block/partition/msdos: detect AIX formatted disks even without 55aa

AIX formatted disks do not always have the MSDOS 55aa signature.
This happens e.g. for unbootable AIX disks.

Up to now, such disks were not recognized as AIX disks, because of the
missing 55aa.  Fix that by inverting the two tests.  Let's first
check for the AIX magic strings, and only if that fails check for
the MSDOS magic word.

Signed-off-by: Philippe De Muyter <[email protected]>
Cc: Andreas Mohr <[email protected]>
Cc: OGAWA Hirofumi <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Olaf Hering <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

block/partitions/msdos.c

@@ -455,14 +455,19 @@ int msdos_partition(struct parsed_partitions *state)
 	data = read_part_sector(state, 0, &sect);
 	if (!data)
 		return -1;
-	if (!msdos_magic_present(data + 510)) {
+
+	/*
+	 * Note order! (some AIX disks, e.g. unbootable kind,
+	 * have no MSDOS 55aa)
+	 */
+	if (aix_magic_present(state, data)) {
 		put_dev_sector(sect);
+		strlcat(state->pp_buf, " [AIX]", PAGE_SIZE);
 		return 0;
 	}
 
-	if (aix_magic_present(state, data)) {
+	if (!msdos_magic_present(data + 510)) {
 		put_dev_sector(sect);
-		strlcat(state->pp_buf, " [AIX]", PAGE_SIZE);
 		return 0;
 	}