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

[Andreas Dilger <adilger@sun.com>]

On Feb 28, 2008  13:32 +0100, Frank Seidel wrote:
> 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.

There is also a similar complaint from users who format ext3 directly on
a disk/LUN without a partition table, because the partition table offset
negatively impacts the performance of the filesystem (causing unaligned
IO on a RAID device).  Turning off this message is very good.

> ---
> From: Frank Seidel <fseidel@suse.de>
> 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@suse.de>
> ---
>  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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Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.