xref: /openbmc/u-boot/fs/fat/fat.c (revision baefb63a)

/*
 * fat.c
 *
 * R/O (V)FAT 12/16/32 filesystem implementation by Marcus Sundberg
 *
 * 2002-07-28 - rjones@nexus-tech.net - ported to ppcboot v1.1.6
 * 2003-03-10 - kharris@nexus-tech.net - ported to uboot
 *
 * SPDX-License-Identifier:	GPL-2.0+
 */

#include <common.h>
#include <blk.h>
#include <config.h>
#include <exports.h>
#include <fat.h>
#include <fs.h>
#include <asm/byteorder.h>
#include <part.h>
#include <malloc.h>
#include <memalign.h>
#include <linux/compiler.h>
#include <linux/ctype.h>

#ifdef CONFIG_SUPPORT_VFAT
static const int vfat_enabled = 1;
#else
static const int vfat_enabled = 0;
#endif

/*
 * Convert a string to lowercase.  Converts at most 'len' characters,
 * 'len' may be larger than the length of 'str' if 'str' is NULL
 * terminated.
 */
static void downcase(char *str, size_t len)
{
	while (*str != '\0' && len--) {
		*str = tolower(*str);
		str++;
	}
}

static struct blk_desc *cur_dev;
static disk_partition_t cur_part_info;

#define DOS_BOOT_MAGIC_OFFSET	0x1fe
#define DOS_FS_TYPE_OFFSET	0x36
#define DOS_FS32_TYPE_OFFSET	0x52

static int disk_read(__u32 block, __u32 nr_blocks, void *buf)
{
	ulong ret;

	if (!cur_dev)
		return -1;

	ret = blk_dread(cur_dev, cur_part_info.start + block, nr_blocks, buf);

	if (ret != nr_blocks)
		return -1;

	return ret;
}

int fat_set_blk_dev(struct blk_desc *dev_desc, disk_partition_t *info)
{
	ALLOC_CACHE_ALIGN_BUFFER(unsigned char, buffer, dev_desc->blksz);

	cur_dev = dev_desc;
	cur_part_info = *info;

	/* Make sure it has a valid FAT header */
	if (disk_read(0, 1, buffer) != 1) {
		cur_dev = NULL;
		return -1;
	}

	/* Check if it's actually a DOS volume */
	if (memcmp(buffer + DOS_BOOT_MAGIC_OFFSET, "\x55\xAA", 2)) {
		cur_dev = NULL;
		return -1;
	}

	/* Check for FAT12/FAT16/FAT32 filesystem */
	if (!memcmp(buffer + DOS_FS_TYPE_OFFSET, "FAT", 3))
		return 0;
	if (!memcmp(buffer + DOS_FS32_TYPE_OFFSET, "FAT32", 5))
		return 0;

	cur_dev = NULL;
	return -1;
}

int fat_register_device(struct blk_desc *dev_desc, int part_no)
{
	disk_partition_t info;

	/* First close any currently found FAT filesystem */
	cur_dev = NULL;

	/* Read the partition table, if present */
	if (part_get_info(dev_desc, part_no, &info)) {
		if (part_no != 0) {
			printf("** Partition %d not valid on device %d **\n",
					part_no, dev_desc->devnum);
			return -1;
		}

		info.start = 0;
		info.size = dev_desc->lba;
		info.blksz = dev_desc->blksz;
		info.name[0] = 0;
		info.type[0] = 0;
		info.bootable = 0;
#if CONFIG_IS_ENABLED(PARTITION_UUIDS)
		info.uuid[0] = 0;
#endif
	}

	return fat_set_blk_dev(dev_desc, &info);
}

/*
 * Extract zero terminated short name from a directory entry.
 */
static void get_name(dir_entry *dirent, char *s_name)
{
	char *ptr;

	memcpy(s_name, dirent->name, 8);
	s_name[8] = '\0';
	ptr = s_name;
	while (*ptr && *ptr != ' ')
		ptr++;
	if (dirent->lcase & CASE_LOWER_BASE)
		downcase(s_name, (unsigned)(ptr - s_name));
	if (dirent->ext[0] && dirent->ext[0] != ' ') {
		*ptr++ = '.';
		memcpy(ptr, dirent->ext, 3);
		if (dirent->lcase & CASE_LOWER_EXT)
			downcase(ptr, 3);
		ptr[3] = '\0';
		while (*ptr && *ptr != ' ')
			ptr++;
	}
	*ptr = '\0';
	if (*s_name == DELETED_FLAG)
		*s_name = '\0';
	else if (*s_name == aRING)
		*s_name = DELETED_FLAG;
}

static int flush_dirty_fat_buffer(fsdata *mydata);
#if !defined(CONFIG_FAT_WRITE)
/* Stub for read only operation */
int flush_dirty_fat_buffer(fsdata *mydata)
{
	(void)(mydata);
	return 0;
}
#endif

/*
 * Get the entry at index 'entry' in a FAT (12/16/32) table.
 * On failure 0x00 is returned.
 */
static __u32 get_fatent(fsdata *mydata, __u32 entry)
{
	__u32 bufnum;
	__u32 offset, off8;
	__u32 ret = 0x00;

	if (CHECK_CLUST(entry, mydata->fatsize)) {
		printf("Error: Invalid FAT entry: 0x%08x\n", entry);
		return ret;
	}

	switch (mydata->fatsize) {
	case 32:
		bufnum = entry / FAT32BUFSIZE;
		offset = entry - bufnum * FAT32BUFSIZE;
		break;
	case 16:
		bufnum = entry / FAT16BUFSIZE;
		offset = entry - bufnum * FAT16BUFSIZE;
		break;
	case 12:
		bufnum = entry / FAT12BUFSIZE;
		offset = entry - bufnum * FAT12BUFSIZE;
		break;

	default:
		/* Unsupported FAT size */
		return ret;
	}

	debug("FAT%d: entry: 0x%08x = %d, offset: 0x%04x = %d\n",
	       mydata->fatsize, entry, entry, offset, offset);

	/* Read a new block of FAT entries into the cache. */
	if (bufnum != mydata->fatbufnum) {
		__u32 getsize = FATBUFBLOCKS;
		__u8 *bufptr = mydata->fatbuf;
		__u32 fatlength = mydata->fatlength;
		__u32 startblock = bufnum * FATBUFBLOCKS;

		/* Cap length if fatlength is not a multiple of FATBUFBLOCKS */
		if (startblock + getsize > fatlength)
			getsize = fatlength - startblock;

		startblock += mydata->fat_sect;	/* Offset from start of disk */

		/* Write back the fatbuf to the disk */
		if (flush_dirty_fat_buffer(mydata) < 0)
			return -1;

		if (disk_read(startblock, getsize, bufptr) < 0) {
			debug("Error reading FAT blocks\n");
			return ret;
		}
		mydata->fatbufnum = bufnum;
	}

	/* Get the actual entry from the table */
	switch (mydata->fatsize) {
	case 32:
		ret = FAT2CPU32(((__u32 *) mydata->fatbuf)[offset]);
		break;
	case 16:
		ret = FAT2CPU16(((__u16 *) mydata->fatbuf)[offset]);
		break;
	case 12:
		off8 = (offset * 3) / 2;
		/* fatbut + off8 may be unaligned, read in byte granularity */
		ret = mydata->fatbuf[off8] + (mydata->fatbuf[off8 + 1] << 8);

		if (offset & 0x1)
			ret >>= 4;
		ret &= 0xfff;
	}
	debug("FAT%d: ret: 0x%08x, entry: 0x%08x, offset: 0x%04x\n",
	       mydata->fatsize, ret, entry, offset);

	return ret;
}

/*
 * Read at most 'size' bytes from the specified cluster into 'buffer'.
 * Return 0 on success, -1 otherwise.
 */
static int
get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
{
	__u32 idx = 0;
	__u32 startsect;
	int ret;

	if (clustnum > 0) {
		startsect = clust_to_sect(mydata, clustnum);
	} else {
		startsect = mydata->rootdir_sect;
	}

	debug("gc - clustnum: %d, startsect: %d\n", clustnum, startsect);

	if ((unsigned long)buffer & (ARCH_DMA_MINALIGN - 1)) {
		ALLOC_CACHE_ALIGN_BUFFER(__u8, tmpbuf, mydata->sect_size);

		printf("FAT: Misaligned buffer address (%p)\n", buffer);

		while (size >= mydata->sect_size) {
			ret = disk_read(startsect++, 1, tmpbuf);
			if (ret != 1) {
				debug("Error reading data (got %d)\n", ret);
				return -1;
			}

			memcpy(buffer, tmpbuf, mydata->sect_size);
			buffer += mydata->sect_size;
			size -= mydata->sect_size;
		}
	} else {
		idx = size / mydata->sect_size;
		ret = disk_read(startsect, idx, buffer);
		if (ret != idx) {
			debug("Error reading data (got %d)\n", ret);
			return -1;
		}
		startsect += idx;
		idx *= mydata->sect_size;
		buffer += idx;
		size -= idx;
	}
	if (size) {
		ALLOC_CACHE_ALIGN_BUFFER(__u8, tmpbuf, mydata->sect_size);

		ret = disk_read(startsect, 1, tmpbuf);
		if (ret != 1) {
			debug("Error reading data (got %d)\n", ret);
			return -1;
		}

		memcpy(buffer, tmpbuf, size);
	}

	return 0;
}

/*
 * Read at most 'maxsize' bytes from 'pos' in the file associated with 'dentptr'
 * into 'buffer'.
 * Update the number of bytes read in *gotsize or return -1 on fatal errors.
 */
__u8 get_contents_vfatname_block[MAX_CLUSTSIZE]
	__aligned(ARCH_DMA_MINALIGN);

static int get_contents(fsdata *mydata, dir_entry *dentptr, loff_t pos,
			__u8 *buffer, loff_t maxsize, loff_t *gotsize)
{
	loff_t filesize = FAT2CPU32(dentptr->size);
	unsigned int bytesperclust = mydata->clust_size * mydata->sect_size;
	__u32 curclust = START(dentptr);
	__u32 endclust, newclust;
	loff_t actsize;

	*gotsize = 0;
	debug("Filesize: %llu bytes\n", filesize);

	if (pos >= filesize) {
		debug("Read position past EOF: %llu\n", pos);
		return 0;
	}

	if (maxsize > 0 && filesize > pos + maxsize)
		filesize = pos + maxsize;

	debug("%llu bytes\n", filesize);

	actsize = bytesperclust;

	/* go to cluster at pos */
	while (actsize <= pos) {
		curclust = get_fatent(mydata, curclust);
		if (CHECK_CLUST(curclust, mydata->fatsize)) {
			debug("curclust: 0x%x\n", curclust);
			debug("Invalid FAT entry\n");
			return 0;
		}
		actsize += bytesperclust;
	}

	/* actsize > pos */
	actsize -= bytesperclust;
	filesize -= actsize;
	pos -= actsize;

	/* align to beginning of next cluster if any */
	if (pos) {
		actsize = min(filesize, (loff_t)bytesperclust);
		if (get_cluster(mydata, curclust, get_contents_vfatname_block,
				(int)actsize) != 0) {
			printf("Error reading cluster\n");
			return -1;
		}
		filesize -= actsize;
		actsize -= pos;
		memcpy(buffer, get_contents_vfatname_block + pos, actsize);
		*gotsize += actsize;
		if (!filesize)
			return 0;
		buffer += actsize;

		curclust = get_fatent(mydata, curclust);
		if (CHECK_CLUST(curclust, mydata->fatsize)) {
			debug("curclust: 0x%x\n", curclust);
			debug("Invalid FAT entry\n");
			return 0;
		}
	}

	actsize = bytesperclust;
	endclust = curclust;

	do {
		/* search for consecutive clusters */
		while (actsize < filesize) {
			newclust = get_fatent(mydata, endclust);
			if ((newclust - 1) != endclust)
				goto getit;
			if (CHECK_CLUST(newclust, mydata->fatsize)) {
				debug("curclust: 0x%x\n", newclust);
				debug("Invalid FAT entry\n");
				return 0;
			}
			endclust = newclust;
			actsize += bytesperclust;
		}

		/* get remaining bytes */
		actsize = filesize;
		if (get_cluster(mydata, curclust, buffer, (int)actsize) != 0) {
			printf("Error reading cluster\n");
			return -1;
		}
		*gotsize += actsize;
		return 0;
getit:
		if (get_cluster(mydata, curclust, buffer, (int)actsize) != 0) {
			printf("Error reading cluster\n");
			return -1;
		}
		*gotsize += (int)actsize;
		filesize -= actsize;
		buffer += actsize;

		curclust = get_fatent(mydata, endclust);
		if (CHECK_CLUST(curclust, mydata->fatsize)) {
			debug("curclust: 0x%x\n", curclust);
			printf("Invalid FAT entry\n");
			return 0;
		}
		actsize = bytesperclust;
		endclust = curclust;
	} while (1);
}

/*
 * Extract the file name information from 'slotptr' into 'l_name',
 * starting at l_name[*idx].
 * Return 1 if terminator (zero byte) is found, 0 otherwise.
 */
static int slot2str(dir_slot *slotptr, char *l_name, int *idx)
{
	int j;

	for (j = 0; j <= 8; j += 2) {
		l_name[*idx] = slotptr->name0_4[j];
		if (l_name[*idx] == 0x00)
			return 1;
		(*idx)++;
	}
	for (j = 0; j <= 10; j += 2) {
		l_name[*idx] = slotptr->name5_10[j];
		if (l_name[*idx] == 0x00)
			return 1;
		(*idx)++;
	}
	for (j = 0; j <= 2; j += 2) {
		l_name[*idx] = slotptr->name11_12[j];
		if (l_name[*idx] == 0x00)
			return 1;
		(*idx)++;
	}

	return 0;
}

/* Calculate short name checksum */
static __u8 mkcksum(const char name[8], const char ext[3])
{
	int i;

	__u8 ret = 0;

	for (i = 0; i < 8; i++)
		ret = (((ret & 1) << 7) | ((ret & 0xfe) >> 1)) + name[i];
	for (i = 0; i < 3; i++)
		ret = (((ret & 1) << 7) | ((ret & 0xfe) >> 1)) + ext[i];

	return ret;
}

/*
 * TODO these should go away once fat_write is reworked to use the
 * directory iterator
 */
__u8 get_dentfromdir_block[MAX_CLUSTSIZE]
	__aligned(ARCH_DMA_MINALIGN);
__u8 do_fat_read_at_block[MAX_CLUSTSIZE]
	__aligned(ARCH_DMA_MINALIGN);

/*
 * Read boot sector and volume info from a FAT filesystem
 */
static int
read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
{
	__u8 *block;
	volume_info *vistart;
	int ret = 0;

	if (cur_dev == NULL) {
		debug("Error: no device selected\n");
		return -1;
	}

	block = malloc_cache_aligned(cur_dev->blksz);
	if (block == NULL) {
		debug("Error: allocating block\n");
		return -1;
	}

	if (disk_read(0, 1, block) < 0) {
		debug("Error: reading block\n");
		goto fail;
	}

	memcpy(bs, block, sizeof(boot_sector));
	bs->reserved = FAT2CPU16(bs->reserved);
	bs->fat_length = FAT2CPU16(bs->fat_length);
	bs->secs_track = FAT2CPU16(bs->secs_track);
	bs->heads = FAT2CPU16(bs->heads);
	bs->total_sect = FAT2CPU32(bs->total_sect);

	/* FAT32 entries */
	if (bs->fat_length == 0) {
		/* Assume FAT32 */
		bs->fat32_length = FAT2CPU32(bs->fat32_length);
		bs->flags = FAT2CPU16(bs->flags);
		bs->root_cluster = FAT2CPU32(bs->root_cluster);
		bs->info_sector = FAT2CPU16(bs->info_sector);
		bs->backup_boot = FAT2CPU16(bs->backup_boot);
		vistart = (volume_info *)(block + sizeof(boot_sector));
		*fatsize = 32;
	} else {
		vistart = (volume_info *)&(bs->fat32_length);
		*fatsize = 0;
	}
	memcpy(volinfo, vistart, sizeof(volume_info));

	if (*fatsize == 32) {
		if (strncmp(FAT32_SIGN, vistart->fs_type, SIGNLEN) == 0)
			goto exit;
	} else {
		if (strncmp(FAT12_SIGN, vistart->fs_type, SIGNLEN) == 0) {
			*fatsize = 12;
			goto exit;
		}
		if (strncmp(FAT16_SIGN, vistart->fs_type, SIGNLEN) == 0) {
			*fatsize = 16;
			goto exit;
		}
	}

	debug("Error: broken fs_type sign\n");
fail:
	ret = -1;
exit:
	free(block);
	return ret;
}

static int get_fs_info(fsdata *mydata)
{
	boot_sector bs;
	volume_info volinfo;
	int ret;

	ret = read_bootsectandvi(&bs, &volinfo, &mydata->fatsize);
	if (ret) {
		debug("Error: reading boot sector\n");
		return ret;
	}

	if (mydata->fatsize == 32) {
		mydata->fatlength = bs.fat32_length;
	} else {
		mydata->fatlength = bs.fat_length;
	}

	mydata->fat_sect = bs.reserved;

	mydata->rootdir_sect = mydata->fat_sect + mydata->fatlength * bs.fats;

	mydata->sect_size = (bs.sector_size[1] << 8) + bs.sector_size[0];
	mydata->clust_size = bs.cluster_size;
	if (mydata->sect_size != cur_part_info.blksz) {
		printf("Error: FAT sector size mismatch (fs=%hu, dev=%lu)\n",
				mydata->sect_size, cur_part_info.blksz);
		return -1;
	}

	if (mydata->fatsize == 32) {
		mydata->data_begin = mydata->rootdir_sect -
					(mydata->clust_size * 2);
		mydata->root_cluster = bs.root_cluster;
	} else {
		mydata->rootdir_size = ((bs.dir_entries[1]  * (int)256 +
					 bs.dir_entries[0]) *
					 sizeof(dir_entry)) /
					 mydata->sect_size;
		mydata->data_begin = mydata->rootdir_sect +
					mydata->rootdir_size -
					(mydata->clust_size * 2);
		mydata->root_cluster =
			sect_to_clust(mydata, mydata->rootdir_sect);
	}

	mydata->fatbufnum = -1;
	mydata->fat_dirty = 0;
	mydata->fatbuf = malloc_cache_aligned(FATBUFSIZE);
	if (mydata->fatbuf == NULL) {
		debug("Error: allocating memory\n");
		return -1;
	}

	if (vfat_enabled)
		debug("VFAT Support enabled\n");

	debug("FAT%d, fat_sect: %d, fatlength: %d\n",
	       mydata->fatsize, mydata->fat_sect, mydata->fatlength);
	debug("Rootdir begins at cluster: %d, sector: %d, offset: %x\n"
	       "Data begins at: %d\n",
	       mydata->root_cluster,
	       mydata->rootdir_sect,
	       mydata->rootdir_sect * mydata->sect_size, mydata->data_begin);
	debug("Sector size: %d, cluster size: %d\n", mydata->sect_size,
	      mydata->clust_size);

	return 0;
}


/*
 * Directory iterator, to simplify filesystem traversal
 *
 * Implements an iterator pattern to traverse directory tables,
 * transparently handling directory tables split across multiple
 * clusters, and the difference between FAT12/FAT16 root directory
 * (contiguous) and subdirectories + FAT32 root (chained).
 *
 * Rough usage:
 *
 *   for (fat_itr_root(&itr, fsdata); fat_itr_next(&itr); ) {
 *      // to traverse down to a subdirectory pointed to by
 *      // current iterator position:
 *      fat_itr_child(&itr, &itr);
 *   }
 *
 * For more complete example, see fat_itr_resolve()
 */

typedef struct {
	fsdata    *fsdata;        /* filesystem parameters */
	unsigned   clust;         /* current cluster */
	int        last_cluster;  /* set once we've read last cluster */
	int        is_root;       /* is iterator at root directory */
	int        remaining;     /* remaining dent's in current cluster */

	/* current iterator position values: */
	dir_entry *dent;          /* current directory entry */
	char       l_name[VFAT_MAXLEN_BYTES];    /* long (vfat) name */
	char       s_name[14];    /* short 8.3 name */
	char      *name;          /* l_name if there is one, else s_name */

	/* storage for current cluster in memory: */
	u8         block[MAX_CLUSTSIZE] __aligned(ARCH_DMA_MINALIGN);
} fat_itr;

static int fat_itr_isdir(fat_itr *itr);

/**
 * fat_itr_root() - initialize an iterator to start at the root
 * directory
 *
 * @itr: iterator to initialize
 * @fsdata: filesystem data for the partition
 * @return 0 on success, else -errno
 */
static int fat_itr_root(fat_itr *itr, fsdata *fsdata)
{
	if (get_fs_info(fsdata))
		return -ENXIO;

	itr->fsdata = fsdata;
	itr->clust = fsdata->root_cluster;
	itr->dent = NULL;
	itr->remaining = 0;
	itr->last_cluster = 0;
	itr->is_root = 1;

	return 0;
}

/**
 * fat_itr_child() - initialize an iterator to descend into a sub-
 * directory
 *
 * Initializes 'itr' to iterate the contents of the directory at
 * the current cursor position of 'parent'.  It is an error to
 * call this if the current cursor of 'parent' is pointing at a
 * regular file.
 *
 * Note that 'itr' and 'parent' can be the same pointer if you do
 * not need to preserve 'parent' after this call, which is useful
 * for traversing directory structure to resolve a file/directory.
 *
 * @itr: iterator to initialize
 * @parent: the iterator pointing at a directory entry in the
 *    parent directory of the directory to iterate
 */
static void fat_itr_child(fat_itr *itr, fat_itr *parent)
{
	fsdata *mydata = parent->fsdata;  /* for silly macros */
	unsigned clustnum = START(parent->dent);

	assert(fat_itr_isdir(parent));

	itr->fsdata = parent->fsdata;
	if (clustnum > 0) {
		itr->clust = clustnum;
		itr->is_root = 0;
	} else {
		itr->clust = parent->fsdata->root_cluster;
		itr->is_root = 1;
	}
	itr->dent = NULL;
	itr->remaining = 0;
	itr->last_cluster = 0;
}

static void *next_cluster(fat_itr *itr)
{
	fsdata *mydata = itr->fsdata;  /* for silly macros */
	int ret;
	u32 sect;

	/* have we reached the end? */
	if (itr->last_cluster)
		return NULL;

	sect = clust_to_sect(itr->fsdata, itr->clust);

	debug("FAT read(sect=%d), clust_size=%d, DIRENTSPERBLOCK=%zd\n",
	      sect, itr->fsdata->clust_size, DIRENTSPERBLOCK);

	/*
	 * NOTE: do_fat_read_at() had complicated logic to deal w/
	 * vfat names that span multiple clusters in the fat16 case,
	 * which get_dentfromdir() probably also needed (and was
	 * missing).  And not entirely sure what fat32 didn't have
	 * the same issue..  We solve that by only caring about one
	 * dent at a time and iteratively constructing the vfat long
	 * name.
	 */
	ret = disk_read(sect, itr->fsdata->clust_size,
			itr->block);
	if (ret < 0) {
		debug("Error: reading block\n");
		return NULL;
	}

	if (itr->is_root && itr->fsdata->fatsize != 32) {
		itr->clust++;
		sect = clust_to_sect(itr->fsdata, itr->clust);
		if (sect - itr->fsdata->rootdir_sect >=
		    itr->fsdata->rootdir_size) {
			debug("cursect: 0x%x\n", itr->clust);
			itr->last_cluster = 1;
		}
	} else {
		itr->clust = get_fatent(itr->fsdata, itr->clust);
		if (CHECK_CLUST(itr->clust, itr->fsdata->fatsize)) {
			debug("cursect: 0x%x\n", itr->clust);
			itr->last_cluster = 1;
		}
	}

	return itr->block;
}

static dir_entry *next_dent(fat_itr *itr)
{
	if (itr->remaining == 0) {
		struct dir_entry *dent = next_cluster(itr);
		unsigned nbytes = itr->fsdata->sect_size *
			itr->fsdata->clust_size;

		/* have we reached the last cluster? */
		if (!dent)
			return NULL;

		itr->remaining = nbytes / sizeof(dir_entry) - 1;
		itr->dent = dent;
	} else {
		itr->remaining--;
		itr->dent++;
	}

	/* have we reached the last valid entry? */
	if (itr->dent->name[0] == 0)
		return NULL;

	return itr->dent;
}

static dir_entry *extract_vfat_name(fat_itr *itr)
{
	struct dir_entry *dent = itr->dent;
	int seqn = itr->dent->name[0] & ~LAST_LONG_ENTRY_MASK;
	u8 chksum, alias_checksum = ((dir_slot *)dent)->alias_checksum;
	int n = 0;

	while (seqn--) {
		char buf[13];
		int idx = 0;

		slot2str((dir_slot *)dent, buf, &idx);

		/* shift accumulated long-name up and copy new part in: */
		memmove(itr->l_name + idx, itr->l_name, n);
		memcpy(itr->l_name, buf, idx);
		n += idx;

		dent = next_dent(itr);
		if (!dent)
			return NULL;
	}

	itr->l_name[n] = '\0';

	chksum = mkcksum(dent->name, dent->ext);

	/* checksum mismatch could mean deleted file, etc.. skip it: */
	if (chksum != alias_checksum) {
		debug("** chksum=%x, alias_checksum=%x, l_name=%s, s_name=%8s.%3s\n",
		      chksum, alias_checksum, itr->l_name, dent->name, dent->ext);
		return NULL;
	}

	return dent;
}

/**
 * fat_itr_next() - step to the next entry in a directory
 *
 * Must be called once on a new iterator before the cursor is valid.
 *
 * @itr: the iterator to iterate
 * @return boolean, 1 if success or 0 if no more entries in the
 *    current directory
 */
static int fat_itr_next(fat_itr *itr)
{
	dir_entry *dent;

	itr->name = NULL;

	while (1) {
		dent = next_dent(itr);
		if (!dent)
			return 0;

		if (dent->name[0] == DELETED_FLAG ||
		    dent->name[0] == aRING)
			continue;

		if (dent->attr & ATTR_VOLUME) {
			if (vfat_enabled &&
			    (dent->attr & ATTR_VFAT) == ATTR_VFAT &&
			    (dent->name[0] & LAST_LONG_ENTRY_MASK)) {
				dent = extract_vfat_name(itr);
				if (!dent)
					continue;
				itr->name = itr->l_name;
				break;
			} else {
				/* Volume label or VFAT entry, skip */
				continue;
			}
		}

		break;
	}

	get_name(dent, itr->s_name);
	if (!itr->name)
		itr->name = itr->s_name;

	return 1;
}

/**
 * fat_itr_isdir() - is current cursor position pointing to a directory
 *
 * @itr: the iterator
 * @return true if cursor is at a directory
 */
static int fat_itr_isdir(fat_itr *itr)
{
	return !!(itr->dent->attr & ATTR_DIR);
}

/*
 * Helpers:
 */

#define TYPE_FILE 0x1
#define TYPE_DIR  0x2
#define TYPE_ANY  (TYPE_FILE | TYPE_DIR)

/**
 * fat_itr_resolve() - traverse directory structure to resolve the
 * requested path.
 *
 * Traverse directory structure to the requested path.  If the specified
 * path is to a directory, this will descend into the directory and
 * leave it iterator at the start of the directory.  If the path is to a
 * file, it will leave the iterator in the parent directory with current
 * cursor at file's entry in the directory.
 *
 * @itr: iterator initialized to root
 * @path: the requested path
 * @type: bitmask of allowable file types
 * @return 0 on success or -errno
 */
static int fat_itr_resolve(fat_itr *itr, const char *path, unsigned type)
{
	const char *next;

	/* chomp any extra leading slashes: */
	while (path[0] && ISDIRDELIM(path[0]))
		path++;

	/* are we at the end? */
	if (strlen(path) == 0) {
		if (!(type & TYPE_DIR))
			return -ENOENT;
		return 0;
	}

	/* find length of next path entry: */
	next = path;
	while (next[0] && !ISDIRDELIM(next[0]))
		next++;

	while (fat_itr_next(itr)) {
		int match = 0;
		unsigned n = max(strlen(itr->name), (size_t)(next - path));

		/* check both long and short name: */
		if (!strncasecmp(path, itr->name, n))
			match = 1;
		else if (itr->name != itr->s_name &&
			 !strncasecmp(path, itr->s_name, n))
			match = 1;

		if (!match)
			continue;

		if (fat_itr_isdir(itr)) {
			/* recurse into directory: */
			fat_itr_child(itr, itr);
			return fat_itr_resolve(itr, next, type);
		} else if (next[0]) {
			/*
			 * If next is not empty then we have a case
			 * like: /path/to/realfile/nonsense
			 */
			debug("bad trailing path: %s\n", next);
			return -ENOENT;
		} else if (!(type & TYPE_FILE)) {
			return -ENOTDIR;
		} else {
			return 0;
		}
	}

	return -ENOENT;
}

int file_fat_detectfs(void)
{
	boot_sector bs;
	volume_info volinfo;
	int fatsize;
	char vol_label[12];

	if (cur_dev == NULL) {
		printf("No current device\n");
		return 1;
	}

#if defined(CONFIG_IDE) || \
    defined(CONFIG_SATA) || \
    defined(CONFIG_SCSI) || \
    defined(CONFIG_CMD_USB) || \
    defined(CONFIG_MMC)
	printf("Interface:  ");
	switch (cur_dev->if_type) {
	case IF_TYPE_IDE:
		printf("IDE");
		break;
	case IF_TYPE_SATA:
		printf("SATA");
		break;
	case IF_TYPE_SCSI:
		printf("SCSI");
		break;
	case IF_TYPE_ATAPI:
		printf("ATAPI");
		break;
	case IF_TYPE_USB:
		printf("USB");
		break;
	case IF_TYPE_DOC:
		printf("DOC");
		break;
	case IF_TYPE_MMC:
		printf("MMC");
		break;
	default:
		printf("Unknown");
	}

	printf("\n  Device %d: ", cur_dev->devnum);
	dev_print(cur_dev);
#endif

	if (read_bootsectandvi(&bs, &volinfo, &fatsize)) {
		printf("\nNo valid FAT fs found\n");
		return 1;
	}

	memcpy(vol_label, volinfo.volume_label, 11);
	vol_label[11] = '\0';
	volinfo.fs_type[5] = '\0';

	printf("Filesystem: %s \"%s\"\n", volinfo.fs_type, vol_label);

	return 0;
}

int fat_exists(const char *filename)
{
	fsdata fsdata;
	fat_itr *itr;
	int ret;

	itr = malloc_cache_aligned(sizeof(fat_itr));
	if (!itr)
		return 0;
	ret = fat_itr_root(itr, &fsdata);
	if (ret)
		goto out;

	ret = fat_itr_resolve(itr, filename, TYPE_ANY);
	free(fsdata.fatbuf);
out:
	free(itr);
	return ret == 0;
}

int fat_size(const char *filename, loff_t *size)
{
	fsdata fsdata;
	fat_itr *itr;
	int ret;

	itr = malloc_cache_aligned(sizeof(fat_itr));
	if (!itr)
		return -ENOMEM;
	ret = fat_itr_root(itr, &fsdata);
	if (ret)
		goto out_free_itr;

	ret = fat_itr_resolve(itr, filename, TYPE_FILE);
	if (ret) {
		/*
		 * Directories don't have size, but fs_size() is not
		 * expected to fail if passed a directory path:
		 */
		free(fsdata.fatbuf);
		fat_itr_root(itr, &fsdata);
		if (!fat_itr_resolve(itr, filename, TYPE_DIR)) {
			*size = 0;
			ret = 0;
		}
		goto out_free_both;
	}

	*size = FAT2CPU32(itr->dent->size);
out_free_both:
	free(fsdata.fatbuf);
out_free_itr:
	free(itr);
	return ret;
}

int file_fat_read_at(const char *filename, loff_t pos, void *buffer,
		     loff_t maxsize, loff_t *actread)
{
	fsdata fsdata;
	fat_itr *itr;
	int ret;

	itr = malloc_cache_aligned(sizeof(fat_itr));
	if (!itr)
		return -ENOMEM;
	ret = fat_itr_root(itr, &fsdata);
	if (ret)
		goto out_free_itr;

	ret = fat_itr_resolve(itr, filename, TYPE_FILE);
	if (ret)
		goto out_free_both;

	printf("reading %s\n", filename);
	ret = get_contents(&fsdata, itr->dent, pos, buffer, maxsize, actread);

out_free_both:
	free(fsdata.fatbuf);
out_free_itr:
	free(itr);
	return ret;
}

int file_fat_read(const char *filename, void *buffer, int maxsize)
{
	loff_t actread;
	int ret;

	ret =  file_fat_read_at(filename, 0, buffer, maxsize, &actread);
	if (ret)
		return ret;
	else
		return actread;
}

int fat_read_file(const char *filename, void *buf, loff_t offset, loff_t len,
		  loff_t *actread)
{
	int ret;

	ret = file_fat_read_at(filename, offset, buf, len, actread);
	if (ret)
		printf("** Unable to read file %s **\n", filename);

	return ret;
}

typedef struct {
	struct fs_dir_stream parent;
	struct fs_dirent dirent;
	fsdata fsdata;
	fat_itr itr;
} fat_dir;

int fat_opendir(const char *filename, struct fs_dir_stream **dirsp)
{
	fat_dir *dir;
	int ret;

	dir = malloc_cache_aligned(sizeof(*dir));
	if (!dir)
		return -ENOMEM;
	memset(dir, 0, sizeof(*dir));

	ret = fat_itr_root(&dir->itr, &dir->fsdata);
	if (ret)
		goto fail_free_dir;

	ret = fat_itr_resolve(&dir->itr, filename, TYPE_DIR);
	if (ret)
		goto fail_free_both;

	*dirsp = (struct fs_dir_stream *)dir;
	return 0;

fail_free_both:
	free(dir->fsdata.fatbuf);
fail_free_dir:
	free(dir);
	return ret;
}

int fat_readdir(struct fs_dir_stream *dirs, struct fs_dirent **dentp)
{
	fat_dir *dir = (fat_dir *)dirs;
	struct fs_dirent *dent = &dir->dirent;

	if (!fat_itr_next(&dir->itr))
		return -ENOENT;

	memset(dent, 0, sizeof(*dent));
	strcpy(dent->name, dir->itr.name);

	if (fat_itr_isdir(&dir->itr)) {
		dent->type = FS_DT_DIR;
	} else {
		dent->type = FS_DT_REG;
		dent->size = FAT2CPU32(dir->itr.dent->size);
	}

	*dentp = dent;

	return 0;
}

void fat_closedir(struct fs_dir_stream *dirs)
{
	fat_dir *dir = (fat_dir *)dirs;
	free(dir->fsdata.fatbuf);
	free(dir);
}

void fat_close(void)
{
}