on WSL systemd status fails output

[Jean-Baptiste Lallement]

[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@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: /

[Jean-Baptiste Lallement (jibel)]

upstream patch

[Jean-Baptiste Lallement (jibel)]

debdiff with patch applied

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "8e57759d6d80ef772d8e17a4559a6797e09af93a.patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]

[Nick Rosbrook (enr0n)]

The fix for this is included in v256, which is currently in oracular-proposed. So we just need to upload the fix to noble.

[Nick Rosbrook (enr0n)]

@jibel - thanks for the patch. I already had it prepped in git, so I will upload that version.