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A mount point /mnt/sub is shadowed by another mount point /mnt. Is it always possible to access the mounted filesystem?

Root access is a given. The system is a reasonably recent Linux.

Example scenario: accessing the branches of an overlay root

The basic sequence of operations is:

mount device1 /mnt/sub
mount device2 /mnt

After this /mnt/sub is a file on device2 (if it exists). The question is how to access files on device1.

Some devices can be mounted twice, so mount device1 /elsewhere would work. But this doesn't work for all devices, in particular not for FUSE filesystems.

This differs from the case where a subdirectory is shadowed by a mount point, but the mount point of the subdirectory is itself visible, and a bind mount can create an unobscured view. In the example above, mount --bind / /elsewhere lets us see the /mnt/sub directory from the root filesystem on /elsewhere/mnt/sub, but this question is about accessing the filesystem on device1.

  • A quick test shows the shadowed directory is accessible using /proc/PID/cwd for a process that was running in it before the shadowing mount. That won't do, I guess? – muru May 11 '16 at 1:06
  • @muru That only works if the directory was the current directory of some process. It doesn't work in general for the scenario that inspired this question, which was accessing a branch of an overlay filesystem that was mounted on /. – Gilles May 11 '16 at 7:27
  • Depending on how early it happened, wouldn't init's CWD still be in the old / in that scenario? It wouldn't work in general since there's no way to make sure a process is started in the old mount point, but with /, init is a good candidate, hopefully? – muru May 11 '16 at 7:41
  • @muru Not if the branch is a separate filesystem rather than a part of the root filesystem. – Gilles May 11 '16 at 7:44
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# unshare --mount  # this opens a sub-shell
# cd /
# umount /mnt

do what thou wilt

# exit  # close the sub-shell
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Looks like device1 is completely shadowed, so it makes no difference* whether it is mounted or not. Unmount, then mount it elsewhere to have access to files.

*Well, I guess it makes no difference, unless there are files from device1 still in use since the time it wasn't shadowed yet. Is that the case?

  • device1 can still be in use even though it's shadowed. The most obvious case is indeed that files are still open on it (or even new files get opened because a process has its current directory on it). Another case is the one in the scenario I link to, where device1 is one of the branches in a union mount. – Gilles Apr 20 '16 at 18:06
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The files living on device1 under /mnt/sub are not accessible through that path at all. Remount the device elsewhere, or make sure that the two devices are mounted in the opposite order.

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