wait: WSTOPPED|WCONTINUED doesn't work if a zombie leader is traced by another process

Even if the main thread is dead the process still can stop/continue.
However, if the leader is ptraced wait_consider_task(ptrace => false)
always skips wait_task_stopped/wait_task_continued, so WSTOPPED or
WCONTINUED can never work for the natural parent in this case.

Move the "A zombie ptracee is only visible to its ptracer" check into the
"if (!delay_group_leader(p))" block.  ->notask_error is cleared by the
"fall through" code below.

This depends on the previous change, wait_task_stopped/continued must be
avoided if !delay_group_leader() and the tracer is ->real_parent.
Otherwise WSTOPPED|WEXITED could wrongly report "stopped" when the child
is already dead (single-threaded or not).  If it is traced by another task
then the "stopped" state is fine until the debugger detaches and reveals a
zombie state.

Stupid test-case:

	void *tfunc(void *arg)
	{
		sleep(1);	// wait for zombie leader
		raise(SIGSTOP);
		exit(0x13);
		return NULL;
	}

	int run_child(void)
	{
		pthread_t thread;

		if (!fork()) {
			int tracee = getppid();

			assert(ptrace(PTRACE_ATTACH, tracee, 0,0) == 0);
			do
				ptrace(PTRACE_CONT, tracee, 0,0);
			while (wait(NULL) > 0);

			return 0;
		}

		sleep(1);	// wait for PTRACE_ATTACH
		assert(pthread_create(&thread, NULL, tfunc, NULL) == 0);
		pthread_exit(NULL);
	}

	int main(void)
	{
		int child, stat;

		child = fork();
		if (!child)
			return run_child();

		assert(child == waitpid(-1, &stat, WSTOPPED));
		assert(stat == 0x137f);

		kill(child, SIGCONT);

		assert(child == waitpid(-1, &stat, WCONTINUED));
		assert(stat == 0xffff);

		assert(child == waitpid(-1, &stat, 0));
		assert(stat == 0x1300);

		return 0;
	}

Without this patch it hangs in waitpid(WSTOPPED), wait_task_stopped() is
never called.

Note: this doesn't fix all problems with a zombie delay_group_leader(),
WCONTINUED | WEXITED check is not exactly right.  debugger can't assume it
will be notified if another thread reaps the whole thread group.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Lennart Poettering <lpoetter@redhat.com>
Cc: Michal Schmidt <mschmidt@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -1380,20 +1380,16 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
 
 	/* slay zombie? */
 	if (p->exit_state == EXIT_ZOMBIE) {
+		/* we don't reap group leaders with subthreads */
+		if (!delay_group_leader(p)) {
 			/*
 			 * A zombie ptracee is only visible to its ptracer.
-		 * Notification and reaping will be cascaded to the real
-		 * parent when the ptracer detaches.
+			 * Notification and reaping will be cascaded to the
+			 * real parent when the ptracer detaches.
 			 */
-		if (likely(!ptrace) && unlikely(p->ptrace)) {
-			/* it will become visible, clear notask_error */
-			wo->notask_error = 0;
-			return 0;
-		}
-
-		/* we don't reap group leaders with subthreads */
-		if (!delay_group_leader(p))
+			if (unlikely(ptrace) || likely(!p->ptrace))
 				return wait_task_zombie(wo, p);
+		}
 
 		/*
 		 * Allow access to stopped/continued state via zombie by