[PATCH v7] fs: FAT: Add support for DOS 1.x formatted volumes

From: Conrad Meyer
Date: Mon Apr 14 2014 - 21:56:13 EST

Add structure for parsed BPB information, struct fat_bios_param_block,
and move all of the deserialization and validation logic from
fat_fill_super() into fat_read_bpb().

Add a 'dos1xfloppy' mount option to infer DOS 2.x BIOS Parameter Block
defaults from block device geometry for ancient floppies and floppy
images, as a fall-back from the default BPB parsing logic.

When fat_read_bpb() finds an invalid FAT filesystem and dos1xfloppy is
set, fall back to fat_read_static_bpb(). fat_read_static_bpb() validates
that the entire BPB is zero, and that the floppy has a DOS-style 8086
code bootstrapping header. Then it fills in default BPB values from
media size and a table.[0]

Media size is assumed to be static for archaic FAT volumes. See also:
[1].

Fixes kernel.org bug #42617.

[0]: https://en.wikipedia.org/wiki/File_Allocation_Table#Exceptions
[1]: http://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html

Signed-off-by: Conrad Meyer <cse.cem@xxxxxxxxx>
---
Changes since v6:
* Update show_options()
* Add option documentation to Doc/filesystems/vfat.txt

Thanks.
---
Documentation/filesystems/vfat.txt | 5 +
fs/fat/fat.h | 28 +++-
fs/fat/inode.c | 318 ++++++++++++++++++++++++++++---------
3 files changed, 274 insertions(+), 77 deletions(-)

diff --git a/Documentation/filesystems/vfat.txt b/Documentation/filesystems/vfat.txt
index 5cf57b3..223c321 100644
--- a/Documentation/filesystems/vfat.txt
+++ b/Documentation/filesystems/vfat.txt
@@ -172,6 +172,11 @@ nfs=stale_rw|nostale_ro
To maintain backward compatibility, '-o nfs' is also accepted,
defaulting to stale_rw

+dos1xfloppy -- If set, use a fallback default BIOS Parameter Block
+ configuration, determined by backing device size. These static
+ parameters match defaults assumed by DOS 1.x for 160 kiB,
+ 180 kiB, 320 kiB, and 360 kiB floppies and floppy images.
+

<bool>: 0,1,yes,no,true,false

diff --git a/fs/fat/fat.h b/fs/fat/fat.h
index 7270bdb..bcb0d67 100644
--- a/fs/fat/fat.h
+++ b/fs/fat/fat.h
@@ -52,7 +52,8 @@ struct fat_mount_options {
usefree:1, /* Use free_clusters for FAT32 */
tz_set:1, /* Filesystem timestamps' offset set */
rodir:1, /* allow ATTR_RO for directory */
- discard:1; /* Issue discard requests on deletions */
+ discard:1, /* Issue discard requests on deletions */
+ dos1xfloppy:1; /* Assume default BPB for DOS 1.x floppies */
};

#define FAT_HASH_BITS 8
@@ -139,6 +140,31 @@ struct fat_slot_info {
struct buffer_head *bh;
};

+/*
+ * A deserialized copy of the on-disk structure laid out in struct
+ * fat_boot_sector.
+ */
+struct fat_bios_param_block {
+ u16 fat_sector_size;
+ u8 fat_sec_per_clus;
+ u16 fat_reserved;
+ u8 fat_fats;
+ u16 fat_dir_entries;
+ u16 fat_sectors;
+ u16 fat_fat_length;
+ u32 fat_total_sect;
+
+ u8 fat16_state;
+ u32 fat16_vol_id;
+
+ u32 fat32_length;
+ u32 fat32_root_cluster;
+ u16 fat32_info_sector;
+ u8 fat32_state;
+ u32 fat32_vol_id;
+
+};
+
static inline struct msdos_sb_info *MSDOS_SB(struct super_block *sb)
{
return sb->s_fs_info;
diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 992e8cb..70ba768 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -35,9 +35,47 @@
#define CONFIG_FAT_DEFAULT_IOCHARSET ""
#endif

+#define KB_IN_SECTORS 2
+
static int fat_default_codepage = CONFIG_FAT_DEFAULT_CODEPAGE;
static char fat_default_iocharset[] = CONFIG_FAT_DEFAULT_IOCHARSET;

+static struct fat_floppy_defaults {
+ unsigned nr_sectors;
+ unsigned sec_per_clus;
+ unsigned dir_entries;
+ unsigned media;
+ unsigned fat_length;
+} floppy_defaults[] = {
+{
+ .nr_sectors = 160 * KB_IN_SECTORS,
+ .sec_per_clus = 1,
+ .dir_entries = 64,
+ .media = 0xFE,
+ .fat_length = 1,
+},
+{
+ .nr_sectors = 180 * KB_IN_SECTORS,
+ .sec_per_clus = 1,
+ .dir_entries = 64,
+ .media = 0xFC,
+ .fat_length = 2,
+},
+{
+ .nr_sectors = 320 * KB_IN_SECTORS,
+ .sec_per_clus = 2,
+ .dir_entries = 112,
+ .media = 0xFF,
+ .fat_length = 1,
+},
+{
+ .nr_sectors = 360 * KB_IN_SECTORS,
+ .sec_per_clus = 2,
+ .dir_entries = 112,
+ .media = 0xFD,
+ .fat_length = 2,
+},
+};

static int fat_add_cluster(struct inode *inode)
{
@@ -931,6 +969,8 @@ static int fat_show_options(struct seq_file *m, struct dentry *root)
seq_puts(m, ",nfs=stale_rw");
if (opts->discard)
seq_puts(m, ",discard");
+ if (opts->dos1xfloppy)
+ seq_puts(m, ",dos1xfloppy");

return 0;
}
@@ -945,7 +985,7 @@ enum {
Opt_uni_xl_no, Opt_uni_xl_yes, Opt_nonumtail_no, Opt_nonumtail_yes,
Opt_obsolete, Opt_flush, Opt_tz_utc, Opt_rodir, Opt_err_cont,
Opt_err_panic, Opt_err_ro, Opt_discard, Opt_nfs, Opt_time_offset,
- Opt_nfs_stale_rw, Opt_nfs_nostale_ro, Opt_err,
+ Opt_nfs_stale_rw, Opt_nfs_nostale_ro, Opt_err, Opt_dos1xfloppy,
};

static const match_table_t fat_tokens = {
@@ -978,6 +1018,7 @@ static const match_table_t fat_tokens = {
{Opt_nfs_stale_rw, "nfs"},
{Opt_nfs_stale_rw, "nfs=stale_rw"},
{Opt_nfs_nostale_ro, "nfs=nostale_ro"},
+ {Opt_dos1xfloppy, "dos1xfloppy"},
{Opt_obsolete, "conv=binary"},
{Opt_obsolete, "conv=text"},
{Opt_obsolete, "conv=auto"},
@@ -1180,6 +1221,9 @@ static int parse_options(struct super_block *sb, char *options, int is_vfat,
case Opt_nfs_nostale_ro:
opts->nfs = FAT_NFS_NOSTALE_RO;
break;
+ case Opt_dos1xfloppy:
+ opts->dos1xfloppy = 1;
+ break;

/* msdos specific */
case Opt_dots:
@@ -1326,6 +1370,169 @@ static unsigned long calc_fat_clusters(struct super_block *sb)
return sbi->fat_length * sb->s_blocksize * 8 / sbi->fat_bits;
}

+static bool fat_bpb_is_zero(struct fat_boot_sector *b)
+{
+ if (get_unaligned_le16(&b->sector_size))
+ return false;
+ if (b->sec_per_clus)
+ return false;
+ if (b->reserved)
+ return false;
+ if (b->fats)
+ return false;
+ if (get_unaligned_le16(&b->dir_entries))
+ return false;
+ if (get_unaligned_le16(&b->sectors))
+ return false;
+ if (b->media)
+ return false;
+ if (b->fat_length)
+ return false;
+ if (b->secs_track)
+ return false;
+ if (b->heads)
+ return false;
+ return true;
+}
+
+static int fat_read_bpb(struct super_block *sb, struct fat_boot_sector *b,
+ int silent, struct fat_bios_param_block *bpb)
+{
+ int error = -EINVAL;
+
+ /* Read in BPB ... */
+ memset(bpb, 0, sizeof(*bpb));
+ bpb->fat_sector_size = get_unaligned_le16(&b->sector_size);
+ bpb->fat_sec_per_clus = b->sec_per_clus;
+ bpb->fat_reserved = le16_to_cpu(b->reserved);
+ bpb->fat_fats = b->fats;
+ bpb->fat_dir_entries = get_unaligned_le16(&b->dir_entries);
+ bpb->fat_sectors = get_unaligned_le16(&b->sectors);
+ bpb->fat_fat_length = le16_to_cpu(b->fat_length);
+ bpb->fat_total_sect = le32_to_cpu(b->total_sect);
+
+ bpb->fat16_state = b->fat16.state;
+ bpb->fat16_vol_id = get_unaligned_le32(b->fat16.vol_id);
+
+ bpb->fat32_length = le32_to_cpu(b->fat32.length);
+ bpb->fat32_root_cluster = le32_to_cpu(b->fat32.root_cluster);
+ bpb->fat32_info_sector = le16_to_cpu(b->fat32.info_sector);
+ bpb->fat32_state = b->fat32.state;
+ bpb->fat32_vol_id = get_unaligned_le32(b->fat32.vol_id);
+
+ /* Validate this looks like a FAT filesystem BPB */
+ if (!bpb->fat_reserved) {
+ if (!silent)
+ fat_msg(sb, KERN_ERR,
+ "bogus number of reserved sectors");
+ goto out;
+ }
+ if (!bpb->fat_fats) {
+ if (!silent)
+ fat_msg(sb, KERN_ERR, "bogus number of FAT structure");
+ goto out;
+ }
+
+ /*
+ * Earlier we checked here that b->secs_track and b->head are nonzero,
+ * but it turns out valid FAT filesystems can have zero there.
+ */
+
+ if (!fat_valid_media(b->media)) {
+ if (!silent)
+ fat_msg(sb, KERN_ERR, "invalid media value (0x%02x)",
+ (unsigned)b->media);
+ goto out;
+ }
+
+ if (!is_power_of_2(bpb->fat_sector_size)
+ || (bpb->fat_sector_size < 512)
+ || (bpb->fat_sector_size > 4096)) {
+ if (!silent)
+ fat_msg(sb, KERN_ERR, "bogus logical sector size %u",
+ (unsigned)bpb->fat_sector_size);
+ goto out;
+ }
+
+ if (!is_power_of_2(bpb->fat_sec_per_clus)) {
+ if (!silent)
+ fat_msg(sb, KERN_ERR, "bogus sectors per cluster %u",
+ (unsigned)bpb->fat_sec_per_clus);
+ goto out;
+ }
+
+ error = 0;
+
+out:
+ return error;
+}
+
+static int fat_read_static_bpb(struct super_block *sb,
+ struct fat_boot_sector *b, int silent,
+ struct fat_bios_param_block *bpb)
+{
+ static const char *notdos1x = "This doesn't look like a DOS 1.x volume";
+
+ struct fat_floppy_defaults *fdefaults = NULL;
+ int error = -EINVAL;
+ sector_t bd_sects;
+ unsigned i;
+
+ bd_sects = i_size_read(sb->s_bdev->bd_inode) / SECTOR_SIZE;
+
+ /* 16-bit DOS 1.x reliably wrote bootstrap short-jmp code */
+ if (b->ignored[0] != 0xeb || b->ignored[2] != 0x90) {
+ if (!silent)
+ fat_msg(sb, KERN_ERR,
+ "%s; no bootstrapping code", notdos1x);
+ goto out;
+ }
+
+ /*
+ * If any value in this region is non-zero, it isn't archaic
+ * DOS.
+ */
+ if (!fat_bpb_is_zero(b)) {
+ if (!silent)
+ fat_msg(sb, KERN_ERR,
+ "%s; DOS 2.x BPB is non-zero", notdos1x);
+ goto out;
+ }
+
+ for (i = 0; i < ARRAY_SIZE(floppy_defaults); i++) {
+ if (floppy_defaults[i].nr_sectors == bd_sects) {
+ fdefaults = &floppy_defaults[i];
+ break;
+ }
+ }
+
+ if (fdefaults == NULL) {
+ if (!silent)
+ fat_msg(sb, KERN_WARNING,
+ "This looks like a DOS 1.x volume, but isn't a recognized floppy size (%llu sectors)",
+ (u64)bd_sects);
+ goto out;
+ }
+
+ if (!silent)
+ fat_msg(sb, KERN_INFO,
+ "This looks like a DOS 1.x volume; assuming default BPB values");
+
+ memset(bpb, 0, sizeof(*bpb));
+ bpb->fat_sector_size = SECTOR_SIZE;
+ bpb->fat_sec_per_clus = fdefaults->sec_per_clus;
+ bpb->fat_reserved = 1;
+ bpb->fat_fats = 2;
+ bpb->fat_dir_entries = fdefaults->dir_entries;
+ bpb->fat_sectors = fdefaults->nr_sectors;
+ bpb->fat_fat_length = fdefaults->fat_length;
+
+ error = 0;
+
+out:
+ return error;
+}
+
/*
* Read the super block of an MS-DOS FS.
*/
@@ -1335,12 +1542,11 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
struct inode *root_inode = NULL, *fat_inode = NULL;
struct inode *fsinfo_inode = NULL;
struct buffer_head *bh;
- struct fat_boot_sector *b;
+ struct fat_bios_param_block bpb;
struct msdos_sb_info *sbi;
u16 logical_sector_size;
u32 total_sectors, total_clusters, fat_clusters, rootdir_sectors;
int debug;
- unsigned int media;
long error;
char buf[50];

@@ -1377,100 +1583,71 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
goto out_fail;
}

- b = (struct fat_boot_sector *) bh->b_data;
- if (!b->reserved) {
- if (!silent)
- fat_msg(sb, KERN_ERR, "bogus number of reserved sectors");
- brelse(bh);
- goto out_invalid;
- }
- if (!b->fats) {
- if (!silent)
- fat_msg(sb, KERN_ERR, "bogus number of FAT structure");
- brelse(bh);
- goto out_invalid;
- }
-
- /*
- * Earlier we checked here that b->secs_track and b->head are nonzero,
- * but it turns out valid FAT filesystems can have zero there.
- */
+ error = fat_read_bpb(sb, (struct fat_boot_sector *)bh->b_data, silent,
+ &bpb);
+ if (error == -EINVAL && sbi->options.dos1xfloppy)
+ error = fat_read_static_bpb(sb,
+ (struct fat_boot_sector *)bh->b_data, silent, &bpb);
+ brelse(bh);

- media = b->media;
- if (!fat_valid_media(media)) {
- if (!silent)
- fat_msg(sb, KERN_ERR, "invalid media value (0x%02x)",
- media);
- brelse(bh);
+ if (error == -EINVAL)
goto out_invalid;
- }
- logical_sector_size = get_unaligned_le16(&b->sector_size);
- if (!is_power_of_2(logical_sector_size)
- || (logical_sector_size < 512)
- || (logical_sector_size > 4096)) {
- if (!silent)
- fat_msg(sb, KERN_ERR, "bogus logical sector size %u",
- logical_sector_size);
- brelse(bh);
- goto out_invalid;
- }
- sbi->sec_per_clus = b->sec_per_clus;
- if (!is_power_of_2(sbi->sec_per_clus)) {
- if (!silent)
- fat_msg(sb, KERN_ERR, "bogus sectors per cluster %u",
- sbi->sec_per_clus);
- brelse(bh);
- goto out_invalid;
- }
+ else if (error)
+ goto out_fail;
+
+ logical_sector_size = bpb.fat_sector_size;
+ sbi->sec_per_clus = bpb.fat_sec_per_clus;

if (logical_sector_size < sb->s_blocksize) {
fat_msg(sb, KERN_ERR, "logical sector size too small for device"
" (logical sector size = %u)", logical_sector_size);
- brelse(bh);
goto out_fail;
}
+
if (logical_sector_size > sb->s_blocksize) {
- brelse(bh);
+ struct buffer_head *bh_resize;

if (!sb_set_blocksize(sb, logical_sector_size)) {
fat_msg(sb, KERN_ERR, "unable to set blocksize %u",
logical_sector_size);
goto out_fail;
}
- bh = sb_bread(sb, 0);
- if (bh == NULL) {
+
+ /* Verify that the larger boot sector is fully readable */
+ bh_resize = sb_bread(sb, 0);
+ if (bh_resize == NULL) {
fat_msg(sb, KERN_ERR, "unable to read boot sector"
" (logical sector size = %lu)",
sb->s_blocksize);
goto out_fail;
}
- b = (struct fat_boot_sector *) bh->b_data;
+ brelse(bh_resize);
}

mutex_init(&sbi->s_lock);
sbi->cluster_size = sb->s_blocksize * sbi->sec_per_clus;
sbi->cluster_bits = ffs(sbi->cluster_size) - 1;
- sbi->fats = b->fats;
+ sbi->fats = bpb.fat_fats;
sbi->fat_bits = 0; /* Don't know yet */
- sbi->fat_start = le16_to_cpu(b->reserved);
- sbi->fat_length = le16_to_cpu(b->fat_length);
+ sbi->fat_start = bpb.fat_reserved;
+ sbi->fat_length = bpb.fat_fat_length;
sbi->root_cluster = 0;
sbi->free_clusters = -1; /* Don't know yet */
sbi->free_clus_valid = 0;
sbi->prev_free = FAT_START_ENT;
sb->s_maxbytes = 0xffffffff;

- if (!sbi->fat_length && b->fat32.length) {
+ if (!sbi->fat_length && bpb.fat32_length) {
struct fat_boot_fsinfo *fsinfo;
struct buffer_head *fsinfo_bh;

/* Must be FAT32 */
sbi->fat_bits = 32;
- sbi->fat_length = le32_to_cpu(b->fat32.length);
- sbi->root_cluster = le32_to_cpu(b->fat32.root_cluster);
+ sbi->fat_length = bpb.fat32_length;
+ sbi->root_cluster = bpb.fat32_root_cluster;

/* MC - if info_sector is 0, don't multiply by 0 */
- sbi->fsinfo_sector = le16_to_cpu(b->fat32.info_sector);
+ sbi->fsinfo_sector = bpb.fat32_info_sector;
if (sbi->fsinfo_sector == 0)
sbi->fsinfo_sector = 1;

@@ -1478,7 +1655,6 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
if (fsinfo_bh == NULL) {
fat_msg(sb, KERN_ERR, "bread failed, FSINFO block"
" (sector = %lu)", sbi->fsinfo_sector);
- brelse(bh);
goto out_fail;
}

@@ -1501,35 +1677,28 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,

/* interpret volume ID as a little endian 32 bit integer */
if (sbi->fat_bits == 32)
- sbi->vol_id = (((u32)b->fat32.vol_id[0]) |
- ((u32)b->fat32.vol_id[1] << 8) |
- ((u32)b->fat32.vol_id[2] << 16) |
- ((u32)b->fat32.vol_id[3] << 24));
+ sbi->vol_id = bpb.fat32_vol_id;
else /* fat 16 or 12 */
- sbi->vol_id = (((u32)b->fat16.vol_id[0]) |
- ((u32)b->fat16.vol_id[1] << 8) |
- ((u32)b->fat16.vol_id[2] << 16) |
- ((u32)b->fat16.vol_id[3] << 24));
+ sbi->vol_id = bpb.fat16_vol_id;

sbi->dir_per_block = sb->s_blocksize / sizeof(struct msdos_dir_entry);
sbi->dir_per_block_bits = ffs(sbi->dir_per_block) - 1;

sbi->dir_start = sbi->fat_start + sbi->fats * sbi->fat_length;
- sbi->dir_entries = get_unaligned_le16(&b->dir_entries);
+ sbi->dir_entries = bpb.fat_dir_entries;
if (sbi->dir_entries & (sbi->dir_per_block - 1)) {
if (!silent)
fat_msg(sb, KERN_ERR, "bogus directory-entries per block"
" (%u)", sbi->dir_entries);
- brelse(bh);
goto out_invalid;
}

rootdir_sectors = sbi->dir_entries
* sizeof(struct msdos_dir_entry) / sb->s_blocksize;
sbi->data_start = sbi->dir_start + rootdir_sectors;
- total_sectors = get_unaligned_le16(&b->sectors);
+ total_sectors = bpb.fat_sectors;
if (total_sectors == 0)
- total_sectors = le32_to_cpu(b->total_sect);
+ total_sectors = bpb.fat_total_sect;

total_clusters = (total_sectors - sbi->data_start) / sbi->sec_per_clus;

@@ -1538,9 +1707,9 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,

/* some OSes set FAT_STATE_DIRTY and clean it on unmount. */
if (sbi->fat_bits == 32)
- sbi->dirty = b->fat32.state & FAT_STATE_DIRTY;
+ sbi->dirty = bpb.fat32_state & FAT_STATE_DIRTY;
else /* fat 16 or 12 */
- sbi->dirty = b->fat16.state & FAT_STATE_DIRTY;
+ sbi->dirty = bpb.fat16_state & FAT_STATE_DIRTY;

/* check that FAT table does not overflow */
fat_clusters = calc_fat_clusters(sb);
@@ -1549,7 +1718,6 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
if (!silent)
fat_msg(sb, KERN_ERR, "count of clusters too big (%u)",
total_clusters);
- brelse(bh);
goto out_invalid;
}

@@ -1562,8 +1730,6 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
if (sbi->prev_free < FAT_START_ENT)
sbi->prev_free = FAT_START_ENT;

- brelse(bh);
-
/* set up enough so that it can read an inode */
fat_hash_init(sb);
dir_hash_init(sb);
--
1.9.0

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