Re: [musl] [bug] Ctrl-Z when process is doing posix_spawn makes the process hard to kill

[Rich Felker <dalias@libc.org>]

On Fri, Jan 17, 2025 at 03:14:03AM +0400, Askar Safin wrote:
> I found a bug both in glibc and musl.
> 
> If a process does posix_spawn+waitpid, then attempting to pause it using Ctrl-Z
> sometimes doesn't work and, worse, makes the process unkillable by usual Ctrl-Z or Ctrl-C.
> 
> The bug is described in full in this glibc issue: https://sourceware.org/bugzilla/show_bug.cgi?id=32565 .
> 
> It is reproducible with musl on the same system I used to reproduce it with glibc (see the link).
> 
> I compiled the code using "x86_64-linux-musl-gcc" wrapper provided by Debian.
> 
> Please, CC me when replying.

OK, I think this should be fixable by, if SIGTSTP is to be SIG_DFL in
the spawned child, setting it to a no-op handler instead of SIG_DFL.
It might actually make sense to just do this for all signals.

Note that SIGSTOP, which is not blockable interceptible or ignorable,
can't be handled this way, but the pid has not yet leaked to anything
at this point, so the only way SIGSTOP can be generated is by a badly
behaved program signaling random pids, which is not a case that needs
to be handled gracefully.

In theory SIGTTIN and SIGTTOU might be hazards too, but I don't think
it's possible for a process to generate them without attempting to
perform io, which the pre-exec child doesn't do. Still handling them
might be a good safety measure in case I'm wrong.

I'll prepare one or more versions of a proposed patch.

Rich