From: Christian Taedcke christian.taedcke@weidmueller.com
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 ---
Changes in v2: - Use get_unaligned_le16 instead of custom macro - Extract FAT32 logic from determine_fat_bits() into read_bootsectandvi() - Rename determine_fat_bits() to determine_legacy_fat_bits()
fs/fat/fat.c | 48 ++++++++++++++++++++++++++++-------------------- include/fat.h | 6 ------ 2 files changed, 28 insertions(+), 26 deletions(-)
diff --git a/fs/fat/fat.c b/fs/fat/fat.c index a3522340ef..c368c3b076 100644 --- a/fs/fat/fat.c +++ b/fs/fat/fat.c @@ -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; } diff --git a/include/fat.h b/include/fat.h index a9756fb4cd..3dce99a23c 100644 --- a/include/fat.h +++ b/include/fat.h @@ -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