When possible, infer DOS 2.x BIOS Parameter Block from block device geometry (for floppies and floppy images). Update in-memory only. We only perform this update when the entire BPB region is zeroed, like produced by DOS 1.x-era FORMAT (and other OEM variations on DOS).
Fixes kernel.org bug #42617. BPB default values are inferred from media size and a table.[0] Media size is assumed to be static for archaic FAT volumes. See also [1]. [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....@gmail.com> --- Diff from v3.14-rc8. Changes since v1: * Check for FAT bootstrap prefix (EB xx 90) to help avoid conflicting with other filesystems * Move default from a switch to a static table Thanks! --- fs/fat/inode.c | 93 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 93 insertions(+) diff --git a/fs/fat/inode.c b/fs/fat/inode.c index 854b578..c31fbdc 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, +}, +{ 0 } }; static int fat_add_cluster(struct inode *inode) { @@ -1246,6 +1284,58 @@ static unsigned long calc_fat_clusters(struct super_block *sb) } /* + * If this FAT filesystem is archaic (lacking a BIOS Parameter Block, ca. DOS + * 1.x), fix it up in-place by creating a DOS 2.x BIOS Parameter Block from + * defaults for the media size. + */ +static void fat_update_archaic_boot_sector(struct super_block *sb, + struct fat_boot_sector *b) +{ + struct fat_floppy_defaults *di; + sector_t bd_sects; + + /* 16-bit DOS 1.x reliably wrote bootstrap short-jmp code */ + if (b->ignored[0] != 0xeb || b->ignored[2] != 0x90) + return; + + /* + * If any value in this region is non-zero, don't assume it is archaic + * DOS. + */ + if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 || + b->reserved != 0 || b->fats != 0 || + get_unaligned_le16(&b->dir_entries) != 0 || + get_unaligned_le16(&b->sectors) != 0 || b->media != 0 || + b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 || + b->secs_track != 0 || b->heads != 0) + return; + + bd_sects = part_nr_sects_read(sb->s_bdev->bd_part); + for (di = floppy_defaults; di->nr_sectors; di++) { + if (di->nr_sectors == bd_sects) + break; + } + if (di->nr_sectors == 0) { + fat_msg(sb, KERN_WARNING, + "DOS volume lacks BPB and isn't a recognized floppy size (%ld sectors)", + (long)bd_sects); + return; + } + + fat_msg(sb, KERN_INFO, + "Volume lacks BPB but looks like archaic DOS; assuming default BPB values"); + + b->sec_per_clus = di->sec_per_clus; + put_unaligned_le16(di->dir_entries, &b->dir_entries); + b->media = di->media; + b->fat_length = cpu_to_le16(di->fat_length); + put_unaligned_le16(SECTOR_SIZE, &b->sector_size); + b->reserved = cpu_to_le16(1); + b->fats = 2; + put_unaligned_le16(bd_sects, &b->sectors); +} + +/* * Read the super block of an MS-DOS FS. */ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat, @@ -1297,6 +1387,8 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat, } b = (struct fat_boot_sector *) bh->b_data; + fat_update_archaic_boot_sector(sb, b); + if (!b->reserved) { if (!silent) fat_msg(sb, KERN_ERR, "bogus number of reserved sectors"); @@ -1364,6 +1456,7 @@ 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; + fat_update_archaic_boot_sector(sb, b); } mutex_init(&sbi->s_lock); -- 1.9.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
