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Date: Fri, 23 Sep 2016 11:50:32 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
        strace-devel@lists.sourceforge.net, Aleksa Sarai <asarai@suse.com>
Subject: Re: strace lockup when tracing exec in go
Message-ID: <20160923095031.GA14923@redhat.com>
References: <20160921152946.GA24210@dhcp22.suse.cz>
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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.

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.