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

[Nick Rosbrook]

The upstream consensus was basically "won't fix", and I don't think we
have a kernel/systemd combo in supported releases that would observe
this (focal has 5.4 kernel but systemd 245, and jammy has kernel 5.15).

** Changed in: systemd (Ubuntu)
       Status: Triaged => Won't Fix

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

Status in systemd:
  Fix Released
Status in systemd package in Ubuntu:
  Won't Fix

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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