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

[Sascha Hauer <s.hauer@pengutronix.de>]

On Mon, Oct 12, 2015 at 01:27:35PM +0300, Peter Mamonov wrote:
> On Fri, 9 Oct 2015 18:11:44 +0200
> Sascha Hauer <s.hauer@pengutronix.de> wrote:
> 
> > 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@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@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@gmail.com>
> > >     Signed-off-by: Sascha Hauer <s.hauer@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?
> 
> No. This is actually the purpose of my patch, since I don't want
> "mount -a" to mount the same partition (FAT on /dev/disk0.0) twice.

I know, and this is valid. It just conflicts with what Franck wants. He
just wants to mount a USB device without having to know if the FAT is on
the raw device or on the first partition.

> 
> > 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.
> 
> Ok. So what is the preferred way to prevent "mount -a" from mounting
> /dev/disk0 and /dev/disk0.0 at the same time?

Sorry, I do not have a solution currently. I'll have a look into it.

Sascha

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