Re: Ensuring that mount(8) will always interpret a filesystem correctly

On Tue, Dec 10, 2024 at 12:11:49AM GMT, Demi Marie Obenour wrote:
> On 12/9/24 5:26 AM, Karel Zak wrote:
> > 
> >  Hi Demi,
> > 
> > On Sat, Dec 07, 2024 at 08:45:32PM GMT, Demi Marie Obenour wrote:
> >> Is there a guarantee that if all data before the filesystem superblock is
> >> zero, and that the filesystem never writes to this region, libblkid (and
> >> thus, presumably, mount(8)) will always mount the filesystem with the
> >> correct filesystem type, even if e.g. someone writes a file containing
> >> a superblock of a different filesystem and the filesystem happens to put
> >> it where that superblock is valid?
> > 
> > the libblkid library offers multiple modes, with "safe mode" being the
> > default for detecting filesystems. In this mode, the library checks
> > for any additional valid superblocks on the device. There are
> > exceptions for certain filesystems on CD/DVD media (such as udf and
> > iso), but for regular filesystems, sharing the same device is not
> > allowed.
> > 
> > There is also an option to specify that a superblock is only valid if
> > no other area is using it (using blkid_probe_set_wiper() and
> > blkid_probe_use_wiper()). However, this is only used for LVM and
> > bcache.
> > 
> > The library does not require that there are zeros before the
> > superblock, as not all mkfs-like programs zero out all areas.
> > 
> > In recent years, there have been no reports of collisions. In the
> > entire history of the library, the only collisions I can recall are
> > with swap areas and luks, and occasionally with poorly detected FAT
> > filesystems (due to the messy design of FAT).
> 
> Was https://github.com/util-linux/util-linux/issues/1305 a
> collision between ZFS and ext4?

Yes, but in this case, ZFS was incorrectly detected. As you can see
from the bug report, blkid ended with an "ambiguous result" error.

> > I believe the situation would be the same even without the
> > Discoverable Partition Specification. The kernel always divides the
> > whole disk into partitions, and libblkid/mount utilizes these
> > partitions. Therefore, the filesystems are automatically separated by
> > the partition table.
> 
> /etc/fstab provides an explicit filesystem type.  The Discoverable
> Partition Specification doesn't.

You can use the "auto" file system type in fstab. It is also common
for people to not use the "-t <type>" option on the mount(8) command
line.

However, if you are paranoid, then specifying the file system type in
fstab and avoiding Discoverable Partitions is a good choice.

    Karel

-- 
 Karel Zak  <[email protected]>
 http://karelzak.blogspot.com