[Bug 2067922] Re: on WSL systemd status fails output

[Nick Rosbrook]

** Changed in: systemd (Ubuntu)
       Status: Fix Committed => Fix Released

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Title:
  on WSL systemd status fails output

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Noble:
  Fix Released

Bug description:
  [Impact]

  WSL 24.04 / systemd 255.4-1ubuntu8

  * Upstream report: https://github.com/microsoft/WSL/issues/8879
  * Upstream PR: https://github.com/systemd/systemd/pull/32534
  * Upstream commit: https://github.com/systemd/systemd-stable/commit/8e57759d6d80ef772d8e17a4559a6797e09af93a.patch

  In WSL, the cgroup.procs PID list for some reason contain a ton of zeros everywhere from unmapped PIDs.
  Whenever cg_read_pid encounters such a zero, it throws an error. This makes systemd near unusable inside of WSL.

  [Test Plan]

  On WSL running Ubuntu 24.04, run systemctl status. On an affected
  system, there will be an error saying "Failed to dump process list
  [...]". On a patched system, the command should work as expected.

  [Where problems could occur]

  This patch adds a flags parameter to cg_read_pid(), and adds a new
  flag, CGROUP_DONT_SKIP_UNMAPPED. The new default behavior of
  cg_read_pid() is that unmapped pids *will* be skipped. When this flag
  is set in a call to cg_read_pid(), encountering an unmapped pid will
  leave the caller to deal with it.

  There are several places where cg_read_pid() is called in systemd, so
  there are several places where this would come up. If we did see
  problems, it would likely because a caller is skipping the unmapped
  pids when it should not be. For callers that do not skip (i.e. pass
  CGROUP_DONT_SKIP_UNMAPPED), they either handle it cleanly, or
  propagate a different error to signal this case.

  [Original Description]

  WSL 24.04 / systemd 255.4-1ubuntu8

  * Upstream report: https://github.com/microsoft/WSL/issues/8879
  * Upstream PR: https://github.com/systemd/systemd/pull/32534
  * Upstream commit: https://github.com/systemd/systemd-stable/commit/8e57759d6d80ef772d8e17a4559a6797e09af93a.patch

  In WSL, the cgroup.procs PID list for some reason contain a ton of zeros everywhere from unmapped PIDs.
  Whenever cg_read_pid encounters such a zero, it throws an error. This makes systemd near unusable inside of WSL.

  # Test Case
  install Ubuntu WSL 24.04 from the MS STore. This version has systemd already enabled.

  ## Expected Behavior
  systemctl status command to work

  ## Actual Behavior
  toor at XPS:~$ systemctl status
  Failed to dump process list for 'XPS', ignoring: Input/output error
  ● XPS
      State: degraded
      Units: 318 loaded (incl. loaded aliases)
       Jobs: 0 queued
     Failed: 1 units
      Since: Sat 2022-09-24 08:43:15 PDT; 14min ago
    systemd: 251.4-1ubuntu6
    Tainted: cgroupsv1
     CGroup: /

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