qemu/block/ssh.c

/*
 * Secure Shell (ssh) backend for QEMU.
 *
 * Copyright (C) 2013 Red Hat Inc., Richard W.M. Jones <rjones@redhat.com>
 *
 * 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 <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

#include <libssh2.h>
#include <libssh2_sftp.h>

#include "block/block_int.h"
#include "qemu/sockets.h"
#include "qemu/uri.h"
#include "qapi/qmp/qint.h"

/* DEBUG_SSH=1 enables the DPRINTF (debugging printf) statements in
 * this block driver code.
 *
 * TRACE_LIBSSH2=<bitmask> enables tracing in libssh2 itself.  Note
 * that this requires that libssh2 was specially compiled with the
 * `./configure --enable-debug' option, so most likely you will have
 * to compile it yourself.  The meaning of <bitmask> is described
 * here: http://www.libssh2.org/libssh2_trace.html
 */
#define DEBUG_SSH     0
#define TRACE_LIBSSH2 0 /* or try: LIBSSH2_TRACE_SFTP */

#define DPRINTF(fmt, ...)                           \
    do {                                            \
        if (DEBUG_SSH) {                            \
            fprintf(stderr, "ssh: %-15s " fmt "\n", \
                    __func__, ##__VA_ARGS__);       \
        }                                           \
    } while (0)

typedef struct BDRVSSHState {
    /* Coroutine. */
    CoMutex lock;

    /* SSH connection. */
    int sock;                         /* socket */
    LIBSSH2_SESSION *session;         /* ssh session */
    LIBSSH2_SFTP *sftp;               /* sftp session */
    LIBSSH2_SFTP_HANDLE *sftp_handle; /* sftp remote file handle */

    /* See ssh_seek() function below. */
    int64_t offset;
    bool offset_op_read;

    /* File attributes at open.  We try to keep the .filesize field
     * updated if it changes (eg by writing at the end of the file).
     */
    LIBSSH2_SFTP_ATTRIBUTES attrs;

    /* Used to warn if 'flush' is not supported. */
    char *hostport;
    bool unsafe_flush_warning;
} BDRVSSHState;

static void ssh_state_init(BDRVSSHState *s)
{
    memset(s, 0, sizeof *s);
    s->sock = -1;
    s->offset = -1;
    qemu_co_mutex_init(&s->lock);
}

static void ssh_state_free(BDRVSSHState *s)
{
    g_free(s->hostport);
    if (s->sftp_handle) {
        libssh2_sftp_close(s->sftp_handle);
    }
    if (s->sftp) {
        libssh2_sftp_shutdown(s->sftp);
    }
    if (s->session) {
        libssh2_session_disconnect(s->session,
                                   "from qemu ssh client: "
                                   "user closed the connection");
        libssh2_session_free(s->session);
    }
    if (s->sock >= 0) {
        close(s->sock);
    }
}

/* Wrappers around error_report which make sure to dump as much
 * information from libssh2 as possible.
 */
static void GCC_FMT_ATTR(2, 3)
session_error_report(BDRVSSHState *s, const char *fs, ...)
{
    va_list args;

    va_start(args, fs);
    error_vprintf(fs, args);

    if ((s)->session) {
        char *ssh_err;
        int ssh_err_code;

        libssh2_session_last_error((s)->session, &ssh_err, NULL, 0);
        /* This is not an errno.  See <libssh2.h>. */
        ssh_err_code = libssh2_session_last_errno((s)->session);

        error_printf(": %s (libssh2 error code: %d)", ssh_err, ssh_err_code);
    }

    va_end(args);
    error_printf("\n");
}

static void GCC_FMT_ATTR(2, 3)
sftp_error_report(BDRVSSHState *s, const char *fs, ...)
{
    va_list args;

    va_start(args, fs);
    error_vprintf(fs, args);

    if ((s)->sftp) {
        char *ssh_err;
        int ssh_err_code;
        unsigned long sftp_err_code;

        libssh2_session_last_error((s)->session, &ssh_err, NULL, 0);
        /* This is not an errno.  See <libssh2.h>. */
        ssh_err_code = libssh2_session_last_errno((s)->session);
        /* See <libssh2_sftp.h>. */
        sftp_err_code = libssh2_sftp_last_error((s)->sftp);

        error_printf(": %s (libssh2 error code: %d, sftp error code: %lu)",
                     ssh_err, ssh_err_code, sftp_err_code);
    }

    va_end(args);
    error_printf("\n");
}

static int parse_uri(const char *filename, QDict *options, Error **errp)
{
    URI *uri = NULL;
    QueryParams *qp = NULL;
    int i;

    uri = uri_parse(filename);
    if (!uri) {
        return -EINVAL;
    }

    if (strcmp(uri->scheme, "ssh") != 0) {
        error_setg(errp, "URI scheme must be 'ssh'");
        goto err;
    }

    if (!uri->server || strcmp(uri->server, "") == 0) {
        error_setg(errp, "missing hostname in URI");
        goto err;
    }

    if (!uri->path || strcmp(uri->path, "") == 0) {
        error_setg(errp, "missing remote path in URI");
        goto err;
    }

    qp = query_params_parse(uri->query);
    if (!qp) {
        error_setg(errp, "could not parse query parameters");
        goto err;
    }

    if(uri->user && strcmp(uri->user, "") != 0) {
        qdict_put(options, "user", qstring_from_str(uri->user));
    }

    qdict_put(options, "host", qstring_from_str(uri->server));

    if (uri->port) {
        qdict_put(options, "port", qint_from_int(uri->port));
    }

    qdict_put(options, "path", qstring_from_str(uri->path));

    /* Pick out any query parameters that we understand, and ignore
     * the rest.
     */
    for (i = 0; i < qp->n; ++i) {
        if (strcmp(qp->p[i].name, "host_key_check") == 0) {
            qdict_put(options, "host_key_check",
                      qstring_from_str(qp->p[i].value));
        }
    }

    query_params_free(qp);
    uri_free(uri);
    return 0;

 err:
    if (qp) {
      query_params_free(qp);
    }
    if (uri) {
      uri_free(uri);
    }
    return -EINVAL;
}

static void ssh_parse_filename(const char *filename, QDict *options,
                               Error **errp)
{
    if (qdict_haskey(options, "user") ||
        qdict_haskey(options, "host") ||
        qdict_haskey(options, "port") ||
        qdict_haskey(options, "path") ||
        qdict_haskey(options, "host_key_check")) {
        error_setg(errp, "user, host, port, path, host_key_check cannot be used at the same time as a file option");
        return;
    }

    parse_uri(filename, options, errp);
}

static int check_host_key_knownhosts(BDRVSSHState *s,
                                     const char *host, int port)
{
    const char *home;
    char *knh_file = NULL;
    LIBSSH2_KNOWNHOSTS *knh = NULL;
    struct libssh2_knownhost *found;
    int ret, r;
    const char *hostkey;
    size_t len;
    int type;

    hostkey = libssh2_session_hostkey(s->session, &len, &type);
    if (!hostkey) {
        ret = -EINVAL;
        session_error_report(s, "failed to read remote host key");
        goto out;
    }

    knh = libssh2_knownhost_init(s->session);
    if (!knh) {
        ret = -EINVAL;
        session_error_report(s, "failed to initialize known hosts support");
        goto out;
    }

    home = getenv("HOME");
    if (home) {
        knh_file = g_strdup_printf("%s/.ssh/known_hosts", home);
    } else {
        knh_file = g_strdup_printf("/root/.ssh/known_hosts");
    }

    /* Read all known hosts from OpenSSH-style known_hosts file. */
    libssh2_knownhost_readfile(knh, knh_file, LIBSSH2_KNOWNHOST_FILE_OPENSSH);

    r = libssh2_knownhost_checkp(knh, host, port, hostkey, len,
                                 LIBSSH2_KNOWNHOST_TYPE_PLAIN|
                                 LIBSSH2_KNOWNHOST_KEYENC_RAW,
                                 &found);
    switch (r) {
    case LIBSSH2_KNOWNHOST_CHECK_MATCH:
        /* OK */
        DPRINTF("host key OK: %s", found->key);
        break;
    case LIBSSH2_KNOWNHOST_CHECK_MISMATCH:
        ret = -EINVAL;
        session_error_report(s, "host key does not match the one in known_hosts (found key %s)",
                             found->key);
        goto out;
    case LIBSSH2_KNOWNHOST_CHECK_NOTFOUND:
        ret = -EINVAL;
        session_error_report(s, "no host key was found in known_hosts");
        goto out;
    case LIBSSH2_KNOWNHOST_CHECK_FAILURE:
        ret = -EINVAL;
        session_error_report(s, "failure matching the host key with known_hosts");
        goto out;
    default:
        ret = -EINVAL;
        session_error_report(s, "unknown error matching the host key with known_hosts (%d)",
                             r);
        goto out;
    }

    /* known_hosts checking successful. */
    ret = 0;

 out:
    if (knh != NULL) {
        libssh2_knownhost_free(knh);
    }
    g_free(knh_file);
    return ret;
}

static unsigned hex2decimal(char ch)
{
    if (ch >= '0' && ch <= '9') {
        return (ch - '0');
    } else if (ch >= 'a' && ch <= 'f') {
        return 10 + (ch - 'a');
    } else if (ch >= 'A' && ch <= 'F') {
        return 10 + (ch - 'A');
    }

    return -1;
}

/* Compare the binary fingerprint (hash of host key) with the
 * host_key_check parameter.
 */
static int compare_fingerprint(const unsigned char *fingerprint, size_t len,
                               const char *host_key_check)
{
    unsigned c;

    while (len > 0) {
        while (*host_key_check == ':')
            host_key_check++;
        if (!qemu_isxdigit(host_key_check[0]) ||
            !qemu_isxdigit(host_key_check[1]))
            return 1;
        c = hex2decimal(host_key_check[0]) * 16 +
            hex2decimal(host_key_check[1]);
        if (c - *fingerprint != 0)
            return c - *fingerprint;
        fingerprint++;
        len--;
        host_key_check += 2;
    }
    return *host_key_check - '\0';
}

static int
check_host_key_hash(BDRVSSHState *s, const char *hash,
                    int hash_type, size_t fingerprint_len)
{
    const char *fingerprint;

    fingerprint = libssh2_hostkey_hash(s->session, hash_type);
    if (!fingerprint) {
        session_error_report(s, "failed to read remote host key");
        return -EINVAL;
    }

    if(compare_fingerprint((unsigned char *) fingerprint, fingerprint_len,
                           hash) != 0) {
        error_report("remote host key does not match host_key_check '%s'",
                     hash);
        return -EPERM;
    }

    return 0;
}

static int check_host_key(BDRVSSHState *s, const char *host, int port,
                          const char *host_key_check)
{
    /* host_key_check=no */
    if (strcmp(host_key_check, "no") == 0) {
        return 0;
    }

    /* host_key_check=md5:xx:yy:zz:... */
    if (strncmp(host_key_check, "md5:", 4) == 0) {
        return check_host_key_hash(s, &host_key_check[4],
                                   LIBSSH2_HOSTKEY_HASH_MD5, 16);
    }

    /* host_key_check=sha1:xx:yy:zz:... */
    if (strncmp(host_key_check, "sha1:", 5) == 0) {
        return check_host_key_hash(s, &host_key_check[5],
                                   LIBSSH2_HOSTKEY_HASH_SHA1, 20);
    }

    /* host_key_check=yes */
    if (strcmp(host_key_check, "yes") == 0) {
        return check_host_key_knownhosts(s, host, port);
    }

    error_report("unknown host_key_check setting (%s)", host_key_check);
    return -EINVAL;
}

static int authenticate(BDRVSSHState *s, const char *user)
{
    int r, ret;
    const char *userauthlist;
    LIBSSH2_AGENT *agent = NULL;
    struct libssh2_agent_publickey *identity;
    struct libssh2_agent_publickey *prev_identity = NULL;

    userauthlist = libssh2_userauth_list(s->session, user, strlen(user));
    if (strstr(userauthlist, "publickey") == NULL) {
        ret = -EPERM;
        error_report("remote server does not support \"publickey\" authentication");
        goto out;
    }

    /* Connect to ssh-agent and try each identity in turn. */
    agent = libssh2_agent_init(s->session);
    if (!agent) {
        ret = -EINVAL;
        session_error_report(s, "failed to initialize ssh-agent support");
        goto out;
    }
    if (libssh2_agent_connect(agent)) {
        ret = -ECONNREFUSED;
        session_error_report(s, "failed to connect to ssh-agent");
        goto out;
    }
    if (libssh2_agent_list_identities(agent)) {
        ret = -EINVAL;
        session_error_report(s, "failed requesting identities from ssh-agent");
        goto out;
    }

    for(;;) {
        r = libssh2_agent_get_identity(agent, &identity, prev_identity);
        if (r == 1) {           /* end of list */
            break;
        }
        if (r < 0) {
            ret = -EINVAL;
            session_error_report(s, "failed to obtain identity from ssh-agent");
            goto out;
        }
        r = libssh2_agent_userauth(agent, user, identity);
        if (r == 0) {
            /* Authenticated! */
            ret = 0;
            goto out;
        }
        /* Failed to authenticate with this identity, try the next one. */
        prev_identity = identity;
    }

    ret = -EPERM;
    error_report("failed to authenticate using publickey authentication "
                 "and the identities held by your ssh-agent");

 out:
    if (agent != NULL) {
        /* Note: libssh2 implementation implicitly calls
         * libssh2_agent_disconnect if necessary.
         */
        libssh2_agent_free(agent);
    }

    return ret;
}

static int connect_to_ssh(BDRVSSHState *s, QDict *options,
                          int ssh_flags, int creat_mode)
{
    int r, ret;
    Error *err = NULL;
    const char *host, *user, *path, *host_key_check;
    int port;

    host = qdict_get_str(options, "host");

    if (qdict_haskey(options, "port")) {
        port = qdict_get_int(options, "port");
    } else {
        port = 22;
    }

    path = qdict_get_str(options, "path");

    if (qdict_haskey(options, "user")) {
        user = qdict_get_str(options, "user");
    } else {
        user = g_get_user_name();
        if (!user) {
            ret = -errno;
            goto err;
        }
    }

    if (qdict_haskey(options, "host_key_check")) {
        host_key_check = qdict_get_str(options, "host_key_check");
    } else {
        host_key_check = "yes";
    }

    /* Construct the host:port name for inet_connect. */
    g_free(s->hostport);
    s->hostport = g_strdup_printf("%s:%d", host, port);

    /* Open the socket and connect. */
    s->sock = inet_connect(s->hostport, &err);
    if (err != NULL) {
        ret = -errno;
        qerror_report_err(err);
        error_free(err);
        goto err;
    }

    /* Create SSH session. */
    s->session = libssh2_session_init();
    if (!s->session) {
        ret = -EINVAL;
        session_error_report(s, "failed to initialize libssh2 session");
        goto err;
    }

#if TRACE_LIBSSH2 != 0
    libssh2_trace(s->session, TRACE_LIBSSH2);
#endif

    r = libssh2_session_handshake(s->session, s->sock);
    if (r != 0) {
        ret = -EINVAL;
        session_error_report(s, "failed to establish SSH session");
        goto err;
    }

    /* Check the remote host's key against known_hosts. */
    ret = check_host_key(s, host, port, host_key_check);
    if (ret < 0) {
        goto err;
    }

    /* Authenticate. */
    ret = authenticate(s, user);
    if (ret < 0) {
        goto err;
    }

    /* Start SFTP. */
    s->sftp = libssh2_sftp_init(s->session);
    if (!s->sftp) {
        session_error_report(s, "failed to initialize sftp handle");
        ret = -EINVAL;
        goto err;
    }

    /* Open the remote file. */
    DPRINTF("opening file %s flags=0x%x creat_mode=0%o",
            path, ssh_flags, creat_mode);
    s->sftp_handle = libssh2_sftp_open(s->sftp, path, ssh_flags, creat_mode);
    if (!s->sftp_handle) {
        session_error_report(s, "failed to open remote file '%s'", path);
        ret = -EINVAL;
        goto err;
    }

    r = libssh2_sftp_fstat(s->sftp_handle, &s->attrs);
    if (r < 0) {
        sftp_error_report(s, "failed to read file attributes");
        return -EINVAL;
    }

    /* Delete the options we've used; any not deleted will cause the
     * block layer to give an error about unused options.
     */
    qdict_del(options, "host");
    qdict_del(options, "port");
    qdict_del(options, "user");
    qdict_del(options, "path");
    qdict_del(options, "host_key_check");

    return 0;

 err:
    if (s->sftp_handle) {
        libssh2_sftp_close(s->sftp_handle);
    }
    s->sftp_handle = NULL;
    if (s->sftp) {
        libssh2_sftp_shutdown(s->sftp);
    }
    s->sftp = NULL;
    if (s->session) {
        libssh2_session_disconnect(s->session,
                                   "from qemu ssh client: "
                                   "error opening connection");
        libssh2_session_free(s->session);
    }
    s->session = NULL;

    return ret;
}

static int ssh_file_open(BlockDriverState *bs, QDict *options, int bdrv_flags)
{
    BDRVSSHState *s = bs->opaque;
    int ret;
    int ssh_flags;

    ssh_state_init(s);

    ssh_flags = LIBSSH2_FXF_READ;
    if (bdrv_flags & BDRV_O_RDWR) {
        ssh_flags |= LIBSSH2_FXF_WRITE;
    }

    /* Start up SSH. */
    ret = connect_to_ssh(s, options, ssh_flags, 0);
    if (ret < 0) {
        goto err;
    }

    /* Go non-blocking. */
    libssh2_session_set_blocking(s->session, 0);

    return 0;

 err:
    if (s->sock >= 0) {
        close(s->sock);
    }
    s->sock = -1;

    return ret;
}

static QEMUOptionParameter ssh_create_options[] = {
    {
        .name = BLOCK_OPT_SIZE,
        .type = OPT_SIZE,
        .help = "Virtual disk size"
    },
    { NULL }
};

static int ssh_create(const char *filename, QEMUOptionParameter *options)
{
    int r, ret;
    Error *local_err = NULL;
    int64_t total_size = 0;
    QDict *uri_options = NULL;
    BDRVSSHState s;
    ssize_t r2;
    char c[1] = { '\0' };

    ssh_state_init(&s);

    /* Get desired file size. */
    while (options && options->name) {
        if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
            total_size = options->value.n;
        }
        options++;
    }
    DPRINTF("total_size=%" PRIi64, total_size);

    uri_options = qdict_new();
    r = parse_uri(filename, uri_options, &local_err);
    if (r < 0) {
        qerror_report_err(local_err);
        error_free(local_err);
        ret = r;
        goto out;
    }

    r = connect_to_ssh(&s, uri_options,
                       LIBSSH2_FXF_READ|LIBSSH2_FXF_WRITE|
                       LIBSSH2_FXF_CREAT|LIBSSH2_FXF_TRUNC, 0644);
    if (r < 0) {
        ret = r;
        goto out;
    }

    if (total_size > 0) {
        libssh2_sftp_seek64(s.sftp_handle, total_size-1);
        r2 = libssh2_sftp_write(s.sftp_handle, c, 1);
        if (r2 < 0) {
            sftp_error_report(&s, "truncate failed");
            ret = -EINVAL;
            goto out;
        }
        s.attrs.filesize = total_size;
    }

    ret = 0;

 out:
    ssh_state_free(&s);
    if (uri_options != NULL) {
        QDECREF(uri_options);
    }
    return ret;
}

static void ssh_close(BlockDriverState *bs)
{
    BDRVSSHState *s = bs->opaque;

    ssh_state_free(s);
}

static int ssh_has_zero_init(BlockDriverState *bs)
{
    BDRVSSHState *s = bs->opaque;
    /* Assume false, unless we can positively prove it's true. */
    int has_zero_init = 0;

    if (s->attrs.flags & LIBSSH2_SFTP_ATTR_PERMISSIONS) {
        if (s->attrs.permissions & LIBSSH2_SFTP_S_IFREG) {
            has_zero_init = 1;
        }
    }

    return has_zero_init;
}

static void restart_coroutine(void *opaque)
{
    Coroutine *co = opaque;

    DPRINTF("co=%p", co);

    qemu_coroutine_enter(co, NULL);
}

/* Always true because when we have called set_fd_handler there is
 * always a request being processed.
 */
static int return_true(void *opaque)
{
    return 1;
}

static coroutine_fn void set_fd_handler(BDRVSSHState *s)
{
    int r;
    IOHandler *rd_handler = NULL, *wr_handler = NULL;
    Coroutine *co = qemu_coroutine_self();

    r = libssh2_session_block_directions(s->session);

    if (r & LIBSSH2_SESSION_BLOCK_INBOUND) {
        rd_handler = restart_coroutine;
    }
    if (r & LIBSSH2_SESSION_BLOCK_OUTBOUND) {
        wr_handler = restart_coroutine;
    }

    DPRINTF("s->sock=%d rd_handler=%p wr_handler=%p", s->sock,
            rd_handler, wr_handler);

    qemu_aio_set_fd_handler(s->sock, rd_handler, wr_handler, return_true, co);
}

static coroutine_fn void clear_fd_handler(BDRVSSHState *s)
{
    DPRINTF("s->sock=%d", s->sock);
    qemu_aio_set_fd_handler(s->sock, NULL, NULL, NULL, NULL);
}

/* A non-blocking call returned EAGAIN, so yield, ensuring the
 * handlers are set up so that we'll be rescheduled when there is an
 * interesting event on the socket.
 */
static coroutine_fn void co_yield(BDRVSSHState *s)
{
    set_fd_handler(s);
    qemu_coroutine_yield();
    clear_fd_handler(s);
}

/* SFTP has a function `libssh2_sftp_seek64' which seeks to a position
 * in the remote file.  Notice that it just updates a field in the
 * sftp_handle structure, so there is no network traffic and it cannot
 * fail.
 *
 * However, `libssh2_sftp_seek64' does have a catastrophic effect on
 * performance since it causes the handle to throw away all in-flight
 * reads and buffered readahead data.  Therefore this function tries
 * to be intelligent about when to call the underlying libssh2 function.
 */
#define SSH_SEEK_WRITE 0
#define SSH_SEEK_READ  1
#define SSH_SEEK_FORCE 2

static void ssh_seek(BDRVSSHState *s, int64_t offset, int flags)
{
    bool op_read = (flags & SSH_SEEK_READ) != 0;
    bool force = (flags & SSH_SEEK_FORCE) != 0;

    if (force || op_read != s->offset_op_read || offset != s->offset) {
        DPRINTF("seeking to offset=%" PRIi64, offset);
        libssh2_sftp_seek64(s->sftp_handle, offset);
        s->offset = offset;
        s->offset_op_read = op_read;
    }
}

static coroutine_fn int ssh_read(BDRVSSHState *s,
                                 int64_t offset, size_t size,
                                 QEMUIOVector *qiov)
{
    ssize_t r;
    size_t got;
    char *buf, *end_of_vec;
    struct iovec *i;

    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);

    ssh_seek(s, offset, SSH_SEEK_READ);

    /* This keeps track of the current iovec element ('i'), where we
     * will write to next ('buf'), and the end of the current iovec
     * ('end_of_vec').
     */
    i = &qiov->iov[0];
    buf = i->iov_base;
    end_of_vec = i->iov_base + i->iov_len;

    /* libssh2 has a hard-coded limit of 2000 bytes per request,
     * although it will also do readahead behind our backs.  Therefore
     * we may have to do repeated reads here until we have read 'size'
     * bytes.
     */
    for (got = 0; got < size; ) {
    again:
        DPRINTF("sftp_read buf=%p size=%zu", buf, end_of_vec - buf);
        r = libssh2_sftp_read(s->sftp_handle, buf, end_of_vec - buf);
        DPRINTF("sftp_read returned %zd", r);

        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
            co_yield(s);
            goto again;
        }
        if (r < 0) {
            sftp_error_report(s, "read failed");
            s->offset = -1;
            return -EIO;
        }
        if (r == 0) {
            /* EOF: Short read so pad the buffer with zeroes and return it. */
            qemu_iovec_memset(qiov, got, 0, size - got);
            return 0;
        }

        got += r;
        buf += r;
        s->offset += r;
        if (buf >= end_of_vec && got < size) {
            i++;
            buf = i->iov_base;
            end_of_vec = i->iov_base + i->iov_len;
        }
    }

    return 0;
}

static coroutine_fn int ssh_co_readv(BlockDriverState *bs,
                                     int64_t sector_num,
                                     int nb_sectors, QEMUIOVector *qiov)
{
    BDRVSSHState *s = bs->opaque;
    int ret;

    qemu_co_mutex_lock(&s->lock);
    ret = ssh_read(s, sector_num * BDRV_SECTOR_SIZE,
                   nb_sectors * BDRV_SECTOR_SIZE, qiov);
    qemu_co_mutex_unlock(&s->lock);

    return ret;
}

static int ssh_write(BDRVSSHState *s,
                     int64_t offset, size_t size,
                     QEMUIOVector *qiov)
{
    ssize_t r;
    size_t written;
    char *buf, *end_of_vec;
    struct iovec *i;

    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);

    ssh_seek(s, offset, SSH_SEEK_WRITE);

    /* This keeps track of the current iovec element ('i'), where we
     * will read from next ('buf'), and the end of the current iovec
     * ('end_of_vec').
     */
    i = &qiov->iov[0];
    buf = i->iov_base;
    end_of_vec = i->iov_base + i->iov_len;

    for (written = 0; written < size; ) {
    again:
        DPRINTF("sftp_write buf=%p size=%zu", buf, end_of_vec - buf);
        r = libssh2_sftp_write(s->sftp_handle, buf, end_of_vec - buf);
        DPRINTF("sftp_write returned %zd", r);

        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
            co_yield(s);
            goto again;
        }
        if (r < 0) {
            sftp_error_report(s, "write failed");
            s->offset = -1;
            return -EIO;
        }
        /* The libssh2 API is very unclear about this.  A comment in
         * the code says "nothing was acked, and no EAGAIN was
         * received!" which apparently means that no data got sent
         * out, and the underlying channel didn't return any EAGAIN
         * indication.  I think this is a bug in either libssh2 or
         * OpenSSH (server-side).  In any case, forcing a seek (to
         * discard libssh2 internal buffers), and then trying again
         * works for me.
         */
        if (r == 0) {
            ssh_seek(s, offset + written, SSH_SEEK_WRITE|SSH_SEEK_FORCE);
            co_yield(s);
            goto again;
        }

        written += r;
        buf += r;
        s->offset += r;
        if (buf >= end_of_vec && written < size) {
            i++;
            buf = i->iov_base;
            end_of_vec = i->iov_base + i->iov_len;
        }

        if (offset + written > s->attrs.filesize)
            s->attrs.filesize = offset + written;
    }

    return 0;
}

static coroutine_fn int ssh_co_writev(BlockDriverState *bs,
                                      int64_t sector_num,
                                      int nb_sectors, QEMUIOVector *qiov)
{
    BDRVSSHState *s = bs->opaque;
    int ret;

    qemu_co_mutex_lock(&s->lock);
    ret = ssh_write(s, sector_num * BDRV_SECTOR_SIZE,
                    nb_sectors * BDRV_SECTOR_SIZE, qiov);
    qemu_co_mutex_unlock(&s->lock);

    return ret;
}

static void unsafe_flush_warning(BDRVSSHState *s, const char *what)
{
    if (!s->unsafe_flush_warning) {
        error_report("warning: ssh server %s does not support fsync",
                     s->hostport);
        if (what) {
            error_report("to support fsync, you need %s", what);
        }
        s->unsafe_flush_warning = true;
    }
}

#ifdef HAS_LIBSSH2_SFTP_FSYNC

static coroutine_fn int ssh_flush(BDRVSSHState *s)
{
    int r;

    DPRINTF("fsync");
 again:
    r = libssh2_sftp_fsync(s->sftp_handle);
    if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
        co_yield(s);
        goto again;
    }
    if (r == LIBSSH2_ERROR_SFTP_PROTOCOL &&
        libssh2_sftp_last_error(s->sftp) == LIBSSH2_FX_OP_UNSUPPORTED) {
        unsafe_flush_warning(s, "OpenSSH >= 6.3");
        return 0;
    }
    if (r < 0) {
        sftp_error_report(s, "fsync failed");
        return -EIO;
    }

    return 0;
}

static coroutine_fn int ssh_co_flush(BlockDriverState *bs)
{
    BDRVSSHState *s = bs->opaque;
    int ret;

    qemu_co_mutex_lock(&s->lock);
    ret = ssh_flush(s);
    qemu_co_mutex_unlock(&s->lock);

    return ret;
}

#else /* !HAS_LIBSSH2_SFTP_FSYNC */

static coroutine_fn int ssh_co_flush(BlockDriverState *bs)
{
    BDRVSSHState *s = bs->opaque;

    unsafe_flush_warning(s, "libssh2 >= 1.4.4");
    return 0;
}

#endif /* !HAS_LIBSSH2_SFTP_FSYNC */

static int64_t ssh_getlength(BlockDriverState *bs)
{
    BDRVSSHState *s = bs->opaque;
    int64_t length;

    /* Note we cannot make a libssh2 call here. */
    length = (int64_t) s->attrs.filesize;
    DPRINTF("length=%" PRIi64, length);

    return length;
}

static BlockDriver bdrv_ssh = {
    .format_name                  = "ssh",
    .protocol_name                = "ssh",
    .instance_size                = sizeof(BDRVSSHState),
    .bdrv_parse_filename          = ssh_parse_filename,
    .bdrv_file_open               = ssh_file_open,
    .bdrv_create                  = ssh_create,
    .bdrv_close                   = ssh_close,
    .bdrv_has_zero_init           = ssh_has_zero_init,
    .bdrv_co_readv                = ssh_co_readv,
    .bdrv_co_writev               = ssh_co_writev,
    .bdrv_getlength               = ssh_getlength,
    .bdrv_co_flush_to_disk        = ssh_co_flush,
    .create_options               = ssh_create_options,
};

static void bdrv_ssh_init(void)
{
    int r;

    r = libssh2_init(0);
    if (r != 0) {
        fprintf(stderr, "libssh2 initialization failed, %d\n", r);
        exit(EXIT_FAILURE);
    }

    bdrv_register(&bdrv_ssh);
}

block_init(bdrv_ssh_init);