that image are included in the calculation.  If @opts contains a
 * backing file that is shared by @in_bs then backing clusters may be omitted
 * from the calculation.
 *
 * If @in_bs is NULL then the calculation includes no allocated clusters
 * unless a preallocation option is given in @opts.
 *
 * Note that @in_bs may use a different BlockDriver from @drv.
 *
 * If an error occurs the @errp pointer is set.
 */
BlockMeasureInfo *bdrv_measure(BlockDriver *drv, QemuOpts *opts,
                               BlockDriverState *in_bs, Error **errp)
{
    IO_CODE();
    if (!drv->bdrv_measure) {
        error_setg(errp, "Block driver '%s' does not support size measurement",
                   drv->format_name);
        return NULL;
    }

    return drv->bdrv_measure(opts, in_bs, errp);
}

/**
 * Return number of sectors on success, -errno on error.
 */
int64_t coroutine_fn bdrv_co_nb_sectors(BlockDriverState *bs)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();

    if (!drv)
        return -ENOMEDIUM;

    if (drv->has_variable_length) {
        int ret = bdrv_co_refresh_total_sectors(bs, bs->total_sectors);
        if (ret < 0) {
            return ret;
        }
    }
    return bs->total_sectors;
}

/**
 * Return length in bytes on success, -errno on error.
 * The length is always a multiple of BDRV_SECTOR_SIZE.
 */
int64_t coroutine_fn bdrv_co_getlength(BlockDriverState *bs)
{
    int64_t ret;
    IO_CODE();

    ret = bdrv_co_nb_sectors(bs);
    if (ret < 0) {
        return ret;
    }
    if (ret > INT64_MAX / BDRV_SECTOR_SIZE) {
        return -EFBIG;
    }
    return ret * BDRV_SECTOR_SIZE;
}

/* return 0 as number of sectors if no device present or error */
void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr)
{
    int64_t nb_sectors = bdrv_nb_sectors(bs);
    IO_CODE();

    *nb_sectors_ptr = nb_sectors < 0 ? 0 : nb_sectors;
}

bool bdrv_is_sg(BlockDriverState *bs)
{
    IO_CODE();
    return bs->sg;
}

/**
 * Return whether the given node supports compressed writes.
 */
bool bdrv_supports_compressed_writes(BlockDriverState *bs)
{
    BlockDriverState *filtered;
    IO_CODE();

    if (!bs->drv || !block_driver_can_compress(bs->drv)) {
        return false;
    }

    filtered = bdrv_filter_bs(bs);
    if (filtered) {
        /*
         * Filters can only forward compressed writes, so we have to
         * check the child.
         */
        return bdrv_supports_compressed_writes(filtered);
    }

    return true;
}

const char *bdrv_get_format_name(BlockDriverState *bs)
{
    IO_CODE();
    return bs->drv ? bs->drv->format_name : NULL;
}

static int qsort_strcmp(const void *a, const void *b)
{
    return strcmp(*(char *const *)a, *(char *const *)b);
}

void bdrv_iterate_format(void (*it)(void *opaque, const char *name),
                         void *opaque, bool read_only)
{
    BlockDriver *drv;
    int count = 0;
    int i;
    const char **formats = NULL;

    GLOBAL_STATE_CODE();

    QLIST_FOREACH(drv, &bdrv_drivers, list) {
        if (drv->format_name) {
            bool found = false;
            int i = count;

            if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, read_only)) {
                continue;
            }

            while (formats && i && !found) {
                found = !strcmp(formats[--i], drv->format_name);
            }

            if (!found) {
                formats = g_renew(const char *, formats, count + 1);
                formats[count++] = drv->format_name;
            }
        }
    }

    for (i = 0; i < (int)ARRAY_SIZE(block_driver_modules); i++) {
        const char *format_name = block_driver_modules[i].format_name;

        if (format_name) {
            bool found = false;
            int j = count;

            if (use_bdrv_whitelist &&
                !bdrv_format_is_whitelisted(format_name, read_only)) {
                continue;
            }

            while (formats && j && !found) {
                found = !strcmp(formats[--j], format_name);
            }

            if (!found) {
                formats = g_renew(const char *, formats, count + 1);
                formats[count++] = format_name;
            }
        }
    }

    qsort(formats, count, sizeof(formats[0]), qsort_strcmp);

    for (i = 0; i < count; i++) {
        it(opaque, formats[i]);
    }

    g_free(formats);
}

/* This function is to find a node in the bs graph */
BlockDriverState *bdrv_find_node(const char *node_name)
{
    BlockDriverState *bs;

    assert(node_name);
    GLOBAL_STATE_CODE();

    QTAILQ_FOREACH(bs, &graph_bdrv_states, node_list) {
        if (!strcmp(node_name, bs->node_name)) {
            return bs;
        }
    }
    return NULL;
}

/* Put this QMP function here so it can access the static graph_bdrv_states. */
BlockDeviceInfoList *bdrv_named_nodes_list(bool flat,
                                           Error **errp)
{
    BlockDeviceInfoList *list;
    BlockDriverState *bs;

    GLOBAL_STATE_CODE();

    list = NULL;
    QTAILQ_FOREACH(bs, &graph_bdrv_states, node_list) {
        BlockDeviceInfo *info = bdrv_block_device_info(NULL, bs, flat, errp);
        if (!info) {
            qapi_free_BlockDeviceInfoList(list);
            return NULL;
        }
        QAPI_LIST_PREPEND(list, info);
    }

    return list;
}

typedef struct XDbgBlockGraphConstructor {
    XDbgBlockGraph *graph;
    GHashTable *graph_nodes;
} XDbgBlockGraphConstructor;

static XDbgBlockGraphConstructor *xdbg_graph_new(void)
{
    XDbgBlockGraphConstructor *gr = g_new(XDbgBlockGraphConstructor, 1);

    gr->graph = g_new0(XDbgBlockGraph, 1);
    gr->graph_nodes = g_hash_table_new(NULL, NULL);

    return gr;
}

static XDbgBlockGraph *xdbg_graph_finalize(XDbgBlockGraphConstructor *gr)
{
    XDbgBlockGraph *graph = gr->graph;

    g_hash_table_destroy(gr->graph_nodes);
    g_free(gr);

    return graph;
}

static uintptr_t xdbg_graph_node_num(XDbgBlockGraphConstructor *gr, void *node)
{
    uintptr_t ret = (uintptr_t)g_hash_table_lookup(gr->graph_nodes, node);

    if (ret != 0) {
        return ret;
    }

    /*
     * Start counting from 1, not 0, because 0 interferes with not-found (NULL)
     * answer of g_hash_table_lookup.
     */
    ret = g_hash_table_size(gr->graph_nodes) + 1;
    g_hash_table_insert(gr->graph_nodes, node, (void *)ret);

    return ret;
}

static void xdbg_graph_add_node(XDbgBlockGraphConstructor *gr, void *node,
                                XDbgBlockGraphNodeType type, const char *name)
{
    XDbgBlockGraphNode *n;

    n = g_new0(XDbgBlockGraphNode, 1);

    n->id = xdbg_graph_node_num(gr, node);
    n->type = type;
    n->name = g_strdup(name);

    QAPI_LIST_PREPEND(gr->graph->nodes, n);
}

static void xdbg_graph_add_edge(XDbgBlockGraphConstructor *gr, void *parent,
                                const BdrvChild *child)
{
    BlockPermission qapi_perm;
    XDbgBlockGraphEdge *edge;
    GLOBAL_STATE_CODE();

    edge = g_new0(XDbgBlockGraphEdge, 1);

    edge->parent = xdbg_graph_node_num(gr, parent);
    edge->child = xdbg_graph_node_num(gr, child->bs);
    edge->name = g_strdup(child->name);

    for (qapi_perm = 0; qapi_perm < BLOCK_PERMISSION__MAX; qapi_perm++) {
        uint64_t flag = bdrv_qapi_perm_to_blk_perm(qapi_perm);

        if (flag & child->perm) {
            QAPI_LIST_PREPEND(edge->perm, qapi_perm);
        }
        if (flag & child->shared_perm) {
            QAPI_LIST_PREPEND(edge->shared_perm, qapi_perm);
        }
    }

    QAPI_LIST_PREPEND(gr->graph->edges, edge);
}


XDbgBlockGraph *bdrv_get_xdbg_block_graph(Error **errp)
{
    BlockBackend *blk;
    BlockJob *job;
    BlockDriverState *bs;
    BdrvChild *child;
    XDbgBlockGraphConstructor *gr = xdbg_graph_new();

    GLOBAL_STATE_CODE();

    for (blk = blk_all_next(NULL); blk; blk = blk_all_next(blk)) {
        char *allocated_name = NULL;
        const char *name = blk_name(blk);

        if (!*name) {
            name = allocated_name = blk_get_attached_dev_id(blk);
        }
        xdbg_graph_add_node(gr, blk, X_DBG_BLOCK_GRAPH_NODE_TYPE_BLOCK_BACKEND,
                           name);
        g_free(allocated_name);
        if (blk_root(blk)) {
            xdbg_graph_add_edge(gr, blk, blk_root(blk));
        }
    }

    WITH_JOB_LOCK_GUARD() {
        for (job = block_job_next_locked(NULL); job;
             job = block_job_next_locked(job)) {
            GSList *el;

            xdbg_graph_add_node(gr, job, X_DBG_BLOCK_GRAPH_NODE_TYPE_BLOCK_JOB,
                                job->job.id);
            for (el = job->nodes; el; el = el->next) {
                xdbg_graph_add_edge(gr, job, (BdrvChild *)el->data);
            }
        }
    }

    QTAILQ_FOREACH(bs, &graph_bdrv_states, node_list) {
        xdbg_graph_add_node(gr, bs, X_DBG_BLOCK_GRAPH_NODE_TYPE_BLOCK_DRIVER,
                           bs->node_name);
        QLIST_FOREACH(child, &bs->children, next) {
            xdbg_graph_add_edge(gr, bs, child);
        }
    }

    return xdbg_graph_finalize(gr);
}

BlockDriverState *bdrv_lookup_bs(const char *device,
                                 const char *node_name,
                                 Error **errp)
{
    BlockBackend *blk;
    BlockDriverState *bs;

    GLOBAL_STATE_CODE();

    if (device) {
        blk = blk_by_name(device);

        if (blk) {
            bs = blk_bs(blk);
            if (!bs) {
                error_setg(errp, "Device '%s' has no medium", device);
            }

            return bs;
        }
    }

    if (node_name) {
        bs = bdrv_find_node(node_name);

        if (bs) {
            return bs;
        }
    }

    error_setg(errp, "Cannot find device=\'%s\' nor node-name=\'%s\'",
                     device ? device : "",
                     node_name ? node_name : "");
    return NULL;
}

/* If 'base' is in the same chain as 'top', return true. Otherwise,
 * return false.  If either argument is NULL, return false. */
bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base)
{

    GLOBAL_STATE_CODE();

    while (top && top != base) {
        top = bdrv_filter_or_cow_bs(top);
    }

    return top != NULL;
}

BlockDriverState *bdrv_next_node(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    if (!bs) {
        return QTAILQ_FIRST(&graph_bdrv_states);
    }
    return QTAILQ_NEXT(bs, node_list);
}

BlockDriverState *bdrv_next_all_states(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    if (!bs) {
        return QTAILQ_FIRST(&all_bdrv_states);
    }
    return QTAILQ_NEXT(bs, bs_list);
}

const char *bdrv_get_node_name(const BlockDriverState *bs)
{
    IO_CODE();
    return bs->node_name;
}

const char *bdrv_get_parent_name(const BlockDriverState *bs)
{
    BdrvChild *c;
    const char *name;
    IO_CODE();

    /* If multiple parents have a name, just pick the first one. */
    QLIST_FOREACH(c, &bs->parents, next_parent) {
        if (c->klass->get_name) {
            name = c->klass->get_name(c);
            if (name && *name) {
                return name;
            }
        }
    }

    return NULL;
}

/* TODO check what callers really want: bs->node_name or blk_name() */
const char *bdrv_get_device_name(const BlockDriverState *bs)
{
    IO_CODE();
    return bdrv_get_parent_name(bs) ?: "";
}

/* This can be used to identify nodes that might not have a device
 * name associated. Since node and device names live in the same
 * namespace, the result is unambiguous. The exception is if both are
 * absent, then this returns an empty (non-null) string. */
const char *bdrv_get_device_or_node_name(const BlockDriverState *bs)
{
    IO_CODE();
    return bdrv_get_parent_name(bs) ?: bs->node_name;
}

int bdrv_get_flags(BlockDriverState *bs)
{
    IO_CODE();
    return bs->open_flags;
}

int bdrv_has_zero_init_1(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    return 1;
}

int bdrv_has_zero_init(BlockDriverState *bs)
{
    BlockDriverState *filtered;
    GLOBAL_STATE_CODE();

    if (!bs->drv) {
        return 0;
    }

    /* If BS is a copy on write image, it is initialized to
       the contents of the base image, which may not be zeroes.  */
    if (bdrv_cow_child(bs)) {
        return 0;
    }
    if (bs->drv->bdrv_has_zero_init) {
        return bs->drv->bdrv_has_zero_init(bs);
    }

    filtered = bdrv_filter_bs(bs);
    if (filtered) {
        return bdrv_has_zero_init(filtered);
    }

    /* safe default */
    return 0;
}

bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs)
{
    IO_CODE();
    if (!(bs->open_flags & BDRV_O_UNMAP)) {
        return false;
    }

    return bs->supported_zero_flags & BDRV_REQ_MAY_UNMAP;
}

void bdrv_get_backing_filename(BlockDriverState *bs,
                               char *filename, int filename_size)
{
    IO_CODE();
    pstrcpy(filename, filename_size, bs->backing_file);
}

int coroutine_fn bdrv_co_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
{
    int ret;
    BlockDriver *drv = bs->drv;
    IO_CODE();
    /* if bs->drv == NULL, bs is closed, so there's nothing to do here */
    if (!drv) {
        return -ENOMEDIUM;
    }
    if (!drv->bdrv_co_get_info) {
        BlockDriverState *filtered = bdrv_filter_bs(bs);
        if (filtered) {
            return bdrv_co_get_info(filtered, bdi);
        }
        return -ENOTSUP;
    }
    memset(bdi, 0, sizeof(*bdi));
    ret = drv->bdrv_co_get_info(bs, bdi);
    if (ret < 0) {
        return ret;
    }

    if (bdi->cluster_size > BDRV_MAX_ALIGNMENT) {
        return -EINVAL;
    }

    return 0;
}

ImageInfoSpecific *bdrv_get_specific_info(BlockDriverState *bs,
                                          Error **errp)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();
    if (drv && drv->bdrv_get_specific_info) {
        return drv->bdrv_get_specific_info(bs, errp);
    }
    return NULL;
}

BlockStatsSpecific *bdrv_get_specific_stats(BlockDriverState *bs)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();
    if (!drv || !drv->bdrv_get_specific_stats) {
        return NULL;
    }
    return drv->bdrv_get_specific_stats(bs);
}

void coroutine_fn bdrv_co_debug_event(BlockDriverState *bs, BlkdebugEvent event)
{
    IO_CODE();
    if (!bs || !bs->drv || !bs->drv->bdrv_co_debug_event) {
        return;
    }

    bs->drv->bdrv_co_debug_event(bs, event);
}

static BlockDriverState *bdrv_find_debug_node(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    while (bs && bs->drv && !bs->drv->bdrv_debug_breakpoint) {
        bs = bdrv_primary_bs(bs);
    }

    if (bs && bs->drv && bs->drv->bdrv_debug_breakpoint) {
        assert(bs->drv->bdrv_debug_remove_breakpoint);
        return bs;
    }

    return NULL;
}

int bdrv_debug_breakpoint(BlockDriverState *bs, const char *event,
                          const char *tag)
{
    GLOBAL_STATE_CODE();
    bs = bdrv_find_debug_node(bs);
    if (bs) {
        return bs->drv->bdrv_debug_breakpoint(bs, event, tag);
    }

    return -ENOTSUP;
}

int bdrv_debug_remove_breakpoint(BlockDriverState *bs, const char *tag)
{
    GLOBAL_STATE_CODE();
    bs = bdrv_find_debug_node(bs);
    if (bs) {
        return bs->drv->bdrv_debug_remove_breakpoint(bs, tag);
    }

    return -ENOTSUP;
}

int bdrv_debug_resume(BlockDriverState *bs, const char *tag)
{
    GLOBAL_STATE_CODE();
    while (bs && (!bs->drv || !bs->drv->bdrv_debug_resume)) {
        bs = bdrv_primary_bs(bs);
    }

    if (bs && bs->drv && bs->drv->bdrv_debug_resume) {
        return bs->drv->bdrv_debug_resume(bs, tag);
    }

    return -ENOTSUP;
}

bool bdrv_debug_is_suspended(BlockDriverState *bs, const char *tag)
{
    GLOBAL_STATE_CODE();
    while (bs && bs->drv && !bs->drv->bdrv_debug_is_suspended) {
        bs = bdrv_primary_bs(bs);
    }

    if (bs && bs->drv && bs->drv->bdrv_debug_is_suspended) {
        return bs->drv->bdrv_debug_is_suspended(bs, tag);
    }

    return false;
}

/* backing_file can either be relative, or absolute, or a protocol.  If it is
 * relative, it must be relative to the chain.  So, passing in bs->filename
 * from a BDS as backing_file should not be done, as that may be relative to
 * the CWD rather than the chain. */
BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
        const char *backing_file)
{
    char *filename_full = NULL;
    char *backing_file_full = NULL;
    char *filename_tmp = NULL;
    int is_protocol = 0;
    bool filenames_refreshed = false;
    BlockDriverState *curr_bs = NULL;
    BlockDriverState *retval = NULL;
    BlockDriverState *bs_below;

    GLOBAL_STATE_CODE();

    if (!bs || !bs->drv || !backing_file) {
        return NULL;
    }

    filename_full     = g_malloc(PATH_MAX);
    backing_file_full = g_malloc(PATH_MAX);

    is_protocol = path_has_protocol(backing_file);

    /*
     * Being largely a legacy function, skip any filters here
     * (because filters do not have normal filenames, so they cannot
     * match anyway; and allowing json:{} filenames is a bit out of
     * scope).
     */
    for (curr_bs = bdrv_skip_filters(bs);
         bdrv_cow_child(curr_bs) != NULL;
         curr_bs = bs_below)
    {
        bs_below = bdrv_backing_chain_next(curr_bs);

        if (bdrv_backing_overridden(curr_bs)) {
            /*
             * If the backing file was overridden, we can only compare
             * directly against the backing node's filename.
             */

            if (!filenames_refreshed) {
                /*
                 * This will automatically refresh all of the
                 * filenames in the rest of the backing chain, so we
                 * only need to do this once.
                 */
                bdrv_refresh_filename(bs_below);
                filenames_refreshed = true;
            }

            if (strcmp(backing_file, bs_below->filename) == 0) {
                retval = bs_below;
                break;
            }
        } else if (is_protocol || path_has_protocol(curr_bs->backing_file)) {
            /*
             * If either of the filename paths is actually a protocol, then
             * compare unmodified paths; otherwise make paths relative.
             */
            char *backing_file_full_ret;

            if (strcmp(backing_file, curr_bs->backing_file) == 0) {
                retval = bs_below;
                break;
            }
            /* Also check against the full backing filename for the image */
            backing_file_full_ret = bdrv_get_full_backing_filename(curr_bs,
                                                                   NULL);
            if (backing_file_full_ret) {
                bool equal = strcmp(backing_file, backing_file_full_ret) == 0;
                g_free(backing_file_full_ret);
                if (equal) {
                    retval = bs_below;
                    break;
                }
            }
        } else {
            /* If not an absolute filename path, make it relative to the current
             * image's filename path */
            filename_tmp = bdrv_make_absolute_filename(curr_bs, backing_file,
                                                       NULL);
            /* We are going to compare canonicalized absolute pathnames */
            if (!filename_tmp || !realpath(filename_tmp, filename_full)) {
                g_free(filename_tmp);
                continue;
            }
            g_free(filename_tmp);

            /* We need to make sure the backing filename we are comparing against
             * is relative to the current image filename (or absolute) */
            filename_tmp = bdrv_get_full_backing_filename(curr_bs, NULL);
            if (!filename_tmp || !realpath(filename_tmp, backing_file_full)) {
                g_free(filename_tmp);
                continue;
            }
            g_free(filename_tmp);

            if (strcmp(backing_file_full, filename_full) == 0) {
                retval = bs_below;
                break;
            }
        }
    }

    g_free(filename_full);
    g_free(backing_file_full);
    return retval;
}

void bdrv_init(void)
{
#ifdef CONFIG_BDRV_WHITELIST_TOOLS
    use_bdrv_whitelist = 1;
#endif
    module_call_init(MODULE_INIT_BLOCK);
}

void bdrv_init_with_whitelist(void)
{
    use_bdrv_whitelist = 1;
    bdrv_init();
}

int bdrv_activate(BlockDriverState *bs, Error **errp)
{
    BdrvChild *child, *parent;
    Error *local_err = NULL;
    int ret;
    BdrvDirtyBitmap *bm;

    GLOBAL_STATE_CODE();

    if (!bs->drv)  {
        return -ENOMEDIUM;
    }

    QLIST_FOREACH(child, &bs->children, next) {
        bdrv_activate(child->bs, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
            return -EINVAL;
        }
    }

    /*
     * Update permissions, they may differ for inactive nodes.
     *
     * Note that the required permissions of inactive images are always a
     * subset of the permissions required after activating the image. This
     * allows us to just get the permissions upfront without restricting
     * bdrv_co_invalidate_cache().
     *
     * It also means that in error cases, we don't have to try and revert to
     * the old permissions (which is an operation that could fail, too). We can
     * just keep the extended permissions for the next time that an activation
     * of the image is tried.
     */
    if (bs->open_flags & BDRV_O_INACTIVE) {
        bs->open_flags &= ~BDRV_O_INACTIVE;
        ret = bdrv_refresh_perms(bs, NULL, errp);
        if (ret < 0) {
            bs->open_flags |= BDRV_O_INACTIVE;
            return ret;
        }

        ret = bdrv_invalidate_cache(bs, errp);
        if (ret < 0) {
            bs->open_flags |= BDRV_O_INACTIVE;
            return ret;
        }

        FOR_EACH_DIRTY_BITMAP(bs, bm) {
            bdrv_dirty_bitmap_skip_store(bm, false);
        }

        ret = bdrv_refresh_total_sectors(bs, bs->total_sectors);
        if (ret < 0) {
            bs->open_flags |= BDRV_O_INACTIVE;
            error_setg_errno(errp, -ret, "Could not refresh total sector count");
            return ret;
        }
    }

    QLIST_FOREACH(parent, &bs->parents, next_parent) {
        if (parent->klass->activate) {
            parent->klass->activate(parent, &local_err);
            if (local_err) {
                bs->open_flags |= BDRV_O_INACTIVE;
                error_propagate(errp, local_err);
                return -EINVAL;
            }
        }
    }

    return 0;
}

int coroutine_fn bdrv_co_invalidate_cache(BlockDriverState *bs, Error **errp)
{
    Error *local_err = NULL;
    IO_CODE();

    assert(!(bs->open_flags & BDRV_O_INACTIVE));
    assert_bdrv_graph_readable();

    if (bs->drv->bdrv_co_invalidate_cache) {
        bs->drv->bdrv_co_invalidate_cache(bs, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
            return -EINVAL;
        }
    }

    return 0;
}

void bdrv_activate_all(Error **errp)
{
    BlockDriverState *bs;
    BdrvNextIterator it;

    GLOBAL_STATE_CODE();

    for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
        AioContext *aio_context = bdrv_get_aio_context(bs);
        int ret;

        aio_context_acquire(aio_context);
        ret = bdrv_activate(bs, errp);
        aio_context_release(aio_context);
        if (ret < 0) {
            bdrv_next_cleanup(&it);
            return;
        }
    }
}

static bool bdrv_has_bds_parent(BlockDriverState *bs, bool only_active)
{
    BdrvChild *parent;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH(parent, &bs->parents, next_parent) {
        if (parent->klass->parent_is_bds) {
            BlockDriverState *parent_bs = parent->opaque;
            if (!only_active || !(parent_bs->open_flags & BDRV_O_INACTIVE)) {
                return true;
            }
        }
    }

    return false;
}

static int bdrv_inactivate_recurse(BlockDriverState *bs)
{
    BdrvChild *child, *parent;
    int ret;
    uint64_t cumulative_perms, cumulative_shared_perms;

    GLOBAL_STATE_CODE();

    if (!bs->drv) {
        return -ENOMEDIUM;
    }

    /* Make sure that we don't inactivate a child before its parent.
     * It will be covered by recursion from the yet active parent. */
    if (bdrv_has_bds_parent(bs, true)) {
        return 0;
    }

    assert(!(bs->open_flags & BDRV_O_INACTIVE));

    /* Inactivate this node */
    if (bs->drv->bdrv_inactivate) {
        ret = bs->drv->bdrv_inactivate(bs);
        if (ret < 0) {
            return ret;
        }
    }

    QLIST_FOREACH(parent, &bs->parents, next_parent) {
        if (parent->klass->inactivate) {
            ret = parent->klass->inactivate(parent);
            if (ret < 0) {
                return ret;
            }
        }
    }

    bdrv_get_cumulative_perm(bs, &cumulative_perms,
                             &cumulative_shared_perms);
    if (cumulative_perms & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED)) {
        /* Our inactive parents still need write access. Inactivation failed. */
        return -EPERM;
    }

    bs->open_flags |= BDRV_O_INACTIVE;

    /*
     * Update permissions, they may differ for inactive nodes.
     * We only tried to loosen restrictions, so errors are not fatal, ignore
     * them.
     */
    bdrv_refresh_perms(bs, NULL, NULL);

    /* Recursively inactivate children */
    QLIST_FOREACH(child, &bs->children, next) {
        ret = bdrv_inactivate_recurse(child->bs);
        if (ret < 0) {
            return ret;
        }
    }

    return 0;
}

int bdrv_inactivate_all(void)
{
    BlockDriverState *bs = NULL;
    BdrvNextIterator it;
    int ret = 0;
    GSList *aio_ctxs = NULL, *ctx;

    GLOBAL_STATE_CODE();

    for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
        AioContext *aio_context = bdrv_get_aio_context(bs);

        if (!g_slist_find(aio_ctxs, aio_context)) {
            aio_ctxs = g_slist_prepend(aio_ctxs, aio_context);
            aio_context_acquire(aio_context);
        }
    }

    for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
        /* Nodes with BDS parents are covered by recursion from the last
         * parent that gets inactivated. Don't inactivate them a second
         * time if that has already happened. */
        if (bdrv_has_bds_parent(bs, false)) {
            continue;
        }
        ret = bdrv_inactivate_recurse(bs);
        if (ret < 0) {
            bdrv_next_cleanup(&it);
            goto out;
        }
    }

out:
    for (ctx = aio_ctxs; ctx != NULL; ctx = ctx->next) {
        AioContext *aio_context = ctx->data;
        aio_context_release(aio_context);
    }
    g_slist_free(aio_ctxs);

    return ret;
}

/**************************************************************/
/* removable device support */

/**
 * Return TRUE if the media is present
 */
bool coroutine_fn bdrv_co_is_inserted(BlockDriverState *bs)
{
    BlockDriver *drv = bs->drv;
    BdrvChild *child;
    IO_CODE();

    if (!drv) {
        return false;
    }
    if (drv->bdrv_co_is_inserted) {
        return drv->bdrv_co_is_inserted(bs);
    }
    QLIST_FOREACH(child, &bs->children, next) {
        if (!bdrv_co_is_inserted(child->bs)) {
            return false;
        }
    }
    return true;
}

/**
 * If eject_flag is TRUE, eject the media. Otherwise, close the tray
 */
void coroutine_fn bdrv_co_eject(BlockDriverState *bs, bool eject_flag)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();

    if (drv && drv->bdrv_co_eject) {
        drv->bdrv_co_eject(bs, eject_flag);
    }
}

/**
 * Lock or unlock the media (if it is locked, the user won't be able
 * to eject it manually).
 */
void coroutine_fn bdrv_co_lock_medium(BlockDriverState *bs, bool locked)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();
    trace_bdrv_lock_medium(bs, locked);

    if (drv && drv->bdrv_co_lock_medium) {
        drv->bdrv_co_lock_medium(bs, locked);
    }
}

/* Get a reference to bs */
void bdrv_ref(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    bs->refcnt++;
}

/* Release a previously grabbed reference to bs.
 * If after releasing, reference count is zero, the BlockDriverState is
 * deleted. */
void bdrv_unref(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    if (!bs) {
        return;
    }
    assert(bs->refcnt > 0);
    if (--bs->refcnt == 0) {
        bdrv_delete(bs);
    }
}

struct BdrvOpBlocker {
    Error *reason;
    QLIST_ENTRY(BdrvOpBlocker) list;
};

bool bdrv_op_is_blocked(BlockDriverState *bs, BlockOpType op, Error **errp)
{
    BdrvOpBlocker *blocker;
    GLOBAL_STATE_CODE();
    assert((int) op >= 0 && op < BLOCK_OP_TYPE_MAX);
    if (!QLIST_EMPTY(&bs->op_blockers[op])) {
        blocker = QLIST_FIRST(&bs->op_blockers[op]);
        error_propagate_prepend(errp, error_copy(blocker->reason),
                                "Node '%s' is busy: ",
                                bdrv_get_device_or_node_name(bs));
        return true;
    }
    return false;
}

void bdrv_op_block(BlockDriverState *bs, BlockOpType op, Error *reason)
{
    BdrvOpBlocker *blocker;
    GLOBAL_STATE_CODE();
    assert((int) op >= 0 && op < BLOCK_OP_TYPE_MAX);

    blocker = g_new0(BdrvOpBlocker, 1);
    blocker->reason = reason;
    QLIST_INSERT_HEAD(&bs->op_blockers[op], blocker, list);
}

void bdrv_op_unblock(BlockDriverState *bs, BlockOpType op, Error *reason)
{
    BdrvOpBlocker *blocker, *next;
    GLOBAL_STATE_CODE();
    assert((int) op >= 0 && op < BLOCK_OP_TYPE_MAX);
    QLIST_FOREACH_SAFE(blocker, &bs->op_blockers[op], list, next) {
        if (blocker->reason == reason) {
            QLIST_REMOVE(blocker, list);
            g_free(blocker);
        }
    }
}

void bdrv_op_block_all(BlockDriverState *bs, Error *reason)
{
    int i;
    GLOBAL_STATE_CODE();
    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
        bdrv_op_block(bs, i, reason);
    }
}

void bdrv_op_unblock_all(BlockDriverState *bs, Error *reason)
{
    int i;
    GLOBAL_STATE_CODE();
    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
        bdrv_op_unblock(bs, i, reason);
    }
}

bool bdrv_op_blocker_is_empty(BlockDriverState *bs)
{
    int i;
    GLOBAL_STATE_CODE();
    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
        if (!QLIST_EMPTY(&bs->op_blockers[i])) {
            return false;
        }
    }
    return true;
}

/*
 * Must not be called while holding the lock of an AioContext other than the
 * current one.
 */
void bdrv_img_create(const char *filename, const char *fmt,
                     const char *base_filename, const char *base_fmt,
                     char *options, uint64_t img_size, int flags, bool quiet,
                     Error **errp)
{
    QemuOptsList *create_opts = NULL;
    QemuOpts *opts = NULL;
    const char *backing_fmt, *backing_file;
    int64_t size;
    BlockDriver *drv, *proto_drv;
    Error *local_err = NULL;
    int ret = 0;

    GLOBAL_STATE_CODE();

    /* Find driver and parse its options */
    drv = bdrv_find_format(fmt);
    if (!drv) {
        error_setg(errp, "Unknown file format '%s'", fmt);
        return;
    }

    proto_drv = bdrv_find_protocol(filename, true, errp);
    if (!proto_drv) {
        return;
    }

    if (!drv->create_opts) {
        error_setg(errp, "Format driver '%s' does not support image creation",
                   drv->format_name);
        return;
    }

    if (!proto_drv->create_opts) {
        error_setg(errp, "Protocol driver '%s' does not support image creation",
                   proto_drv->format_name);
        return;
    }

    /* Create parameter list */
    create_opts = qemu_opts_append(create_opts, drv->create_opts);
    create_opts = qemu_opts_append(create_opts, proto_drv->create_opts);

    opts = qemu_opts_create(create_opts, NULL, 0, &error_abort);

    /* Parse -o options */
    if (options) {
        if (!qemu_opts_do_parse(opts, options, NULL, errp)) {
            goto out;
        }
    }

    if (!qemu_opt_get(opts, BLOCK_OPT_SIZE)) {
        qemu_opt_set_number(opts, BLOCK_OPT_SIZE, img_size, &error_abort);
    } else if (img_size != UINT64_C(-1)) {
        error_setg(errp, "The image size must be specified only once");
        goto out;
    }

    if (base_filename) {
        if (!qemu_opt_set(opts, BLOCK_OPT_BACKING_FILE, base_filename,
                          NULL)) {
            error_setg(errp, "Backing file not supported for file format '%s'",
                       fmt);
            goto out;
        }
    }

    if (base_fmt) {
        if (!qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, base_fmt, NULL)) {
            error_setg(errp, "Backing file format not supported for file "
                             "format '%s'", fmt);
            goto out;
        }
    }

    backing_file = qemu_opt_get(opts, BLOCK_OPT_BACKING_FILE);
    if (backing_file) {
        if (!strcmp(filename, backing_file)) {
            error_setg(errp, "Error: Trying to create an image with the "
                             "same filename as the backing file");
            goto out;
        }
        if (backing_file[0] == '\0') {
            error_setg(errp, "Expected backing file name, got empty string");
            goto out;
        }
    }

    backing_fmt = qemu_opt_get(opts, BLOCK_OPT_BACKING_FMT);

    /* The size for the image must always be specified, unless we have a backing
     * file and we have not been forbidden from opening it. */
    size = qemu_opt_get_size(opts, BLOCK_OPT_SIZE, img_size);
    if (backing_file && !(flags & BDRV_O_NO_BACKING)) {
        BlockDriverState *bs;
        char *full_backing;
        int back_flags;
        QDict *backing_options = NULL;

        full_backing =
            bdrv_get_full_backing_filename_from_filename(filename, backing_file,
                                                         &local_err);
        if (local_err) {
            goto out;
        }
        assert(full_backing);

        /*
         * No need to do I/O here, which allows us to open encrypted
         * backing images without needing the secret
         */
        back_flags = flags;
        back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
        back_flags |= BDRV_O_NO_IO;

        backing_options = qdict_new();
        if (backing_fmt) {
            qdict_put_str(backing_options, "driver", backing_fmt);
        }
        qdict_put_bool(backing_options, BDRV_OPT_FORCE_SHARE, true);

        bs = bdrv_open(full_backing, NULL, backing_options, back_flags,
                       &local_err);
        g_free(full_backing);
        if (!bs) {
            error_append_hint(&local_err, "Could not open backing image.\n");
            goto out;
        } else {
            if (!backing_fmt) {
                error_setg(&local_err,
                           "Backing file specified without backing format");
                error_append_hint(&local_err, "Detected format of %s.",
                                  bs->drv->format_name);
                goto out;
            }
            if (size == -1) {
                /* Opened BS, have no size */
                size = bdrv_getlength(bs);
                if (size < 0) {
                    error_setg_errno(errp, -size, "Could not get size of '%s'",
                                     backing_file);
                    bdrv_unref(bs);
                    goto out;
                }
                qemu_opt_set_number(opts, BLOCK_OPT_SIZE, size, &error_abort);
            }
            bdrv_unref(bs);
        }
        /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
    } else if (backing_file && !backing_fmt) {
        error_setg(&local_err,
                   "Backing file specified without backing format");
        goto out;
    }

    if (size == -1) {
        error_setg(errp, "Image creation needs a size parameter");
        goto out;
    }

    if (!quiet) {
        printf("Formatting '%s', fmt=%s ", filename, fmt);
        qemu_opts_print(opts, " ");
        puts("");
        fflush(stdout);
    }

    ret = bdrv_create(drv, filename, opts, &local_err);

    if (ret == -EFBIG) {
        /* This is generally a better message than whatever the driver would
         * deliver (especially because of the cluster_size_hint), since that
         * is most probably not much different from "image too large". */
        const char *cluster_size_hint = "";
        if (qemu_opt_get_size(opts, BLOCK_OPT_CLUSTER_SIZE, 0)) {
            cluster_size_hint = " (try using a larger cluster size)";
        }
        error_setg(errp, "The image size is too large for file format '%s'"
                   "%s", fmt, cluster_size_hint);
        error_free(local_err);
        local_err = NULL;
    }

out:
    qemu_opts_del(opts);
    qemu_opts_free(create_opts);
    error_propagate(errp, local_err);
}

AioContext *bdrv_get_aio_context(BlockDriverState *bs)
{
    IO_CODE();
    return bs ? bs->aio_context : qemu_get_aio_context();
}

AioContext *coroutine_fn bdrv_co_enter(BlockDriverState *bs)
{
    Coroutine *self = qemu_coroutine_self();
    AioContext *old_ctx = qemu_coroutine_get_aio_context(self);
    AioContext *new_ctx;
    IO_CODE();

    /*
     * Increase bs->in_flight to ensure that this operation is completed before
     * moving the node to a different AioContext. Read new_ctx only afterwards.
     */
    bdrv_inc_in_flight(bs);

    new_ctx = bdrv_get_aio_context(bs);
    aio_co_reschedule_self(new_ctx);
    return old_ctx;
}

void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx)
{
    IO_CODE();
    aio_co_reschedule_self(old_ctx);
    bdrv_dec_in_flight(bs);
}

void coroutine_fn bdrv_co_lock(BlockDriverState *bs)
{
    AioContext *ctx = bdrv_get_aio_context(bs);

    /* In the main thread, bs->aio_context won't change concurrently */
    assert(qemu_get_current_aio_context() == qemu_get_aio_context());

    /*
     * We're in coroutine context, so we already hold the lock of the main
     * loop AioContext. Don't lock it twice to avoid deadlocks.
     */
    assert(qemu_in_coroutine());
    if (ctx != qemu_get_aio_context()) {
        aio_context_acquire(ctx);
    }
}

void coroutine_fn bdrv_co_unlock(BlockDriverState *bs)
{
    AioContext *ctx = bdrv_get_aio_context(bs);

    assert(qemu_in_coroutine());
    if (ctx != qemu_get_aio_context()) {
        aio_context_release(ctx);
    }
}

static void bdrv_do_remove_aio_context_notifier(BdrvAioNotifier *ban)
{
    GLOBAL_STATE_CODE();
    QLIST_REMOVE(ban, list);
    g_free(ban);
}

static void bdrv_detach_aio_context(BlockDriverState *bs)
{
    BdrvAioNotifier *baf, *baf_tmp;

    assert(!bs->walking_aio_notifiers);
    GLOBAL_STATE_CODE();
    bs->walking_aio_notifiers = true;
    QLIST_FOREACH_SAFE(baf, &bs->aio_notifiers, list, baf_tmp) {
        if (baf->deleted) {
            bdrv_do_remove_aio_context_notifier(baf);
        } else {
            baf->detach_aio_context(baf->opaque);
        }
    }
    /* Never mind iterating again to check for ->deleted.  bdrv_close() will
     * remove remaining aio notifiers if we aren't called again.
     */
    bs->walking_aio_notifiers = false;

    if (bs->drv && bs->drv->bdrv_detach_aio_context) {
        bs->drv->bdrv_detach_aio_context(bs);
    }

    if (bs->quiesce_counter) {
        aio_enable_external(bs->aio_context);
    }
    bs->aio_context = NULL;
}

static void bdrv_attach_aio_context(BlockDriverState *bs,
                                    AioContext *new_context)
{
    BdrvAioNotifier *ban, *ban_tmp;
    GLOBAL_STATE_CODE();

    if (bs->quiesce_counter) {
        aio_disable_external(new_context);
    }

    bs->aio_context = new_context;

    if (bs->drv && bs->drv->bdrv_attach_aio_context) {
        bs->drv->bdrv_attach_aio_context(bs, new_context);
    }

    assert(!bs->walking_aio_notifiers);
    bs->walking_aio_notifiers = true;
    QLIST_FOREACH_SAFE(ban, &bs->aio_notifiers, list, ban_tmp) {
        if (ban->deleted) {
            bdrv_do_remove_aio_context_notifier(ban);
        } else {
            ban->attached_aio_context(new_context, ban->opaque);
        }
    }
    bs->walking_aio_notifiers = false;
}

typedef struct BdrvStateSetAioContext {
    AioContext *new_ctx;
    BlockDriverState *bs;
} BdrvStateSetAioContext;

static bool bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx,
                                           GHashTable *visited,
                                           Transaction *tran,
                                           Error **errp)
{
    GLOBAL_STATE_CODE();
    if (g_hash_table_contains(visited, c)) {
        return true;
    }
    g_hash_table_add(visited, c);

    /*
     * A BdrvChildClass that doesn't handle AioContext changes cannot
     * tolerate any AioContext changes
     */
    if (!c->klass->change_aio_ctx) {
        char *user = bdrv_child_user_desc(c);
        error_setg(errp, "Changing iothreads is not supported by %s", user);
        g_free(user);
        return false;
    }
    if (!c->klass->change_aio_ctx(c, ctx, visited, tran, errp)) {
        assert(!errp || *errp);
        return false;
    }
    return true;
}

bool bdrv_child_change_aio_context(BdrvChild *c, AioContext *ctx,
                                   GHashTable *visited, Transaction *tran,
                                   Error **errp)
{
    GLOBAL_STATE_CODE();
    if (g_hash_table_contains(visited, c)) {
        return true;
    }
    g_hash_table_add(visited, c);
    return bdrv_change_aio_context(c->bs, ctx, visited, tran, errp);
}

static void bdrv_set_aio_context_clean(void *opaque)
{
    BdrvStateSetAioContext *state = (BdrvStateSetAioContext *) opaque;
    BlockDriverState *bs = (BlockDriverState *) state->bs;

    /* Paired with bdrv_drained_begin in bdrv_change_aio_context() */
    bdrv_drained_end(bs);

    g_free(state);
}

static void bdrv_set_aio_context_commit(void *opaque)
{
    BdrvStateSetAioContext *state = (BdrvStateSetAioContext *) opaque;
    BlockDriverState *bs = (BlockDriverState *) state->bs;
    AioContext *new_context = state->new_ctx;
    AioContext *old_context = bdrv_get_aio_context(bs);

    /*
     * Take the old AioContex when detaching it from bs.
     * At this point, new_context lock is already acquired, and we are now
     * also taking old_context. This is safe as long as bdrv_detach_aio_context
     * does not call AIO_POLL_WHILE().
     */
    if (old_context != qemu_get_aio_context()) {
        aio_context_acquire(old_context);
    }
    bdrv_detach_aio_context(bs);
    if (old_context != qemu_get_aio_context()) {
        aio_context_release(old_context);
    }
    bdrv_attach_aio_context(bs, new_context);
}

static TransactionActionDrv set_aio_context = {
    .commit = bdrv_set_aio_context_commit,
    .clean = bdrv_set_aio_context_clean,
};

/*
 * Changes the AioContext used for fd handlers, timers, and BHs by this
 * BlockDriverState and all its children and parents.
 *
 * Must be called from the main AioContext.
 *
 * The caller must own the AioContext lock for the old AioContext of bs, but it
 * must not own the AioContext lock for new_context (unless new_context is the
 * same as the current context of bs).
 *
 * @visited will accumulate all visited BdrvChild objects. The caller is
 * responsible for freeing the list afterwards.
 */
static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
                                    GHashTable *visited, Transaction *tran,
                                    Error **errp)
{
    BdrvChild *c;
    BdrvStateSetAioContext *state;

    GLOBAL_STATE_CODE();

    if (bdrv_get_aio_context(bs) == ctx) {
        return true;
    }

    QLIST_FOREACH(c, &bs->parents, next_parent) {
        if (!bdrv_parent_change_aio_context(c, ctx, visited, tran, errp)) {
            return false;
        }
    }

    QLIST_FOREACH(c, &bs->children, next) {
        if (!bdrv_child_change_aio_context(c, ctx, visited, tran, errp)) {
            return false;
        }
    }

    state = g_new(BdrvStateSetAioContext, 1);
    *state = (BdrvStateSetAioContext) {
        .new_ctx = ctx,
        .bs = bs,
    };

    /* Paired with bdrv_drained_end in bdrv_set_aio_context_clean() */
    bdrv_drained_begin(bs);

    tran_add(tran, &set_aio_context, state);

    return true;
}

/*
 * Change bs's and recursively all of its parents' and children's AioContext
 * to the given new context, returning an error if that isn't possible.
 *
 * If ignore_child is not NULL, that child (and its subgraph) will not
 * be touched.
 *
 * This function still requires the caller to take the bs current
 * AioContext lock, otherwise draining will fail since AIO_WAIT_WHILE
 * assumes the lock is always held if bs is in another AioContext.
 * For the same reason, it temporarily also holds the new AioContext, since
 * bdrv_drained_end calls BDRV_POLL_WHILE that assumes the lock is taken too.
 * Therefore the new AioContext lock must not be taken by the caller.
 */
int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
                                BdrvChild *ignore_child, Error **errp)
{
    Transaction *tran;
    GHashTable *visited;
    int ret;
    AioContext *old_context = bdrv_get_aio_context(bs);
    GLOBAL_STATE_CODE();

    /*
     * Recursion phase: go through all nodes of the graph.
     * Take care of checking that all nodes support changing AioContext
     * and drain them, builing a linear list of callbacks to run if everything
     * is successful (the transaction itself).
     */
    tran = tran_new();
    visited = g_hash_table_new(NULL, NULL);
    if (ignore_child) {
        g_hash_table_add(visited, ignore_child);
    }
    ret = bdrv_change_aio_context(bs, ctx, visited, tran, errp);
    g_hash_table_destroy(visited);

    /*
     * Linear phase: go through all callbacks collected in the transaction.
     * Run all callbacks collected in the recursion to switch all nodes
     * AioContext lock (transaction commit), or undo all changes done in the
     * recursion (transaction abort).
     */

    if (!ret) {
        /* Just run clean() callbacks. No AioContext changed. */
        tran_abort(tran);
        return -EPERM;
    }

    /*
     * Release old AioContext, it won't be needed anymore, as all
     * bdrv_drained_begin() have been called already.
     */
    if (qemu_get_aio_context() != old_context) {
        aio_context_release(old_context);
    }

    /*
     * Acquire new AioContext since bdrv_drained_end() is going to be called
     * after we switched all nodes in the new AioContext, and the function
     * assumes that the lock of the bs is always taken.
     */
    if (qemu_get_aio_context() != ctx) {
        aio_context_acquire(ctx);
    }

    tran_commit(tran);

    if (qemu_get_aio_context() != ctx) {
        aio_context_release(ctx);
    }

    /* Re-acquire the old AioContext, since the caller takes and releases it. */
    if (qemu_get_aio_context() != old_context) {
        aio_context_acquire(old_context);
    }

    return 0;
}

void bdrv_add_aio_context_notifier(BlockDriverState *bs,
        void (*attached_aio_context)(AioContext *new_context, void *opaque),
        void (*detach_aio_context)(void *opaque), void *opaque)
{
    BdrvAioNotifier *ban = g_new(BdrvAioNotifier, 1);
    *ban = (BdrvAioNotifier){
        .attached_aio_context = attached_aio_context,
        .detach_aio_context   = detach_aio_context,
        .opaque               = opaque
    };
    GLOBAL_STATE_CODE();

    QLIST_INSERT_HEAD(&bs->aio_notifiers, ban, list);
}

void bdrv_remove_aio_context_notifier(BlockDriverState *bs,
                                      void (*attached_aio_context)(AioContext *,
                                                                   void *),
                                      void (*detach_aio_context)(void *),
                                      void *opaque)
{
    BdrvAioNotifier *ban, *ban_next;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH_SAFE(ban, &bs->aio_notifiers, list, ban_next) {
        if (ban->attached_aio_context == attached_aio_context &&
            ban->detach_aio_context   == detach_aio_context   &&
            ban->opaque               == opaque               &&
            ban->deleted              == false)
        {
            if (bs->walking_aio_notifiers) {
                ban->deleted = true;
            } else {
                bdrv_do_remove_aio_context_notifier(ban);
            }
            return;
        }
    }

    abort();
}

int bdrv_amend_options(BlockDriverState *bs, QemuOpts *opts,
                       BlockDriverAmendStatusCB *status_cb, void *cb_opaque,
                       bool force,
                       Error **errp)
{
    GLOBAL_STATE_CODE();
    if (!bs->drv) {
        error_setg(errp, "Node is ejected");
        return -ENOMEDIUM;
    }
    if (!bs->drv->bdrv_amend_options) {
        error_setg(errp, "Block driver '%s' does not support option amendment",
                   bs->drv->format_name);
        return -ENOTSUP;
    }
    return bs->drv->bdrv_amend_options(bs, opts, status_cb,
                                       cb_opaque, force, errp);
}

/*
 * This function checks whether the given @to_replace is allowed to be
 * replaced by a node that always shows the same data as @bs.  This is
 * used for example to verify whether the mirror job can replace
 * @to_replace by the target mirrored from @bs.
 * To be replaceable, @bs and @to_replace may either be guaranteed to
 * always show the same data (because they are only connected through
 * filters), or some driver may allow replacing one of its children
 * because it can guarantee that this child's data is not visible at
 * all (for example, for dissenting quorum children that have no other
 * parents).
 */
bool bdrv_recurse_can_replace(BlockDriverState *bs,
                              BlockDriverState *to_replace)
{
    BlockDriverState *filtered;

    GLOBAL_STATE_CODE();

    if (!bs || !bs->drv) {
        return false;
    }

    if (bs == to_replace) {
        return true;
    }

    /* See what the driver can do */
    if (bs->drv->bdrv_recurse_can_replace) {
        return bs->drv->bdrv_recurse_can_replace(bs, to_replace);
    }

    /* For filters without an own implementation, we can recurse on our own */
    filtered = bdrv_filter_bs(bs);
    if (filtered) {
        return bdrv_recurse_can_replace(filtered, to_replace);
    }

    /* Safe default */
    return false;
}

/*
 * Check whether the given @node_name can be replaced by a node that
 * has the same data as @parent_bs.  If so, return @node_name's BDS;
 * NULL otherwise.
 *
 * @node_name must be a (recursive) *child of @parent_bs (or this
 * function will return NULL).
 *
 * The result (whether the node can be replaced or not) is only valid
 * for as long as no graph or permission changes occur.
 */
BlockDriverState *check_to_replace_node(BlockDriverState *parent_bs,
                                        const char *node_name, Error **errp)
{
    BlockDriverState *to_replace_bs = bdrv_find_node(node_name);
    AioContext *aio_context;

    GLOBAL_STATE_CODE();

    if (!to_replace_bs) {
        error_setg(errp, "Failed to find node with node-name='%s'", node_name);
        return NULL;
    }

    aio_context = bdrv_get_aio_context(to_replace_bs);
    aio_context_acquire(aio_context);

    if (bdrv_op_is_blocked(to_replace_bs, BLOCK_OP_TYPE_REPLACE, errp)) {
        to_replace_bs = NULL;
        goto out;
    }

    /* We don't want arbitrary node of the BDS chain to be replaced only the top
     * most non filter in order to prevent data corruption.
     * Another benefit is that this tests exclude backing files which are
     * blocked by the backing blockers.
     */
    if (!bdrv_recurse_can_replace(parent_bs, to_replace_bs)) {
        error_setg(errp, "Cannot replace '%s' by a node mirrored from '%s', "
                   "because it cannot be guaranteed that doing so would not "
                   "lead to an abrupt change of visible data",
                   node_name, parent_bs->node_name);
        to_replace_bs = NULL;
        goto out;
    }

out:
    aio_context_release(aio_context);
    return to_replace_bs;
}

/**
 * Iterates through the list of runtime option keys that are said to
 * be "strong" for a BDS.  An option is called "strong" if it changes
 * a BDS's data.  For example, the null block driver's "size" and
 * "read-zeroes" options are strong, but its "latency-ns" option is
 * not.
 *
 * If a key returned by this function ends with a dot, all options
 * starting with that prefix are strong.
 */
static const char *const *strong_options(BlockDriverState *bs,
                                         const char *const *curopt)
{
    static const char *const global_options[] = {
        "driver", "filename", NULL
    };

    if (!curopt) {
        return &global_options[0];
    }

    curopt++;
    if (curopt == &global_options[ARRAY_SIZE(global_options) - 1] && bs->drv) {
        curopt = bs->drv->strong_runtime_opts;
    }

    return (curopt && *curopt) ? curopt : NULL;
}

/**
 * Copies all strong runtime options from bs->options to the given
 * QDict.  The set of strong option keys is determined by invoking
 * strong_options().
 *
 * Returns true iff any strong option was present in bs->options (and
 * thus copied to the target QDict) with the exception of "filename"
 * and "driver".  The caller is expected to use this value to decide
 * whether the existence of strong options prevents the generation of
 * a plain filename.
 */
static bool append_strong_runtime_options(QDict *d, BlockDriverState *bs)
{
    bool found_any = false;
    const char *const *option_name = NULL;

    if (!bs->drv) {
        return false;
    }

    while ((option_name = strong_options(bs, option_name))) {
        bool option_given = false;

        assert(strlen(*option_name) > 0);
        if ((*option_name)[strlen(*option_name) - 1] != '.') {
            QObject *entry = qdict_get(bs->options, *option_name);
            if (!entry) {
                continue;
            }

            qdict_put_obj(d, *option_name, qobject_ref(entry));
            option_given = true;
        } else {
            const QDictEntry *entry;
            for (entry = qdict_first(bs->options); entry;
                 entry = qdict_next(bs->options, entry))
            {
                if (strstart(qdict_entry_key(entry), *option_name, NULL)) {
                    qdict_put_obj(d, qdict_entry_key(entry),
                                  qobject_ref(qdict_entry_value(entry)));
                    option_given = true;
                }
            }
        }

        /* While "driver" and "filename" need to be included in a JSON filename,
         * their existence does not prohibit generation of a plain filename. */
        if (!found_any && option_given &&
            strcmp(*option_name, "driver") && strcmp(*option_name, "filename"))
        {
            found_any = true;
        }
    }

    if (!qdict_haskey(d, "driver")) {
        /* Drivers created with bdrv_new_open_driver() may not have a
         * @driver option.  Add it here. */
        qdict_put_str(d, "driver", bs->drv->format_name);
    }

    return found_any;
}

/* Note: This function may return false positives; it may return true
 * even if opening the backing file specified by bs's image header
 * would result in exactly bs->backing. */
static bool bdrv_backing_overridden(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    if (bs->backing) {
        return strcmp(bs->auto_backing_file,
                      bs->backing->bs->filename);
    } else {
        /* No backing BDS, so if the image header reports any backing
         * file, it must have been suppressed */
        return bs->auto_backing_file[0] != '\0';
    }
}

/* Updates the following BDS fields:
 *  - exact_filename: A filename which may be used for opening a block device
 *                    which (mostly) equals the given BDS (even without any
 *                    other options; so reading and writing must return the same
 *                    results, but caching etc. may be different)
 *  - full_open_options: Options which, when given when opening a block device
 *                       (without a filename), result in a BDS (mostly)
 *                       equalling the given one
 *  - filename: If exact_filename is set, it is copied here. Otherwise,
 *              full_open_options is converted to a JSON object, prefixed with
 *              "json:" (for use through the JSON pseudo protocol) and put here.
 */
void bdrv_refresh_filename(BlockDriverState *bs)
{
    BlockDriver *drv = bs->drv;
    BdrvChild *child;
    BlockDriverState *primary_child_bs;
    QDict *opts;
    bool backing_overridden;
    bool generate_json_filename; /* Whether our default implementation should
                                    fill exact_filename (false) or not (true) */

    GLOBAL_STATE_CODE();

    if (!drv) {
        return;
    }

    /* This BDS's file name may depend on any of its children's file names, so
     * refresh those first */
    QLIST_FOREACH(child, &bs->children, next) {
        bdrv_refresh_filename(child->bs);
    }

    if (bs->implicit) {
        /* For implicit nodes, just copy everything from the single child */
        child = QLIST_FIRST(&bs->children);
        assert(QLIST_NEXT(child, next) == NULL);

        pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
                child->bs->exact_filename);
        pstrcpy(bs->filename, sizeof(bs->filename), child->bs->filename);

        qobject_unref(bs->full_open_options);
        bs->full_open_options = qobject_ref(child->bs->full_open_options);

        return;
    }

    backing_overridden = bdrv_backing_overridden(bs);

    if (bs->open_flags & BDRV_O_NO_IO) {
        /* Without I/O, the backing file does not change anything.
         * Therefore, in such a case (primarily qemu-img), we can
         * pretend the backing file has not been overridden even if
         * it technically has been. */
        backing_overridden = false;
    }

    /* Gather the options QDict */
    opts = qdict_new();
    generate_json_filename = append_strong_runtime_options(opts, bs);
    generate_json_filename |= backing_overridden;

    if (drv->bdrv_gather_child_options) {
        /* Some block drivers may not want to present all of their children's
         * options, or name them differently from BdrvChild.name */
        drv->bdrv_gather_child_options(bs, opts, backing_overridden);
    } else {
        QLIST_FOREACH(child, &bs->children, next) {
            if (child == bs->backing && !backing_overridden) {
                /* We can skip the backing BDS if it has not been overridden */
                continue;
            }

            qdict_put(opts, child->name,
                      qobject_ref(child->bs->full_open_options));
        }

        if (backing_overridden && !bs->backing) {
            /* Force no backing file */
            qdict_put_null(opts, "backing");
        }
    }

    qobject_unref(bs->full_open_options);
    bs->full_open_options = opts;

    primary_child_bs = bdrv_primary_bs(bs);

    if (drv->bdrv_refresh_filename) {
        /* Obsolete information is of no use here, so drop the old file name
         * information before refreshing it */
        bs->exact_filename[0] = '\0';

        drv->bdrv_refresh_filename(bs);
    } else if (primary_child_bs) {
        /*
         * Try to reconstruct valid information from the underlying
         * file -- this only works for format nodes (filter nodes
         * cannot be probed and as such must be selected by the user
         * either through an options dict, or through a special
         * filename which the filter driver must construct in its
         * .bdrv_refresh_filename() implementation).
         */

        bs->exact_filename[0] = '\0';

        /*
         * We can use the underlying file's filename if:
         * - it has a filename,
         * - the current BDS is not a filter,
         * - the file is a protocol BDS, and
         * - opening that file (as this BDS's format) will automatically create
         *   the BDS tree we have right now, that is:
         *   - the user did not significantly change this BDS's behavior with
         *     some explicit (strong) options
         *   - no non-file child of this BDS has been overridden by the user
         *   Both of these conditions are represented by generate_json_filename.
         */
        if (primary_child_bs->exact_filename[0] &&
            primary_child_bs->drv->bdrv_file_open &&
            !drv->is_filter && !generate_json_filename)
        {
            strcpy(bs->exact_filename, primary_child_bs->exact_filename);
        }
    }

    if (bs->exact_filename[0]) {
        pstrcpy(bs->filename, sizeof(bs->filename), bs->exact_filename);
    } else {
        GString *json = qobject_to_json(QOBJECT(bs->full_open_options));
        if (snprintf(bs->filename, sizeof(bs->filename), "json:%s",
                     json->str) >= sizeof(bs->filename)) {
            /* Give user a hint if we truncated things. */
            strcpy(bs->filename + sizeof(bs->filename) - 4, "...");
        }
        g_string_free(json, true);
    }
}

char *bdrv_dirname(BlockDriverState *bs, Error **errp)
{
    BlockDriver *drv = bs->drv;
    BlockDriverState *child_bs;

    GLOBAL_STATE_CODE();

    if (!drv) {
        error_setg(errp, "Node '%s' is ejected", bs->node_name);
        return NULL;
    }

    if (drv->bdrv_dirname) {
        return drv->bdrv_dirname(bs, errp);
    }

    child_bs = bdrv_primary_bs(bs);
    if (child_bs) {
        return bdrv_dirname(child_bs, errp);
    }

    bdrv_refresh_filename(bs);
    if (bs->exact_filename[0] != '\0') {
        return path_combine(bs->exact_filename, "");
    }

    error_setg(errp, "Cannot generate a base directory for %s nodes",
               drv->format_name);
    return NULL;
}

/*
 * Hot add/remove a BDS's child. So the user can take a child offline when
 * it is broken and take a new child online
 */
void bdrv_add_child(BlockDriverState *parent_bs, BlockDriverState *child_bs,
                    Error **errp)
{
    GLOBAL_STATE_CODE();
    if (!parent_bs->drv || !parent_bs->drv->bdrv_add_child) {
        error_setg(errp, "The node %s does not support adding a child",
                   bdrv_get_device_or_node_name(parent_bs));
        return;
    }

    if (!QLIST_EMPTY(&child_bs->parents)) {
        error_setg(errp, "The node %s already has a parent",
                   child_bs->node_name);
        return;
    }

    parent_bs->drv->bdrv_add_child(parent_bs, child_bs, errp);
}

void bdrv_del_child(BlockDriverState *parent_bs, BdrvChild *child, Error **errp)
{
    BdrvChild *tmp;

    GLOBAL_STATE_CODE();
    if (!parent_bs->drv || !parent_bs->drv->bdrv_del_child) {
        error_setg(errp, "The node %s does not support removing a child",
                   bdrv_get_device_or_node_name(parent_bs));
        return;
    }

    QLIST_FOREACH(tmp, &parent_bs->children, next) {
        if (tmp == child) {
            break;
        }
    }

    if (!tmp) {
        error_setg(errp, "The node %s does not have a child named %s",
                   bdrv_get_device_or_node_name(parent_bs),
                   bdrv_get_device_or_node_name(child->bs));
        return;
    }

    parent_bs->drv->bdrv_del_child(parent_bs, child, errp);
}

int bdrv_make_empty(BdrvChild *c, Error **errp)
{
    BlockDriver *drv = c->bs->drv;
    int ret;

    GLOBAL_STATE_CODE();
    assert(c->perm & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED));

    if (!drv->bdrv_make_empty) {
        error_setg(errp, "%s does not support emptying nodes",
                   drv->format_name);
        return -ENOTSUP;
    }

    ret = drv->bdrv_make_empty(c->bs);
    if (ret < 0) {
        error_setg_errno(errp, -ret, "Failed to empty %s",
                         c->bs->filename);
        return ret;
    }

    return 0;
}

/*
 * Return the child that @bs acts as an overlay for, and from which data may be
 * copied in COW or COR operations.  Usually this is the backing file.
 */
BdrvChild *bdrv_cow_child(BlockDriverState *bs)
{
    IO_CODE();

    if (!bs || !bs->drv) {
        return NULL;
    }

    if (bs->drv->is_filter) {
        return NULL;
    }

    if (!bs->backing) {
        return NULL;
    }

    assert(bs->backing->role & BDRV_CHILD_COW);
    return bs->backing;
}

/*
 * If @bs acts as a filter for exactly one of its children, return
 * that child.
 */
BdrvChild *bdrv_filter_child(BlockDriverState *bs)
{
    BdrvChild *c;
    IO_CODE();

    if (!bs || !bs->drv) {
        return NULL;
    }

    if (!bs->drv->is_filter) {
        return NULL;
    }

    /* Only one of @backing or @file may be used */
    assert(!(bs->backing && bs->file));

    c = bs->backing ?: bs->file;
    if (!c) {
        return NULL;
    }

    assert(c->role & BDRV_CHILD_FILTERED);
    return c;
}

/*
 * Return either the result of bdrv_cow_child() or bdrv_filter_child(),
 * whichever is non-NULL.
 *
 * Return NULL if both are NULL.
 */
BdrvChild *bdrv_filter_or_cow_child(BlockDriverState *bs)
{
    BdrvChild *cow_child = bdrv_cow_child(bs);
    BdrvChild *filter_child = bdrv_filter_child(bs);
    IO_CODE();

    /* Filter nodes cannot have COW backing files */
    assert(!(cow_child && filter_child));

    return cow_child ?: filter_child;
}

/*
 * Return the primary child of this node: For filters, that is the
 * filtered child.  For other nodes, that is usually the child storing
 * metadata.
 * (A generally more helpful description is that this is (usually) the
 * child that has the same filename as @bs.)
 *
 * Drivers do not necessarily have a primary child; for example quorum
 * does not.
 */
BdrvChild *bdrv_primary_child(BlockDriverState *bs)
{
    BdrvChild *c, *found = NULL;
    IO_CODE();

    QLIST_FOREACH(c, &bs->children, next) {
        if (c->role & BDRV_CHILD_PRIMARY) {
            assert(!found);
            found = c;
        }
    }

    return found;
}

static BlockDriverState *bdrv_do_skip_filters(BlockDriverState *bs,
                                              bool stop_on_explicit_filter)
{
    BdrvChild *c;

    if (!bs) {
        return NULL;
    }

    while (!(stop_on_explicit_filter && !bs->implicit)) {
        c = bdrv_filter_child(bs);
        if (!c) {
            /*
             * A filter that is embedded in a working block graph must
             * have a child.  Assert this here so this function does
             * not return a filter node that is not expected by the
             * caller.
             */
            assert(!bs->drv || !bs->drv->is_filter);
            break;
        }
        bs = c->bs;
    }
    /*
     * Note that this treats nodes with bs->drv == NULL as not being
     * filters (bs->drv == NULL should be replaced by something else
     * anyway).
     * The advantage of this behavior is that this function will thus
     * always return a non-NULL value (given a non-NULL @bs).
     */

    return bs;
}

/*
 * Return the first BDS that has not been added implicitly or that
 * does not have a filtered child down the chain starting from @bs
 * (including @bs itself).
 */
BlockDriverState *bdrv_skip_implicit_filters(BlockDriverState *bs)
{
    GLOBAL_STATE_CODE();
    return bdrv_do_skip_filters(bs, true);
}

/*
 * Return the first BDS that does not have a filtered child down the
 * chain starting from @bs (including @bs itself).
 */
BlockDriverState *bdrv_skip_filters(BlockDriverState *bs)
{
    IO_CODE();
    return bdrv_do_skip_filters(bs, false);
}

/*
 * For a backing chain, return the first non-filter backing image of
 * the first non-filter image.
 */
BlockDriverState *bdrv_backing_chain_next(BlockDriverState *bs)
{
    IO_CODE();
    return bdrv_skip_filters(bdrv_cow_bs(bdrv_skip_filters(bs)));
}

/**
 * Check whether [offset, offset + bytes) overlaps with the cached
 * block-status data region.
 *
 * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`,
 * which is what bdrv_bsc_is_data()'s interface needs.
 * Otherwise, *pnum is not touched.
 */
static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs,
                                           int64_t offset, int64_t bytes,
                                           int64_t *pnum)
{
    BdrvBlockStatusCache *bsc = qatomic_rcu_read(&bs->block_status_cache);
    bool overlaps;

    overlaps =
        qatomic_read(&bsc->valid) &&
        ranges_overlap(offset, bytes, bsc->data_start,
                       bsc->data_end - bsc->data_start);

    if (overlaps && pnum) {
        *pnum = bsc->data_end - offset;
    }

    return overlaps;
}

/**
 * See block_int.h for this function's documentation.
 */
bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum)
{
    IO_CODE();
    RCU_READ_LOCK_GUARD();
    return bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum);
}

/**
 * See block_int.h for this function's documentation.
 */
void bdrv_bsc_invalidate_range(BlockDriverState *bs,
                               int64_t offset, int64_t bytes)
{
    IO_CODE();
    RCU_READ_LOCK_GUARD();

    if (bdrv_bsc_range_overlaps_locked(bs, offset, bytes, NULL)) {
        qatomic_set(&bs->block_status_cache->valid, false);
    }
}

/**
 * See block_int.h for this function's documentation.
 */
void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes)
{
    BdrvBlockStatusCache *new_bsc = g_new(BdrvBlockStatusCache, 1);
    BdrvBlockStatusCache *old_bsc;
    IO_CODE();

    *new_bsc = (BdrvBlockStatusCache) {
        .valid = true,
        .data_start = offset,
        .data_end = offset + bytes,
    };

    QEMU_LOCK_GUARD(&bs->bsc_modify_lock);

    old_bsc = qatomic_rcu_read(&bs->block_status_cache);
    qatomic_rcu_set(&bs->block_status_cache, new_bsc);
    if (old_bsc) {
        g_free_rcu(old_bsc, rcu);
    }
}