linux-kernel - Re: strace lockup when tracing exec in go

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Message-ID: <44178008-720e-0858-c9e8-d23d2087dcc7@suse.com>
Date:   Thu, 22 Sep 2016 18:20:49 +1000
From:   Aleksa Sarai <asarai@...e.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        strace-devel@...ts.sourceforge.net, Oleg Nesterov <oleg@...hat.com>
Subject: Re: strace lockup when tracing exec in go

> So I've stared into do_notify_parent some more and the following was
> just very confusing
>
> 	if (!tsk->ptrace && sig == SIGCHLD &&
> 	    (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
> 	     (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))) {
> 		/*
> 		 * We are exiting and our parent doesn't care.  POSIX.1
> 		 * defines special semantics for setting SIGCHLD to SIG_IGN
> 		 * or setting the SA_NOCLDWAIT flag: we should be reaped
> 		 * automatically and not left for our parent's wait4 call.
> 		 * Rather than having the parent do it as a magic kind of
> 		 * signal handler, we just set this to tell do_exit that we
> 		 * can be cleaned up without becoming a zombie.  Note that
> 		 * we still call __wake_up_parent in this case, because a
> 		 * blocked sys_wait4 might now return -ECHILD.
> 		 *
> 		 * Whether we send SIGCHLD or not for SA_NOCLDWAIT
> 		 * is implementation-defined: we do (if you don't want
> 		 * it, just use SIG_IGN instead).
> 		 */
> 		autoreap = true;
> 		if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN)
> 			sig = 0;
> 	}
>
> it tries to prevent from what I am seeing in a way. If the SIGCHLD is
> ignored then it just does autoreap and everything is fine. But this
> doesn't seem to be the case here. In fact we are not sending the signal
> because sig_task_ignored is true resp. sig_handler_ignored which can
> fail even for handler == SIG_DFL && sig_kernel_ignore() and SIGCHLD
> seems to be in SIG_KERNEL_IGNORE_MASK. So I've tried

I was looking at the same code this morning. I thought maybe we should 
drop the !tsk->ptrace condition (or make it so that the condition still 
succeeds if the tracer also happens to be tsk->real_parent) -- since 
this is only happening when the process is being traced? I tried this 
and the issue still persists, but I didn't apply your other proposed 
change to this conditional.

Or am I misunderstanding what tsk->ptrace refers to?

-- 
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/