systemd-sysusers fails with "Failed to set up credentials: Invalid argument" on s390x

[DaanDeMeyer]

systemd version the issue has been seen with

main branch

Used distribution

Fedora

Linux kernel version used (uname -a)

5.14.18-300.fc35.x86_64

CPU architecture issue was seen on

s390x

Expected behaviour you didn't see

systemd-sysusers succeeds

Unexpected behaviour you saw

systemd-sysusers fails with "systemd-sysusers.service: Failed to set up credentials: Invalid argument"

Steps to reproduce the problem

Try to boot a s390x mkosi container

Additional program output to the terminal or log subsystem illustrating the issue

Failed to fork: Invalid argument
systemd-sysusers.service: Failed to set up credentials: Invalid argument

This happens using qemu user mode emulation. Can't say for sure it also happens on a real s390x machine. I confirmed it's clone() that fails with EINVAL when CLONE_NEWNS is set. If I remove CLONE_NEWNS the call to clone() succeeds.

[poettering]

uh, user mode emulation in qemu is notoriously poor. They need to hook up every single syscall individually for each arch, and there are plenty of mistakes. I'd always assume it's a problem in qemu rather than our code unless proven otherwise...

[DaanDeMeyer]

It seems so. I found https://github.com/qemu/qemu/blob/14d02cfbe4adaeebe7cb833a8cc71191352cf03b/linux-user/syscall.c#L188 which mentions they cannot support CLONE_NEWNS.

I do wish they'd gone for one of the operation not supported error codes in that case but let's close here as it's not systemd's problem.

[poettering]

They should just add a fallback: if the flags combo looks like something that glibc doesn't support they should just call the syscall directly anyway, since apparently that's what the app did.