I recently came across a problem with an ext2 grub boot partition that wasn't
being recognised by blkid. Turns out that libblkid was detecting both FAT
and ext2 file systems. The partition was originally formatted as FAT (by
WinNT) and then reformatted as ext2 by gparted-livecd-0.3.4-11.
The problem is with shlibs/blkid/src/probers/vfat.c. At the end of this file
the struct blkid_idinfo is declared with some magic search strings. In
particular, 2 patterns are defined to match the single byte jmp/jxx opcodes
at the start of a DOS boot sector. Because the partition was once formatted
as a bootable DOS partition it matches these patterns and consequently the
function probe_vfat is called. For these simple pattern matches the function
probe_fat_nomagic is called to filter out false positives. In normal
circumstances only MSDOS 2 and earlier floppies should be detected by this
function. A very important feature of these disks is the boot signature -
bytes 0x55, 0xaa at the end of the 1st sector which indicate to the BIOS that
the disk is bootable. All MSDOS floppies have these bytes but the function
probe_fat_nomagic doesn't check for them.
When I used gparted to reformat this DOS partition to ext2 it overwrote the
MSDOS system name (offset 3), the FAT16 magic signature (offset 0x36) and the
boot signature at offset 0x1fe but left the initial jmp opcode and BPB
intact. This causes the simple pattern match and BPB check to falsely detect
a FAT filesystem.
To fix this I wrote this small patch to check for the 0x55, 0xaa bytes at the
end of the sector:
--- util-linux-ng-2.16.1/shlibs/blkid/src/probers/vfat.c 2009-08-21
15:59:30.000000000 +0200
+++ util-linux-ng-2.16.1-new/shlibs/blkid/src/probers/vfat.c 2009-09-30
14:12:40.000000000 +0200
@@ -58,14 +58,16 @@ struct msdos_super_block {
/* 0e*/ uint16_t ms_reserved;
/* 10*/ uint8_t ms_fats;
/* 11*/ unsigned char ms_dir_entries[2];
-/* 13*/ unsigned char ms_sectors[2];
+/* 13*/ unsigned char ms_sectors[2]; /* =0 iff V3 or later */
/* 15*/ unsigned char ms_media;
-/* 16*/ uint16_t ms_fat_length;
+/* 16*/ uint16_t ms_fat_length; /* Sectors per FAT */
/* 18*/ uint16_t ms_secs_track;
/* 1a*/ uint16_t ms_heads;
/* 1c*/ uint32_t ms_hidden;
-/* 20*/ uint32_t ms_total_sect;
-/* 24*/ unsigned char ms_unknown[3];
+/* V3 BPB */
+/* 20*/ uint32_t ms_total_sect; /* iff ms_sectors == 0 */
+/* V4 BPB */
+/* 24*/ unsigned char ms_unknown[3]; /* Phys drive no., resvd, V4 sig (0x29)
*/
/* 27*/ unsigned char ms_serno[4];
/* 2b*/ unsigned char ms_label[11];
/* 36*/ unsigned char ms_magic[8];
@@ -143,6 +145,10 @@ static int probe_fat_nomagic(blkid_probe
if (!ms)
return -1;
+ /* Old floppies have a valid MBR signature */
+ if (ms->ms_pmagic[0] != 0x55 || ms->ms_pmagic[1] != 0xAA)
+ return 1;
+
/* heads check */
if (ms->ms_heads == 0)
return 1;
Signed-off-by: Lawrence Rust
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