Re: [PATCH 1/2] pidfd: verify task is alive when printing fdinfo

On Tue, Oct 15, 2019 at 04:43:57PM +0200, Oleg Nesterov wrote:

On 10/15, Christian Brauner wrote:

quoted

+static inline bool task_alive(struct pid *pid)
+{
+	bool alive = true;
+
+	rcu_read_lock();
+	if (!pid_task(pid, PIDTYPE_PID))
+		alive = false;
+	rcu_read_unlock();
+
+	return alive;
+}

Well, the usage of rcu_read_lock/unlock looks confusing to me...

I mean, this helper does not need rcu lock at all. Except
rcu_dereference_check() will complain.

Yep, I think we have another codepath were the rcu locks might be purely
cosmetic so I thought it's not a big deal (see below).

	static inline bool task_alive(struct pid *pid)
	{
		bool alive;

		/* shut up rcu_dereference_check() */
		rcu_lock_acquire(&rcu_lock_map);
		alive = !!pid_task(pid, PIDTYPE_PID));
		rcu_lock_release(&rcu_lock_map);

		return alive;
	}

looks more clear imo.

But in fact I'd suggest to simply use !hlist_empty(&pid->tasks[PIDTYPE_PID])
in pidfd_show_fdinfo() and do not add a new helper.

Sounds good to me. But can't we then just do something similar just with
!hlist_empty(&pid->tasks[PIDTYPE_TGID])

in v5.4-rc3:kernel/pid.c:pidfd_open():514-517 ?

or would this be problematic because of de_thread()?

Thanks!
Christian