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disk: part_dos: correctly detect DOS PBR 

The signature 0x55 0xAA in bytes 510 and 511 of the first sector can either
indicate a DOS partition table of the first sector of a FAT file system.

The current code tries to check if the partition table is valid by looking
at the boot indicator of the partition entries. But first of all it does
not count from 0 to 3 but only from 0 to 2. And second it misses to
increment the pointer for the partition entry.

If it is a FAT file system can be discovered by looking for the text 'FAT'
at offset 0x36 or 'FAT32' at offset 0x52. In a DOS PBR there are no
partition entries, so those bytes are undefined. Don't require the byte at
offset 0x1BE to differ from 0x00 and 0x80.

With the patch the logic is changed as follows:

If the partition table has either an invalid boot flag for any partition or
has no partition at all, check if the first sector is a DOS PBR by looking
at the FAT* signature.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
This commit is contained in:
Heinrich Schuchardt 2019-10-15 20:43:42 +02:00 committed by Tom Rini
parent 79c84de468
commit 34856b0f1c
1 changed files with 26 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

41
disk/part_dos.c
View File

@ -67,28 +67,39 @@ static int test_block_type(unsigned char *buffer)
{
int slot;
struct dos_partition *p;
int part_count = 0;
if((buffer[DOS_PART_MAGIC_OFFSET + 0] != 0x55) ||
(buffer[DOS_PART_MAGIC_OFFSET + 1] != 0xaa) ) {
return (-1);
} /* no DOS Signature at all */
p = (struct dos_partition *)&buffer[DOS_PART_TBL_OFFSET];
for (slot = 0; slot < 3; slot++) {
if (p->boot_ind != 0 && p->boot_ind != 0x80) {
if (!slot &&
(strncmp((char *)&buffer[DOS_PBR_FSTYPE_OFFSET],
"FAT", 3) == 0 ||
strncmp((char *)&buffer[DOS_PBR32_FSTYPE_OFFSET],
"FAT32", 5) == 0)) {
return DOS_PBR; /* is PBR */
} else {
return -1;
}
}
}
return DOS_MBR; /* Is MBR */
}
/* Check that the boot indicators are valid and count the partitions. */
for (slot = 0; slot < 4; ++slot, ++p) {
if (p->boot_ind != 0 && p->boot_ind != 0x80)
break;
if (p->sys_ind)
++part_count;
}
/*
* If the partition table is invalid or empty,
* check if this is a DOS PBR
*/
if (slot != 4 || !part_count) {
if (!strncmp((char *)&buffer[DOS_PBR_FSTYPE_OFFSET],
"FAT", 3) ||
!strncmp((char *)&buffer[DOS_PBR32_FSTYPE_OFFSET],
"FAT32", 5))
return DOS_PBR; /* This is a DOS PBR and not an MBR */
}
if (slot == 4)
return DOS_MBR; /* This is an DOS MBR */
/* This is neither a DOS MBR nor a DOS PBR */
return -1;
}
static int part_test_dos(struct blk_desc *dev_desc)
{