linux-kernel - Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT

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Date:   Mon, 25 Apr 2022 16:35:37 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        ebiederm@...ssion.com, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 2/5] sched,ptrace: Fix ptrace_check_attach() vs
 PREEMPT_RT

On 04/21, Peter Zijlstra wrote:
>
> +static void clear_traced_quiesce(void)
> +{
> +	spin_lock_irq(&current->sighand->siglock);
> +	WARN_ON_ONCE(!(current->jobctl & JOBCTL_TRACED_QUIESCE));

This WARN_ON_ONCE() doesn't look right, the task can be killed right
after ptrace_stop() sets JOBCTL_TRACED | JOBCTL_TRACED_QUIESCE and
drops siglock.

> @@ -2290,14 +2303,26 @@ static int ptrace_stop(int exit_code, in
>  		/*
>  		 * Don't want to allow preemption here, because
>  		 * sys_ptrace() needs this task to be inactive.
> -		 *
> -		 * XXX: implement read_unlock_no_resched().
>  		 */
>  		preempt_disable();
>  		read_unlock(&tasklist_lock);
> -		cgroup_enter_frozen();
> +		cgroup_enter_frozen(); // XXX broken on PREEMPT_RT !!!
> +
> +		/*
> +		 * JOBCTL_TRACE_QUIESCE bridges the gap between
> +		 * set_current_state(TASK_TRACED) above and schedule() below.
> +		 * There must not be any blocking (specifically anything that
> +		 * touched ->saved_state on PREEMPT_RT) between here and
> +		 * schedule().
> +		 *
> +		 * ptrace_check_attach() relies on this with its
> +		 * wait_task_inactive() usage.
> +		 */
> +		clear_traced_quiesce();

Well, I think it should be called earlier under tasklist_lock,
before preempt_disable() above.

We need tasklist_lock to protect ->parent, debugger can be killed
and go away right after read_unlock(&tasklist_lock).

Still trying to convince myself everything is right with
JOBCTL_STOPPED/TRACED ...

Oleg.