pidfd: improve uapi when task isn't found

We currently report EINVAL whenever a struct pid has no tasked attached
anymore thereby conflating two concepts:

(1) The task has already been reaped.
(2) The caller requested a pidfd for a thread-group leader but the pid
actually references a struct pid that isn't used as a thread-group
leader.

This is causing issues for non-threaded workloads as in [1].

This patch tries to allow userspace to distinguish between (1) and (2).
This is racy of course but that shouldn't matter.

Link: https://github.com/systemd/systemd/pull/36982 [1]
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

kernel/fork.c

···
  */
 int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
 {
-	bool thread = flags & PIDFD_THREAD;
+	int err = 0;
 
-	if (!pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID))
-		return -EINVAL;
+	if (!(flags & PIDFD_THREAD)) {
+		/*
+		 * If this is struct pid isn't used as a thread-group
+		 * leader pid but the caller requested to create a
+		 * thread-group leader pidfd then report ENOENT to the
+		 * caller as a hint.
+		 */
+		if (!pid_has_task(pid, PIDTYPE_TGID))
+			err = -ENOENT;
+	}
+
+	/*
+	 * If this wasn't a thread-group leader struct pid or the task
+	 * got reaped in the meantime report -ESRCH to userspace.
+	 */
+	if (!pid_has_task(pid, PIDTYPE_PID))
+		err = -ESRCH;
+	if (err)
+		return err;
 
 	return __pidfd_prepare(pid, flags, ret);
 }