Re: Generic way to detect qemu linux-user emulation

[Lawrence Hunter <lawrence.hunter@codethink.co.uk>]

Hello,

I don't have anything to offer around cython but I am also noticing this 
posix_spawn issue with systemd >= tag v255 running inside a cross-arch 
container. systemd specifically is fine under full QEMU or on an ARM64 
host but when ran cross and therefore through qemu-user I see systemd 
failing to fork off any processes i.e. immediately after posix_spawn the 
child process is dying with SIGCHLD.

```
[  OK  ] Reached target rpcbind.target.
Wed 2026-03-25 14:08:10 UTC src/core/service.c:1627: rpcbind.service: 
Will spawn child (service_enter_start): /usr/sbin/rpcbind
Wed 2026-03-25 14:08:10 UTC src/core/service.c:1658: rpcbind.service: 
Passing 3 fds to service
Wed 2026-03-25 14:08:11 UTC src/core/execute.c:345: rpcbind.service: 
About to execute: /usr/sbin/rpcbind "\$RPCBIND_OPTIONS" -w -f
Wed 2026-03-25 14:08:11 UTC src/core/execute.c:472: rpcbind.service: 
Forked /usr/sbin/rpcbind as 37
Wed 2026-03-25 14:08:11 UTC src/shared/fdset.c:71: Closing set fd 40 
(socket:[1181504])
Wed 2026-03-25 14:08:11 UTC src/shared/fdset.c:71: Closing set fd 39 
(socket:[1181503])
Wed 2026-03-25 14:08:11 UTC src/shared/fdset.c:71: Closing set fd 38 
(socket:[1181501])
Wed 2026-03-25 14:08:11 UTC src/shared/fdset.c:71: Closing set fd 41 
(socket:[1193529])
Wed 2026-03-25 14:08:11 UTC src/core/service.c:1269: rpcbind.service: 
Changed dead -> start
          Starting rpcbind.service...
Wed 2026-03-25 14:08:11 UTC src/basic/log.c:1456: Received SIGCHLD from 
PID 37 (11).
Wed 2026-03-25 14:08:11 UTC src/core/manager.c:2804: Child 37 (11) died 
(code=exited, status=1/FAILURE)
Wed 2026-03-25 14:08:11 UTC src/core/manager.c:2769: init.scope: Child 
37 belongs to init.scope.
Wed 2026-03-25 14:08:11 UTC src/core/manager.c:2769: rpcbind.service: 
Child 37 belongs to rpcbind.service.
Wed 2026-03-25 14:08:11 UTC src/core/unit.c:6066: rpcbind.service: Main 
process exited, code=exited, status=1/FAILURE
Wed 2026-03-25 14:08:11 UTC src/core/unit.c:6024: rpcbind.service: 
Failed with result 'exit-code'.
Wed 2026-03-25 14:08:11 UTC src/core/service.c:1980: rpcbind.service: 
Service will not restart (restart setting)
Wed 2026-03-25 14:08:11 UTC src/core/service.c:1269: rpcbind.service: 
Changed start -> failed
Wed 2026-03-25 14:08:11 UTC src/core/job.c:997: rpcbind.service: Job 91 
rpcbind.service/start finished, result=failed
[FAILED] Failed to start rpcbind.service.
See 'systemctl status rpcbind.service' for details.
```

I have narrowed it down to 
https://github.com/systemd/systemd/commit/bb5232b6a3b8af075ee06cc87416e5f49a6170d3 
as being the change which breaks qemu-user mode with a bisect, and the 
key bit seems to be around 
https://github.com/systemd/systemd/blob/7e37e01768e2f223750ead2c9e08b4490243b8d1/src/basic/process-util.c#L2110 
other fixes I found online around credentials or protect/restrictions do 
not seem to make a difference.

This can be reproduced by:
```
docker run --privileged --platform linux/arm64 -it fedora:40 /bin/bash
$ dnf install -y systemd
$ exec /sbin/init
```

Any help around this issue would be greatly helpful too.

Best,
Lawrence

On 2025-03-18 10:18, Andreas Schwab wrote:
> Is there a generic way for a program to detect that is it being run
> inside the linux-user emulation?
> 
> The purpose for that would be to work around limitations of the
> emulation, like CLONE_VFORK being unsupported.  For example, python >=
> 3.13 needs to avoid using posix_spawn in that case, because the
> emulation of CLONE_VFORK as a true fork makes it impossible for it to
> report errors back to the parent process.
> 
> --
> Andreas Schwab, SUSE Labs, schwab@suse.de
> GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA 
> B9D7
> "And now for something completely different."