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fs: fat: calculate FAT type based on cluster count

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This fixes an issue where the FAT type (FAT12, FAT16) is not
correctly detected, e.g. when the BPB field BS_FilSysType contains the
valid value "FAT     ".

According to the FAT spec the field BS_FilSysType has only
informational character and does not determine the FAT type.

The logic of this code is based on the linux kernel implementation
from the file fs/fat/inode.c function fat_fill_super().

For details about FAT see http://elm-chan.org/docs/fat_e.html

Signed-off-by: Christian Taedcke <christian.taedcke@weidmueller.com>
This commit is contained in:
Christian Taedcke 2023-11-15 13:44:18 +01:00 • committed by Tom Rini
parent 601d33f31a
commit 08f622a127
2 changed files with 28 additions and 26 deletions
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48
fs/fat/fat.c
View file

@ -26,6 +26,9 @@
#include <linux/compiler.h>
#include <linux/ctype.h>
/* maximum number of clusters for FAT12 */
#define MAX_FAT12 0xFF4
/*
* Convert a string to lowercase. Converts at most 'len' characters,
* 'len' may be larger than the length of 'str' if 'str' is NULL
@ -484,6 +487,27 @@ static __u8 mkcksum(struct nameext *nameext)
return ret;
}
/*
* Determine if the FAT type is FAT12 or FAT16
*
* Based on fat_fill_super() from the Linux kernel's fs/fat/inode.c
*/
static int determine_legacy_fat_bits(const boot_sector *bs)
{
u16 fat_start = bs->reserved;
u32 dir_start = fat_start + bs->fats * bs->fat_length;
u32 rootdir_sectors = get_unaligned_le16(bs->dir_entries) *
sizeof(dir_entry) /
get_unaligned_le16(bs->sector_size);
u32 data_start = dir_start + rootdir_sectors;
u16 sectors = get_unaligned_le16(bs->sectors);
u32 total_sectors = sectors ? sectors : bs->total_sect;
u32 total_clusters = (total_sectors - data_start) /
bs->cluster_size;
return (total_clusters > MAX_FAT12) ? 16 : 12;
}
/*
* Read boot sector and volume info from a FAT filesystem
*/
@ -518,7 +542,7 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
bs->total_sect = FAT2CPU32(bs->total_sect);
/* FAT32 entries */
if (bs->fat_length == 0) {
if (!bs->fat_length && bs->fat32_length) {
/* Assume FAT32 */
bs->fat32_length = FAT2CPU32(bs->fat32_length);
bs->flags = FAT2CPU16(bs->flags);
@ -529,25 +553,10 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
*fatsize = 32;
} else {
vistart = (volume_info *)&(bs->fat32_length);
*fatsize = 0;
*fatsize = determine_legacy_fat_bits(bs);
}
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");
goto exit;
fail:
ret = -1;
exit:
@ -1157,9 +1166,8 @@ int file_fat_detectfs(void)
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);
printf("Filesystem: FAT%d \"%s\"\n", fatsize, vol_label);
return 0;
}

6
include/fat.h
View file

@ -34,12 +34,6 @@ struct disk_partition;
/* Maximum number of entry for long file name according to spec */
#define MAX_LFN_SLOT 20
/* Filesystem identifiers */
#define FAT12_SIGN "FAT12 "
#define FAT16_SIGN "FAT16 "
#define FAT32_SIGN "FAT32 "
#define SIGNLEN 8
/* File attributes */
#define ATTR_RO 1
#define ATTR_HIDDEN 2