How is findmnt able to list bind mounts?

Question

A lot of people keep saying that Linux does not keep information about bind mounts, so there is no way to get a list of them and their sources. Here are some examples:

• from one of the the comments here:

  IIRC this information is not kept anywhere: after mount --bind, the two copies are equivalent, there isn't one that's more “original” than the other. After all there could be no original if you'd already unmounted /mnt.

• from an answer on this site:

  So the only way to remember what mounts were bind mounts is the log of mount commands left in /etc/mtab. A bind mount operation is indicated by the bind mount option (which causes the filesystem type to be ignored). But mount has no option to list only filesystems mounted with a particular set of sets of options.

• from a Debian bug report:

  This is intentional. Both mount points are fully equal in all ways so the kernel does not keep any flags to differentiate them.

The above is nonsense though. The tool findmnt is able to list the source paths of bind mounts (in the form of device[source-path]; I'm also trying to get it to list just the source path and not the device). If the Linux kernel is to maintain a bind mount, then that information has to be stored somewhere, otherwise it couldn't know that /home is bound to /users. So where is this data? Is it stored in some obscure region in RAM? Does findmnt look in /proc somewhere?

Answer 1

You've misunderstood a little; the two mount points are equal in terms of permissions, flags, etc because the bind effectively redirects access from one path to another. But they are still distinct.

If you look at /proc/self/mountinfo you'll see the kernel view of the mount world for this process (namespaces make things more complicated; there's not just one view of the mount table).

man 5 proc will explain the format of this file, but you can see the tree hierarchy and where bind mounts have their "parent". This is the file that findmnt parses.

Answer 2

Linux does not keep the information about which mount was a bind mount. It does keep information about all mounts including bind mounts.

It's quite similar to hard links. Mounts link to filesystems like filenames link to inodes. The only differences are that mounts also have per-mountpoint flags and may refer to a subdirectory of the target filesystem instead of the filesystem root.

When you create a hard link the file system doesn't save which filename was the original one and which was the hard link. Both simply refer to the same inode. If you unlink the original file the situation is indistinguishable from if you directly created the file with the second filename.

Back to bind mounts: The kernel keeps a table that contains the filesystem (identified by a major:minor number pair), the mountpoint, the path relative to the filesystem root and some flags. You can access this list by looking at /proc/self/mountinfo. (It gets more complicated when namespaces are involved, as @stephen-harris mentioned). findmnt parses this list.

If your root is /dev/sda1 with the major:minor 8:1 and you run mount --bind /a /b /proc/self/mountinfo will contain lines similar to this:

1 0 8:1 / / rw - ext4 /dev/sda1 rw,errors=remount-ro
2 1 8:1 /a /b rw - ext4 /dev/sda1 rw,errors=remount-ro

If your /home is /dev/sda2 with the major:minor 8:2 and you run mount --bind /home /users it will look like this:

1 0 8:1 / / rw - ext4 /dev/sda1 rw,errors=remount-ro
2 1 8:2 / /home rw - ext4 /dev/sda2 rw
3 1 8:2 / /users rw - ext4 /dev/sda2 rw

The relevant columns for your question are the third, the fourth and the fifth ones. These are the filesystem id (for real filesystems it's the same as the device major:minor; for virtual filesystems like tmpfs it's [0:counter]), the path relative to the filesystem root that is bound to the mountpoint (usually / for normal mounts, may be anything for bind mounts) and the mountpoint.
For the meaning of remaining columns see the Linux kernel documentation.

findmnt calls the source path relative to the filesystem root "FSROOT". You can use findmnt -o TARGET,FSROOT to get it. If you want the absolute source path you probably need to parse /proc/self/mountinfo by yourself and combine the information about the mounts for the same filesystem.

For more information see my answer to "List only bind mounts".