lkml.org 
[lkml]   [2014]   [Mar]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] fs: FAT: Add support for DOS 1.x formatted volumes
Date
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@gmail.com>
---
Diff is from Linus' v3.14-rc8.

I am a kernel newbie, apologies in advance for newbie mistakes. Let me know
what is wrong and I will fix it and learn.

Thanks!
---
fs/fat/inode.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 62 insertions(+)

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 854b578..f3686d6 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -35,6 +35,8 @@
#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;

@@ -1246,6 +1248,63 @@ 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)
+{
+ sector_t bd_sects;
+
+ 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);
+ switch (bd_sects) {
+ case 160 * KB_IN_SECTORS:
+ b->sec_per_clus = 1;
+ put_unaligned_le16(64, &b->dir_entries);
+ b->media = 0xFE;
+ b->fat_length = cpu_to_le16(1);
+ break;
+ case 180 * KB_IN_SECTORS:
+ b->sec_per_clus = 1;
+ put_unaligned_le16(64, &b->dir_entries);
+ b->media = 0xFC;
+ b->fat_length = cpu_to_le16(2);
+ break;
+ case 320 * KB_IN_SECTORS:
+ b->sec_per_clus = 2;
+ put_unaligned_le16(112, &b->dir_entries);
+ b->media = 0xFF;
+ b->fat_length = cpu_to_le16(1);
+ break;
+ case 360 * KB_IN_SECTORS:
+ b->sec_per_clus = 2;
+ put_unaligned_le16(112, &b->dir_entries);
+ b->media = 0xFD;
+ b->fat_length = cpu_to_le16(2);
+ break;
+ default:
+ fat_msg(sb, KERN_WARNING,
+ "DOS volume lacks BPB and isn't a recognized floppy size (%ld sectors)",
+ (long)bd_sects);
+ return;
+ }
+
+ 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 +1356,8 @@ 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 +1425,7 @@ 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


\
 
 \ /
  Last update: 2014-03-29 00:01    [W:0.080 / U:5.828 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site