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Date:   Wed, 27 Apr 2022 16:56:47 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, rjw@rjwysocki.net, mingo@kernel.org,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, mgorman@suse.de, bigeasy@linutronix.de,
        Will Deacon <will@kernel.org>, tj@kernel.org,
        linux-pm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Richard Weinberger <richard@nod.at>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        linux-um@lists.infradead.org, Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>,
        inux-xtensa@linux-xtensa.org, Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with
 siglock held
Message-ID: <20220427145646.GC17421@redhat.com>
References: <878rrrh32q.fsf_-_@email.froward.int.ebiederm.org>
 <20220426225211.308418-6-ebiederm@xmission.com>
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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...

Oleg.