From 78419f261750efecfcde9a3b3c91b828821256ef Mon Sep 17 00:00:00 2001 From: Karel Zak Date: Thu, 1 Oct 2009 15:14:48 +0200 Subject: [PATCH] libblkid: fix non-magic FAT detection MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit On Wed, Sep 30, 2009 at 03:15:41PM +0200, Lawrence Rust wrote: > 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. Note that the msdos/vfat superblock comments has been suggested by Lawrence. Reported-by: Lawrence Rust Signed-off-by: Karel Zak --- shlibs/blkid/src/superblocks/vfat.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) diff --git a/shlibs/blkid/src/superblocks/vfat.c b/shlibs/blkid/src/superblocks/vfat.c index 9c20ebd9..633e009e 100644 --- a/shlibs/blkid/src/superblocks/vfat.c +++ b/shlibs/blkid/src/superblocks/vfat.c @@ -46,7 +46,7 @@ struct vfat_super_block { /* 47*/ unsigned char vs_label[11]; /* 52*/ unsigned char vs_magic[8]; /* 5a*/ unsigned char vs_dummy2[164]; -/*1fe*/ unsigned char vs_pmagic[2]; +/* fe*/ unsigned char vs_pmagic[2]; }; /* Yucky misaligned values */ @@ -58,19 +58,21 @@ 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]; -/* 3d*/ unsigned char ms_dummy2[192]; -/*1fe*/ unsigned char ms_pmagic[2]; +/* 3e*/ unsigned char ms_dummy2[192]; +/* fe*/ unsigned char ms_pmagic[2]; }; struct vfat_dir_entry { @@ -138,6 +140,15 @@ static unsigned char *search_fat_label(struct vfat_dir_entry *dir, int count) static int probe_fat_nomagic(blkid_probe pr, const struct blkid_idmag *mag) { struct msdos_super_block *ms; + unsigned char *buf; + + buf = blkid_probe_get_sector(pr, 0); + if (!buf) + return -1; + + /* Old floppies have a valid MBR signature */ + if (buf[510] != 0x55 || buf[511] != 0xAA) + return 1; ms = blkid_probe_get_sb(pr, mag, struct msdos_super_block); if (!ms) -- 2.39.5