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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 28 Feb 2008 13:32:01 +0100
Frank Seidel <fseidel@suse.de> wrote:

> 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) {

fs/partitions/msdos.c: In function 'msdos_partition':
fs/partitions/msdos.c:446: warning: comparison is always true due to limited range of data type

didn't you get this?

The reason is that FAT_VALID_MEDIA() is bogus:

#define FAT_VALID_MEDIA(x)      ((0xF8 <= (x) && (x) <= 0xFF) || (x) == 0xF0)

It appears that the on-disk field which FAT_VALID_MEDIA() is designed to
test is only 8-bit, so the comparison with 0xff is pointless.  The only
existing caller of FAT_VALID_MEDIA() cheats by copying the value into a
local unsigned int first.

So I'll leave things as they are for now, but I'd ask that someone can
confirm that we should simply remove the 0xff test from FAT_VALID_MEDIA()?