linux-kernel - Re: [PATCH 6/9] signal: Always call do_notify_parent

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Message-ID: <20220427150053.GD17421@redhat.com>
Date:   Wed, 27 Apr 2022 17:00:54 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        inux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with
 siglock held

On 04/27, Oleg Nesterov wrote:
>
> On 04/26, Eric W. Biederman wrote:
> >
> > @@ -2209,6 +2213,34 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
> >  		spin_lock_irq(&current->sighand->siglock);
> >  	}
> >
> > +	/* Don't stop if current is not ptraced */
> > +	if (unlikely(!current->ptrace))
> > +		return (clear_code) ? 0 : exit_code;
> > +
> > +	/*
> > +	 * If @why is CLD_STOPPED, we're trapping to participate in a group
> > +	 * stop.  Do the bookkeeping.  Note that if SIGCONT was delievered
> > +	 * across siglock relocks since INTERRUPT was scheduled, PENDING
> > +	 * could be clear now.  We act as if SIGCONT is received after
> > +	 * TASK_TRACED is entered - ignore it.
> > +	 */
> > +	if (why == CLD_STOPPED && (current->jobctl & JOBCTL_STOP_PENDING))
> > +		gstop_done = task_participate_group_stop(current);
> > +
> > +	/*
> > +	 * Notify parents of the stop.
> > +	 *
> > +	 * While ptraced, there are two parents - the ptracer and
> > +	 * the real_parent of the group_leader.  The ptracer should
> > +	 * know about every stop while the real parent is only
> > +	 * interested in the completion of group stop.  The states
> > +	 * for the two don't interact with each other.  Notify
> > +	 * separately unless they're gonna be duplicates.
> > +	 */
> > +	do_notify_parent_cldstop(current, true, why);
> > +	if (gstop_done && ptrace_reparented(current))
> > +		do_notify_parent_cldstop(current, false, why);
>
> This doesn't look right too. The parent should be notified only after
> we set __state = TASK_TRACED and ->exit code.
>
> Suppose that debugger sleeps in do_wait(). do_notify_parent_cldstop()
> wakes it up, debugger calls wait_task_stopped() and then it will sleep
> again, task_stopped_code() returns 0.
>
> This can be probably fixed if you remove the lockless (fast path)
> task_stopped_code() check in wait_task_stopped(), but this is not
> nice performance-wise...

On the other hand, I don't understand why did you move the callsite
of do_notify_parent_cldstop() up... just don't do this?

Oleg.