linux-kernel - Re: [PATCH 9/9] ptrace: Don't change _

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Date:   Wed, 27 Apr 2022 17:41:59 +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 9/9] ptrace: Don't change __state

On 04/26, Eric W. Biederman wrote:
>
>  static void ptrace_unfreeze_traced(struct task_struct *task)
>  {
> -	if (READ_ONCE(task->__state) != __TASK_TRACED)
> +	if (!(READ_ONCE(task->jobctl) & JOBCTL_DELAY_WAKEKILL))
>  		return;
>
>  	WARN_ON(!task->ptrace || task->parent != current);
> @@ -213,11 +213,10 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
>  	 * Recheck state under the lock to close this race.
>  	 */
>  	spin_lock_irq(&task->sighand->siglock);

Now that we do not check __state = __TASK_TRACED, we need lock_task_sighand().
The tracee can be already woken up by ptrace_resume(), but it is possible that
it didn't clear DELAY_WAKEKILL yet.

Now, before we take ->siglock, the tracee can exit and another thread can do
wait() and reap this task.

Also, I think the comment above should be updated. I agree, it makes sense to
re-check JOBCTL_DELAY_WAKEKILL under siglock just for clarity, but we no longer
need to do this to close the race; jobctl &= ~JOBCTL_DELAY_WAKEKILL and
wake_up_state() are safe even if JOBCTL_DELAY_WAKEKILL was already cleared.

> @@ -2307,6 +2307,7 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
>
>  	/* LISTENING can be set only during STOP traps, clear it */
>  	current->jobctl &= ~JOBCTL_LISTENING;
> +	current->jobctl &= ~JOBCTL_DELAY_WAKEKILL;

minor, but

	current->jobctl &= ~(JOBCTL_LISTENING | JOBCTL_DELAY_WAKEKILL);

looks better.

Oleg.