Re: [RFC][PATCH] fcntl: F_[SG]ETOWN_TID

[Oleg Nesterov <oleg@redhat.com>]

On 07/31, Peter Zijlstra wrote:
>
> In order to direct the SIGIO signal to a particular thread of a
> multi-threaded application we cannot, like suggested by the manpage, put
> a TID into the regular fcntl(F_SETOWN) call. It will still be send to
> the whole process of which that thread is part.
>
> Since people do want to properly direct SIGIO we introduce F_SETOWN_TID,
> which functions similarly to F_SETOWN, except positive arguments are
> interpreted as TIDs and negative arguments are interpreted as PIDs.

I think this is correct. But,

> @@ -431,6 +474,7 @@ static void send_sigio_to_task(struct task_struct *p,
>  			       int fd,
>  			       int reason)
>  {
> +	int (*send_sig)(int, struct siginfo *, struct task_struct *);
>  	/*
>  	 * F_SETSIG can change ->signum lockless in parallel, make
>  	 * sure we read it once and use the same value throughout.
> @@ -440,6 +484,8 @@ static void send_sigio_to_task(struct task_struct *p,
>  	if (!sigio_perm(p, fown, signum))
>  		return;
>
> +	send_sig = fown->task_only ? send_sig_info : group_send_sig_info;

this is ugly.

I do not blame your patch, I blame signal.c which has a lot of helpers
to send a signal but it is never possible to find the right one.

I think we need a new trivial helper,

	int xxx(int sig, struct siginfo *info, struct task_struct *p, bool group)
	{
		unsigned long flags;
		int ret = -ESRCH;

		if (lock_task_sighand(p, &flags)) {
			ret = send_signal(sig, info, p, group);
			unlock_task_sighand(p, &flags);
		}

		return ret;
	}

send_sigio_to_task() can just do: send_signal(..., !fown->task_only).

group_send_sig_info(), send_sig_info() should use this helper too.


Also. without the new helper, F_SETOWN does check_kill_permission()
while F_SETOWN_TID does not. This doesn't really matter, but this
looks a bit odd.

Note that send_sigio_to_task() does not need check_kill_permission().
We use either SEND_SIG_PRIV or SI_FROMKERNEL() signals, so we never
actually check permissions. Even if we did, it would be just wrong
to deny the signal here using current_cred().


Peter, may I suggest to to re-diff your patch on top of the trivial
patch I am going to send a bit later today?

Oleg.