linux-kernel - Re: [RFC][PATCH 07/11] signal: Deliver group signals via PIDTYPE_TGID not PIDTYPE

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Message-ID: <20180716125144.GA18262@redhat.com>
Date:   Mon, 16 Jul 2018 14:51:45 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Wen Yang <wen.yang99@....com.cn>,
        majiang <ma.jiang@....com.cn>
Subject: Re: [RFC][PATCH 07/11] signal: Deliver group signals via
 PIDTYPE_TGID not PIDTYPE_PID

On 07/10, Eric W. Biederman wrote:
>
> Now that we can make the distinction use PIDTYPE_TGID rather than
> PIDTYPE_PID.

Wai, wait, this doesn't look right...

> There is no immediate effect as they point point at the
> same task,

How so? pid_task(pid, PIDTYPE_TGID) will return NULL unless this pid is actually
a group leader's pid,

> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1315,7 +1315,7 @@ int kill_pid_info(int sig, struct siginfo *info, struct pid *pid)
>
>  	for (;;) {
>  		rcu_read_lock();
> -		p = pid_task(pid, PIDTYPE_PID);
> +		p = pid_task(pid, PIDTYPE_TGID);
>  		if (p)
>  			error = group_send_sig_info(sig, info, p);

So, currently kill(pid_nr) always works, even if pid_nr is a sub-thread's tid.

After this change kill(2) will always fail with -ESRCH in this case.

Or I am totally confused?

> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -347,12 +347,11 @@ int posix_timer_event(struct k_itimer *timr, int si_private)
>  	 */
>  	timr->sigq->info.si_sys_private = si_private;
>  
> +	shared = !(timr->it_sigev_notify & SIGEV_THREAD_ID);
>  	rcu_read_lock();
> -	task = pid_task(timr->it_pid, PIDTYPE_PID);
> -	if (task) {
> -		shared = !(timr->it_sigev_notify & SIGEV_THREAD_ID);
> +	task = pid_task(timr->it_pid, shared ? PIDTYPE_TGID : PIDTYPE_PID);

This looks fine, afaics without SIGEV_THREAD_ID ->it_pid is alwats task_tgid().

Oleg.