__ptrace_detach: avoid task_detached(), check do_notify_parent()

__ptrace_detach() relies on the current obscure behaviour of
do_notify_parent(tsk) which changes tsk->exit_signal if this child
should be silently reaped. That is why we check task_detached(), it
is true if the task is sub-thread, or it is the group_leader but
its exit_signal was changed by do_notify_parent().

This is confusing, change the code to rely on !thread_group_leader()
or the value returned by do_notify_parent().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>

kernel/ptrace.c

@@ -370,25 +370,28 @@ static int ignoring_children(struct sighand_struct *sigh)
  */
 static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
 {
+	bool dead;
+
 	__ptrace_unlink(p);
 
-	if (p->exit_state == EXIT_ZOMBIE) {
-		if (!task_detached(p) && thread_group_empty(p)) {
+	if (p->exit_state != EXIT_ZOMBIE)
+		return false;
+
+	dead = !thread_group_leader(p);
+
+	if (!dead && thread_group_empty(p)) {
 		if (!same_thread_group(p->real_parent, tracer))
-				do_notify_parent(p, p->exit_signal);
+			dead = do_notify_parent(p, p->exit_signal);
 		else if (ignoring_children(tracer->sighand)) {
 			__wake_up_parent(p, tracer);
 			p->exit_signal = -1;
+			dead = true;
 		}
 	}
-		if (task_detached(p)) {
 	/* Mark it as in the process of being reaped. */
+	if (dead)
 		p->exit_state = EXIT_DEAD;
-			return true;
-		}
-	}
-
-	return false;
+	return dead;
 }
 
 static int ptrace_detach(struct task_struct *child, unsigned int data)