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Date:   Tue, 10 May 2022 16:23:18 +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>,
        linux-xtensa@linux-xtensa.org, Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, linux-ia64@vger.kernel.org
Subject: Re: [PATCH v4 10/12] ptrace: Don't change __state
Message-ID: <20220510142202.GB23277@redhat.com>
References: <87a6bv6dl6.fsf_-_@email.froward.int.ebiederm.org>
 <20220505182645.497868-10-ebiederm@xmission.com>
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On 05/05, Eric W. Biederman wrote:
>
>  static void ptrace_unfreeze_traced(struct task_struct *task)
>  {
> -	if (READ_ONCE(task->__state) != __TASK_TRACED)
> -		return;
> -
> -	WARN_ON(!task->ptrace || task->parent != current);
> +	unsigned long flags;
>  
>  	/*
> -	 * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
> -	 * Recheck state under the lock to close this race.
> +	 * The child may be awake and may have cleared
> +	 * JOBCTL_PTRACE_FROZEN (see ptrace_resume).  The child will
> +	 * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew.
>  	 */
> -	spin_lock_irq(&task->sighand->siglock);
> -	if (READ_ONCE(task->__state) == __TASK_TRACED) {
> +	if (lock_task_sighand(task, &flags)) {

But I still think that a lockless

	if (!(task->jobctl & JOBCTL_PTRACE_FROZEN))
		return;

check at the start of ptrace_unfreeze_traced() makes sense to avoid
lock_task_sighand() if possible.

And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with
JOBCTL_TRACED to make this optimization work better. The same for
ptrace_signal_wake_up().

Oleg.