Re: [PATCH] pasta: Save errno on signal handler entry, restore on return when needed

[Paul Holzinger <pholzing@redhat.com>]

On 02/08/2024 15:34, Stefano Brivio wrote:
> Ed reported this:
>
>    # Error: pasta failed with exit code 1:
>    # Couldn't drop cap 3 from bounding set
>    # : No child processes
>
> in a Podman CI run with tests being run in parallel. The error message
> itself, by the way, is fixed by commit 1cd773081f12 ("log: Drop
> newlines in the middle of the perror()-like messages"), but how can we
> possibly get ECHILD as failure code for prctl()?
>
> Well, we don't, but if we exit early enough, pasta_child_handler()
> might run before we're even done with isolation steps, and it calls
> waitid(), which sets errno. We need to restore it before returning
> from the signal handler (if we return after calling functions that
> might set it), as signal-safety(7) also implies:
>
>         Fetching and setting the value of errno is async-signal-safe
>         provided that the signal handler saves errno on entry and
>         restores its value before returning.
>
> Eventually, we'll probably need to switch to signalfd(2) the day we
> want to implement multithreading, but this will do for the moment.
>
> Reported-by: Ed Santiago <santiago@redhat.com>
> Link: https://github.com/containers/podman/issues/23478
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Paul Holzinger <pholzing@redhat.com>