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

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Date:   Fri, 23 Sep 2016 13:23:38 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        strace-devel@...ts.sourceforge.net, Aleksa Sarai <asarai@...e.com>
Subject: Re: strace lockup when tracing exec in go

On Fri 23-09-16 11:50:32, Oleg Nesterov wrote:
> Sorry for delay, I was offline. I'll try to return to this problem next
> week, currently I can't even read this thread but at first glance the
> proposed patch(es) do not look right...
> 
> On 09/21, Michal Hocko wrote:
> >
> > The further investigation shown that the tracer (strace) is stuck
> > waiting for cred_guard_mutex
> > [<0000000000000000>] mm_access+0x22/0xa0
> > [<0000000000000000>] process_vm_rw_core.isra.1+0x112/0x6c0
> > [<0000000000000000>] process_vm_rw+0xab/0x110
> > [<0000000000000000>] SyS_process_vm_readv+0x15/0x20
> > [<0000000000000000>] system_call_fastpath+0x16/0x1b
> > [<0000000000000000>] 0x7f186f031c3a
> > [<0000000000000000>] 0xffffffffffffffff
> ...
> > this however doesn't happen because both threads which are dead
> > are zombies waiting to be reaped by the parent and to call their
> > release_task->__exit_signal.
> 
> Yes, I know, this was already reported. And so far I do not see any
> reasonable fix. I _think_ that the "real" fix should rework the
> security_bprm_* helpers, but unlikely this is possible.

Rework them to not rely on the cred_guard_mutex?  Is there any way to
workaround this in the strace code?

> The trivial test-case:
> 
> 	void *thread(void *arg)
> 	{
> 		ptrace(PTRACE_TRACEME, 0,0,0);
> 		return NULL;
> 	}
> 
> 	int main(void)
> 	{
> 		int pid = fork();
> 
> 		if (!pid) {
> 			pthread_t pt;
> 			pthread_create(&pt, NULL, thread, NULL);
> 			pthread_join(pt, NULL);
> 			execlp("echo", "echo", "passed", NULL);
> 		}
> 
> 		sleep(1);
> 		// or anything else which needs ->cred_guard_mutex,
> 		// say open(/proc/$pid/mem)
> 		ptrace(PTRACE_ATTACH, pid, 0,0);
> 		kill(pid, SIGCONT);
> 
> 		return 0;
> 	}
> 
> Oleg.

-- 
Michal Hocko
SUSE Labs