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)
