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Date: Tue, 30 Jun 2020 00:43:23 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Potential deadlock in pthread_kill()

On Mon, Jun 29, 2020 at 09:19:08PM -0700, Hydro Flask wrote:
> Hello all,
> 
> Noticed something while reading some code today. pthread_kill() is
> specified by POSIX to be async signal safe but I noticed that in
> musl's implementation if a signal occurs while the "killlock" is
> held and the signal handler calls pthread_kill() on the same target
> thread, a deadlock will occur. Is this intentional?
> 
>         int pthread_kill(pthread_t t, int sig)
>         {
>                 int r;
>                 LOCK(t->killlock);
>                 r = t->tid ? -__syscall(SYS_tkill, t->tid, sig)
>                         : (sig+0U >= _NSIG ? EINVAL : 0);
>                 UNLOCK(t->killlock);
>                 return r;
>         }
> 
> Thank you for your attention.

Thanks. It looks like this case was overlooked in the pthread_cancel
fix that was commit 060ed9367337cbbd59a9e5e638a1c2f460192f25. The
possibility of blocking signals was even mentioned there but deemed
unnecessary.

A simpler/lighter fix might be, before the lock,

	if (t==__pthread_self())
		return -__syscall(SYS_tkill, t->tid, sig);

since no lock is needed if targeting self; t->tid is necessarily valid
in that case.

One concern I just had was interaction with fork (also a nasty AS-safe
function), but if fork is called from a signal handler during
pthread_kill, it's no different from the signal handler running just
before pthread_kill; the result is targeting an invalid (in the child)
pthread_t, which thereby has undefined behavior. So, while ugly, I
think this is ok.

Note that raise() *does* need to block signals here, because there is
no explicit pthread_t argument and thus the interaction with fork is
well-defined.

Rich

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