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From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: [musl] Potential deadlock in pthread_kill()
Date: Tue, 30 Jun 2020 00:43:23 -0400	[thread overview]
Message-ID: <20200630044323.GD6430@brightrain.aerifal.cx> (raw)
In-Reply-To: <0217b8838100175725993b0ed0114ee7@thelig.ht>

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

  reply	other threads:[~2020-06-30  4:43 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-30  4:19 Hydro Flask
2020-06-30  4:43 ` Rich Felker [this message]
2020-06-30  6:19   ` Hydro Flask
2020-06-30  9:26     ` Rich Felker
2020-06-30 14:58       ` Markus Wichmann
2020-06-30 16:28         ` Hydro Flask
2020-06-30 19:28           ` Dmitry Samersoff
2020-06-30 19:54             ` Rich Felker
2020-06-30 21:00               ` Hydro Flask
2020-07-06 22:00                 ` Rich Felker
2020-07-06 22:14                   ` Hydro Flask
2020-07-06 22:22                     ` Rich Felker
2020-07-06 22:37                       ` Hydro Flask

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