From: Pali Rohár <pali.rohar@gmail.com>
Date: Mon, 5 Jul 2021 13:09:12 +0000 (+0200)
Subject: libblkid: vfat: Fix reading FAT16 boot label and serial id
X-Git-Tag: v2.38-rc1~372^2~1
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=575b559254a48751e916e8814b2c631335825705;p=thirdparty%2Futil-linux.git

libblkid: vfat: Fix reading FAT16 boot label and serial id

Older FAT16 variants do not have to contain boot label or serial id. Boot
sign 0x28 indicates that only serial id is present and boot sign 0x29
indicates that both boot label and serial id is present. Other boot sign
values (e.g. zero) indicates older FAT16 variant without boot label and
boot sign.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
---

diff --git a/libblkid/src/superblocks/vfat.c b/libblkid/src/superblocks/vfat.c
index 9b23ea9008..72e9b78c6c 100644
--- a/libblkid/src/superblocks/vfat.c
+++ b/libblkid/src/superblocks/vfat.c
@@ -53,6 +53,7 @@ struct vfat_super_block {
 
 /* Yucky misaligned values */
 struct msdos_super_block {
+/* DOS 2.0 BPB */
 /* 00*/	unsigned char	ms_ignored[3];
 /* 03*/	unsigned char	ms_sysid[8];
 /* 0b*/	unsigned char	ms_sector_size[2];
@@ -63,16 +64,21 @@ struct msdos_super_block {
 /* 13*/	unsigned char	ms_sectors[2]; /* =0 iff V3 or later */
 /* 15*/	unsigned char	ms_media;
 /* 16*/	uint16_t	ms_fat_length; /* Sectors per FAT */
+/* DOS 3.0 BPB */
 /* 18*/	uint16_t	ms_secs_track;
 /* 1a*/	uint16_t	ms_heads;
 /* 1c*/	uint32_t	ms_hidden;
-/* V3 BPB */
+/* DOS 3.31 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) */
+/* DOS 3.4 EBPB */
+/* 24*/	unsigned char	ms_drive_number;
+/* 25*/	unsigned char	ms_boot_flags;
+/* 26*/	unsigned char	ms_ext_boot_sign; /* 0x28 - DOS 3.4 EBPB; 0x29 - DOS 4.0 EBPB */
 /* 27*/	unsigned char	ms_serno[4];
+/* DOS 4.0 EBPB */
 /* 2b*/	unsigned char	ms_label[11];
 /* 36*/	unsigned char   ms_magic[8];
+/* padding */
 /* 3e*/	unsigned char	ms_dummy2[0x1fe - 0x3e];
 /*1fe*/	unsigned char	ms_pmagic[2];
 } __attribute__((packed));
@@ -324,8 +330,11 @@ static int probe_vfat(blkid_probe pr, const struct blkid_idmag *mag)
 			vol_label = vol_label_buf;
 		}
 
-		boot_label = ms->ms_label;
-		vol_serno = ms->ms_serno;
+		if (ms->ms_ext_boot_sign == 0x29)
+			boot_label = ms->ms_label;
+
+		if (ms->ms_ext_boot_sign == 0x28 || ms->ms_ext_boot_sign == 0x29)
+			vol_serno = ms->ms_serno;
 
 		blkid_probe_set_value(pr, "SEC_TYPE", (unsigned char *) "msdos",
                               sizeof("msdos"));