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ptrace: do_wait(traced_leader_killed_by_mt_exec) can block forever

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Test-case:

	void *tfunc(void *arg)
	{
		execvp("true", NULL);
		return NULL;
	}

	int main(void)
	{
		int pid;

		if (fork()) {
			pthread_t t;

			kill(getpid(), SIGSTOP);

			pthread_create(&t, NULL, tfunc, NULL);

			for (;;)
				pause();
		}

		pid = getppid();
		assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0);

		while (wait(NULL) > 0)
			ptrace(PTRACE_CONT, pid, 0,0);

		return 0;
	}

It is racy, exit_notify() does __wake_up_parent() too. But in the
likely case it triggers the problem: de_thread() does release_task()
and the old leader goes away without the notification, the tracer
sleeps in do_wait() without children/tracees.

Change de_thread() to do __wake_up_parent(traced_leader->parent).
Since it is already EXIT_DEAD we can do this without ptrace_unlink(),
EXIT_DEAD threads do not exist from do_wait's pov.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Oleg Nesterov 2011-07-21 20:00:43 +02:00
parent 8a35241803
commit eac1b5e57d
1 changed files with 8 additions and 0 deletions
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8
fs/exec.c
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@ -967,6 +967,14 @@ static int de_thread(struct task_struct *tsk)
BUG_ON(leader->exit_state != EXIT_ZOMBIE);
leader->exit_state = EXIT_DEAD;
/*
* We are going to release_task()->ptrace_unlink() silently,
* the tracer can sleep in do_wait(). EXIT_DEAD guarantees
* the tracer wont't block again waiting for this thread.
*/
if (unlikely(leader->ptrace))
__wake_up_parent(leader, leader->parent);
write_unlock_irq(&tasklist_lock);
release_task(leader);