[RFC] common: filetype: is_fat_or_mbr() considered harmful

[Sascha Hauer]

On Fri, Oct 09, 2015 at 03:40:37PM +0300, Peter Mamonov wrote:
> On Fri, 9 Oct 2015 10:06:24 +0200
> Sascha Hauer <s.hauer at pengutronix.de> wrote:
> 
> > Hi Peter,
> > 
> > On Wed, Oct 07, 2015 at 09:03:56PM +0300, Peter Mamonov wrote:
> > > Deleted pieces of code detect MBR-containig device as a FAT-type
> > > device, if it's first partition contains a FAT filesystem. So, one
> > > can mount the first partition of a hard drive containing FAT FS
> > > using the following command: barebox: mount /dev/ata0.0 /mnt/0
> > > as well as this one:
> > > 	barebox: mount /dev/ata0 /mnt/1
> > > Both commands mount the same FS.
> > > 
> > > This behaviour causes automount (mount -a) to mount FAT FS
> > > on a first partition twice:
> > > 	barebox: mount
> > > 	none on / type ramfs
> > > 	none on /dev type devfs
> > > 	/dev/ata0 on /mnt/ata0 type fat
> > > 	/dev/ata0.0 on /mnt/ata0.0 type fat
> > > 	/dev/ata0.1 on /mnt/ata0.1 type ext4
> > 
> > This is_fat_or_mbr mechanism never worked very well and had funny side
> > effects. Would be nice to get rid of it.
> > Simply removing this option is not a solution though, we have to find
> > a proper way to keep the current feature and make it more sane.
> 
> Ok, the patch comment is misleading a bit. I do not propose to get rid
> of the is_fat_or_mbr() completely. However, I do not see the point
> to check for a FAT FS, after the device was correctly detected as an
> MBR-type device:
> 
> enum filetype file_name_detect_type(const char *filename)
>        ... 
>        type = file_detect_type(buf, ret);
>  
>        if (type == filetype_mbr) {
>                /*
>                 * Get the first partition start sector
>                 * and check for FAT in it
>                 */
>                is_fat_or_mbr(buf, &bootsec);
>                ret = lseek(fd, (bootsec) * 512, SEEK_SET);
>                if (ret < 0)
>                        goto err_out;
>                ret = read(fd, buf, 512);
>                if (ret < 0)
>                        goto err_out;
>                type = is_fat_or_mbr((u8 *)buf, NULL);
>        }
> 
> 
> The deleted code snippet was introduced by this patch:
> 
> commit 010ee209b75c5732ae4144e3ee9ce14158193c1f
> Author: Franck Jullien <franck.jullien at gmail.com>
> Date:   Wed Sep 19 13:09:01 2012 +0200
> 
>     filetype: Improve FAT detection
>     
>     We may have some disk with MBR as a first sector. In this case, the
>     current FAT check returns an error. However, the FAT sector exist
>     and the MBR can tell us where it is.
>     
>     This patch add to file_name_detect_type function the ability to find
>     the FAT boot sector on the first sector of the first partition in
>     case it is not on sector 0.
>     
>     It also introduce is_fat_or_mbr to check if a buffer is a FAT boot
>     or MBR sector
>     
>     Signed-off-by: Franck Jullien <franck.jullien at gmail.com>
>     Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
> 
> According to the patch message it was introduced to workaround FAT
> detection. However, after deletion of the code I'm still able to detect
> and mount FAT-containig partiotions.

But can you mount /dev/disk0 if this disk contains a partition table and
the FAT is on /dev/disk0.0? This is what the patch is about. The problem
the patches solved is that when you plug in a USB drive then you don't
know whether a FAT is directly on the device or if the device is
partitioned. You want to be able to mount both ways with the same
command, so no matter if the FAT is on /dev/disk0 or /dev/disk0.0 you
can mount both using /dev/disk0.
The way this problem is solved currently is not very good, we should
find a better way.

Sascha

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