ptrace(2) debugging

[Konstantin Belousov]

On Sun, Nov 24, 2019 at 12:01:04AM -0600, Kyle Evans wrote:
> Hi,
> 
> I'm working on implementing `reptyr -T` on FreeBSD because I'm pretty
> bad about starting long-running jobs outside of tmux and often desire
> to reparent these jobs into tmux. I've gotten to a point where it's
> getting stuck in waitpid(2) when attempting to work over the session
> leader to ignore SIGHUP. The chain of operations looks roughly like
> this:
> 
> PT_ATTACH -> waitpid -> kill(SIGCONT) -> PT_TO_SCE -> waitpid ->
> PT_TO_SCE -> waitpid
> 
> Each of the waitpids are paired with a PT_LWPINFO. The first waitpid
> observes SIGSTOP. The second waitpid observes SIGCONT. I would expect
> the third to observe PL_FLAG_SCE on ptrace_lwpinfo->pl_flags, but
> instead it actually hangs as the target process is now sleep-inhibited
> and stuck in "pause" wchan.
> 
> I've uploaded a truss excerpt at [0] in case it's helpful -- pid=10204
> is the process I'm reparenting, initially just attached/detached to
> make sure reptyr *can* do this. pid=10187 is the sshd that it's
> running under, and pid=10188 is the shell running under that.
> 
> Anyone have good advice on debugging this? It seems like it might be
> some kind of kernel bug, as it's already done this same dance once
> before when grabbing sshd and my attempts to distill it down to a
> simple test case failed. The FreeBSD part of reptyr needed some love,
> though, so that can't be discounted either.
> 
> Thanks,
> 
> Kyle Evans
> 
> [0] https://people.freebsd.org/~kevans/truss.log
How much work would be to provide a self-contained standalone test ?