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

[Demi Marie Obenour <demiobenour@gmail.com>]

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?

>> The motivation for this message is that systemd-gpt-generator generates
>> mountpoints based on Discoverable Partition Specification GUIDs.  These
>> indicate the mountpoint of the partition but not the filesystem type.
> 
> Filesystem auto-detection is a common feature. The situation is
> similar to having an "auto" fstype in fstab. The systemd-gpt-generator
> simply identifies the partition as "/usr" (or any other mountpoint)
> and the rest is usual scenario.> 
>> If a correctly-produced filesystem image will always continue to be
>> recognized as the correct type, this is fine.  Otherwise, an unlucky
>> combination of writes to the filesystem and filesystem allocation decisions
>> could cause the filesystem to start being mounted as the wrong type, which
>> would be very bad.  According to https://github.com/util-linux/util-linux/issues/1305,
>> libblkid can indeed probe for subsequent superblocks after the first one it
>> finds.
> 
> 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.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)