Children of systemd user instance can't see root user

[Adrian Vovk <adrianvovk@xxxxxxxxx>]

Hello

I've got a problem in my systemd --user instance that I can't quite
grok nor can I explain it very well. Essentially I have no idea what
could possibly be going on. Hoping someone here can help.

Basically, the OS boots and I can log in. Once I do, I experience the
following symptoms:

- Nothing setuid, like /usr/bin/pkexec, works. They all report that
they must be owned by root
- Upon further investigation it turns out all files owned by root/root
is now owned by nobody/nobody
- Dumping the filesystem (unsquashfs -ll /dev/sdX) reveals that the
files are, in fact, owned by root/root on disk
- When executing from outside of my systemd --user instance (i.e. log
in from tty)
  - `id` reports `uid=1000(adrian) gid=100(users) groups=100(users),997(wheel)`
  - `id adrian` reports the same
  - Files that are supposed to be owned by root/root are owned by root/root
  - Files that are supposed to be owned by adrian/users are owned by
adrian/users
- When forked from the systemd --user instance (i.e. in
gnome-terminal, or inside `systemd-run --user -S` on the TTY)
  - `id` reports `uid=1000(adrian) gid=100(users)
groups=100(users),65534(nobody)`
  - `id adrian` reports correctly `uid=1000(adrian) gid=100(users)
groups=100(users),997(wheel)`
  - Files that are supposed to be owned by root/root are owned by nobody/nobody
  - Files that are supposed to be owned by adrian/users are owned by
adrian/users
- If I perform the same experiment but logged in as `root` instead of
`adrian`, the situation reverses: files actually owned by root/root
appear owned by root/root, but files actually owned by adrian/users
appear owned by nobody/nobody

I initially suspected that something is wrong with my PAM
configuration, but it works correctly if I try the commands inside a
shell created with `systemd-run -S --uid=adrian
--property=PAMName=systemd-user`. So, the environment that the systemd
--user instance is executed in is good, but then everything systemd
itself forks off is broken.

Making user@1000.service run w/ debug logging revealed nothing. The
first mention of anything related to this issue is systemd-tmpfiles
quitting because of an unsafe transition from /run/user (owned by
nobody) to /run/user/1000 (owned by adrian).

I'm running a git checkout of systemd 254 at commit
969eb0390f4a94fd95b828ede0588f6c00b293ed.

Does anyone have any idea what could possibly be going on? If you need
more info I'm happy to provide.

Thanks,
Adrian