[Bug 1918328] [NEW] kernel emits error "Bad value for 'hidepid'" with new systemd on old kernel

[Dan Streetman]

Public bug reported:

When using a newer systemd (>=247) and older kernel (<5.8), systemd
attempts to mount /proc using a value for the 'hidepid' parameter that
the older kernel doesn't understand, which causes the kernel to emit a
warning message like:

[4083297.870684] proc: Bad value for 'hidepid'

This is harmless, as systemd falls back to the 'old' mount method,
however the logged error message can be confusing.

Since there is no ubuntu release where the newer systemd might be
directly used with a kernel older than 5.8, this can only reasonably
happen in a container on a older ubuntu release. For example, on a focal
release with the 5.4 kernel, a hirsute container can be created, and
when it starts up the host dmesg will log the kernel error messages
shown above.

Note that if using a lxd container, as there is a bug where lxd prevents
mounting any fs (which mostly breaks systemd), you must set the
'security.nesting' parameter for the lxd container, e.g.:

$ lxc launch ubuntu-daily:hirsute test-h
$ lxc set config test-h security.nesting true


Note that this hasn't been fixed upstream, and as it's purely cosmetic, it's possible it won't get fixed upstream.

** Affects: systemd
     Importance: Unknown
         Status: Unknown

** Affects: systemd (Ubuntu)
     Importance: Wishlist
         Status: New

** Bug watch added: github.com/systemd/systemd/issues #16896
   https://github.com/systemd/systemd/issues/16896

** Also affects: systemd via
   https://github.com/systemd/systemd/issues/16896
   Importance: Unknown
       Status: Unknown

** Changed in: systemd (Ubuntu)
   Importance: Undecided => Wishlist

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https://bugs.launchpad.net/bugs/1918328

Title:
  kernel emits error "Bad value for 'hidepid'" with new systemd on old
  kernel

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

Bug description:
  When using a newer systemd (>=247) and older kernel (<5.8), systemd
  attempts to mount /proc using a value for the 'hidepid' parameter that
  the older kernel doesn't understand, which causes the kernel to emit a
  warning message like:

  [4083297.870684] proc: Bad value for 'hidepid'

  This is harmless, as systemd falls back to the 'old' mount method,
  however the logged error message can be confusing.

  Since there is no ubuntu release where the newer systemd might be
  directly used with a kernel older than 5.8, this can only reasonably
  happen in a container on a older ubuntu release. For example, on a
  focal release with the 5.4 kernel, a hirsute container can be created,
  and when it starts up the host dmesg will log the kernel error
  messages shown above.

  Note that if using a lxd container, as there is a bug where lxd
  prevents mounting any fs (which mostly breaks systemd), you must set
  the 'security.nesting' parameter for the lxd container, e.g.:

  $ lxc launch ubuntu-daily:hirsute test-h
  $ lxc set config test-h security.nesting true

  
  Note that this hasn't been fixed upstream, and as it's purely cosmetic, it's possible it won't get fixed upstream.

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