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 <kzak@xxxxxxxxxx> http://karelzak.blogspot.com
