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Message-Id: <1396120235-27844-1-git-send-email-cse.cem@gmail.com>
Date: Sat, 29 Mar 2014 12:10:35 -0700
From: Conrad Meyer <cemeyer@...edu>
To: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc: linux-kernel@...r.kernel.org, Mark <markk@...ra.co.uk>,
Conrad Meyer <cse.cem@...il.com>
Subject: [PATCH v2] fs: FAT: Add support for DOS 1.x formatted volumes
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.cem@...il.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
--
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