[RFC][PATCH] fs/partitions/msdos: directly check if FAT boot sector

[Frank Seidel]

Hi,

i received a complaint that some FAT formated medias
(e.g. sd memory cards) trigger a "unknown partition table" message even though
there is no partition table and they work correctly, while in general
(when e.g. formated with mkdosfs or even Windows Vista) this message is not
shown.
Currently this seems only to happen when the medias get formatted with
Windows XP (and possibly Win 2000). Then the boot indicator byte contains
garbage (part of text message) and so do the other parts checked by
msdos_paritition which then later triggers this message.

Would the patch below be appropriate to solve this issue/calm those users?
It works ok here for the medias i could test.

Any feedback is very welcome. :-)

Thanks,
Frank
---
From: Frank Seidel <fseidel@xxxxxxx>
Subject: detect fat media without partition table correctly
References: novell bug #364365

Most fat formatted media without partition table contains
zeros in the boot indication and the other tested bytes
and so falls through the checks in msdos_partition, leading
it to return with 1 (all is fine).
But some (e.g. WinXP formatted) fat fomated medias don't
use boot_ind and so the check fails and causes a
"unkown partition table" warning eventhough there is none
and everything would be fine.
This additional check directly verifies if there is a
fat formatted medium without a partition table.

Signed-off-by: Frank Seidel <fseidel@xxxxxxx>
---
 fs/partitions/msdos.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/fs/partitions/msdos.c
+++ b/fs/partitions/msdos.c
@@ -18,7 +18,7 @@
  *
  *  Re-organised Feb 1998 Russell King
  */
-
+#include <linux/msdos_fs.h>
 
 #include "check.h"
 #include "msdos.h"
@@ -419,6 +419,7 @@ int msdos_partition(struct parsed_partit
 	Sector sect;
 	unsigned char *data;
 	struct partition *p;
+	struct fat_boot_sector *fb;
 	int slot;
 
 	data = read_dev_sector(bdev, 0, &sect);
@@ -441,6 +442,12 @@ int msdos_partition(struct parsed_partit
 	 * partition table. Reject this in case the boot indicator
 	 * is not 0 or 0x80.
 	 */
+	fb = (struct fat_boot_sector *) data;
+	if (fb->reserved && fb->fats && FAT_VALID_MEDIA(fb->media)) {
+		printk("\n");
+		put_dev_sector(sect);
+		return 1;
+	}
 	p = (struct partition *) (data + 0x1be);
 	for (slot = 1; slot <= 4; slot++, p++) {
 		if (p->boot_ind != 0 && p->boot_ind != 0x80) {
--
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