Re: [PATCH linux-next v2] signal: clarify __send_signal_locked comment in do_notify_parent

[Oleg Nesterov <oleg@redhat.com>]

On 07/29, fan.yu9@zte.com.cn wrote:
>
> @@ -2252,8 +2252,10 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
>  			sig = 0;
>  	}
>  	/*
> -	 * Send with __send_signal as si_pid and si_uid are in the
> -	 * parent's namespaces.
> +	 * Use __send_signal_locked() instead of send_signal_locked()
> +	 * because si_pid and si_uid are already in the parent's
> +	 * namespace. send_signal_locked() would incorrectly modify
> +	 * them when crossing PID/user namespaces.
>  	 */

Well, Thomas doesn't like the idea to kill this comment, I won't argue.

However, this comment still looks confusing to me, and I don't know how to
make it more clear. Yes, send_signal_locked() may, say, clear info->si_pid
but not "because si_pid and si_uid are already in the parent's namespace".

There are several obvious reasons not to use send_signal_locked():

	1. do_notify_parent() has already correctly filled si_pid/si_uid,
	   the "has_si_pid_and_uid()" checks in send_signal_locked() are
	   pointless.

	   That is why I think this comment should simply die.

	2. send_signal_locked() assumes that different namespaces mean
	   "From an ancestor namespace", but in this case the child can
	   send a signal to the parent namespace while "from parent ns"
	   is not possible.

	3. send_signal_locked() assumes that "current" is a) the sender
	   and b) alive task. Both assumptions may be wrong if "current"
	   is the last exiting thread which calls do_notify_parent() from
	   release_task().

	   In this case task_pid_nr_ns(current, task_active_pid_ns(parent))
	   will return 0 because current->thread_pid is already NULL, and
	   send_signal_locked() will misinterpret this as "from parent ns"
	   and clear si_pid.

	   But imo, it is simply unsafe to use send_signal_locked() in this
	   case, even if currently nothing "really bad" can happen.

OTOH. This patch doesn't make the comment more confusing, plus it removes
the reference to __send_signal() which no longer exists, so let me ack
this patch and forget this surprisingly long discussion ;)

Acked-by: Oleg Nesterov <oleg@redhat.com>