[Bug 1979952] Re: Bind mount to NFS mount fails on Ubuntu 22.04

[Philipp Wendler]

The fix https://github.com/systemd/systemd/pull/24122 is now merged
upstream. The change should be easy to backport, is it possible that
this could be done for Ubuntu?

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Title:
  Bind mount to NFS mount fails on Ubuntu 22.04

Status in systemd:
  Unknown
Status in systemd package in Ubuntu:
  New

Bug description:
  I have the following setup:
  - /data is an NFS mount.
  - /home is a bind mount to /data/home.

  Configured in /etc/fstab with these lines:

  $FILE_SERVER:/data /data nfs defaults 0 0
  /data/home /home none bind,_netdev,x-systemd.requires-mounts-for=/data 0 0

  This has worked with the same configuration for me on at least Ubuntu 18.04 and Ubuntu 20.04, but on Ubuntu 22.04 the mount of /home fails when attempted by systemd.
  So not only is /home not mounted after boot, but also when I run "sudo systemctl start home.mount", it fails.

  The journal entries are this:

  sudo[1316]:  wendler : TTY=pts/0 ; PWD=/home/wendler ; USER=root ; COMMAND=/usr/bin/systemctl start home.mount
  sudo[1316]: pam_unix(sudo:session): session opened for user root(uid=0) by wendler(uid=1000)
  systemd[1]: home.mount: Directory /home to mount over is not empty, mounting anyway.
  systemd[1]: home.mount: Failed to make bind mount source '/data/home': Permission denied
  systemd[1]: home.mount: Failed to run 'mount' task: Permission denied
  systemd[1]: home.mount: Failed with result 'resources'.
  systemd[1]: Failed to mount /home.

  However, when I run "sudo mount /home" it works.

  Now the weird thing is that after I have mounted /home manually once
  and unmounted it again, then "sudo systemctl start home.mount"
  suddenly also works! But of course only until the next reboot.

  And even if I just do "ls /data" once (either as root or as my user),
  it also makes "sudo systemctl start home.mount" start working!

  To be fully clear:

  - Booting the system (/home fails to mount)
  - sudo systemctl start home.mount  # fails
  - sudo ls /data  # shows correct output
  - sudo systemctl start home.mount  # works!

  
  Additional information:

  The NFS export for /data has root_squash set and if I remove this
  option, the bind mount of /home works as it should. However, both
  /data and /data/home have mode rwxr-xr-x, so root is able to enter and
  read these directories even despite root_squash.

  It is not a network or mount-order problem. Not only is the bind mount
  to /home correctly attempted after /data is mounted during boot,
  remember that the bind mount also fails when I try it with "sudo
  systemctl start home.mount" minutes afterwards.

  Versions (system is a recent installation with all updates applied):
  - Ubuntu 22.04
  - Linux 5.15.0-40-generic
  - systemd 249.11-0ubuntu3.3
  - util-linux 2.37.2-4ubuntu3

  This appears on both Ubuntu 22.04 machines that I have (a hardware
  machine and a VM).

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