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

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Date:   Fri, 29 Apr 2022 00:50:09 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>, 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>,
        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

Peter, you know, it is very difficult to me to discuss the changes
in the 2 unfinished series and not loose the context ;) Plus I am
already sleeping. But I'll try to reply anyway.

On 04/29, Peter Zijlstra wrote:
>
> On Thu, Apr 28, 2022 at 10:59:57PM +0200, Oleg Nesterov wrote:
> > If it is on top of JOBCTL_TRACED/DELAY_WAKEKILL changes (yours or Eric's),
> > then it can't help - SIGKILL can come right after the tracee drops siglock
> > and calls schedule().
>
> But by that time it will already have set TRACED and signal_wake_up()
> wil clear it, no?

No. JOBCTL_DELAY_WAKEKILL is already set, this means that signal_wake_up()
will remove TASK_WAKEKILL from the "state" passed to signal_wake_up_state()
and this is fine and correct, this mean thats ttwu() won't change ->__state.

But this also mean that wake_up_state() will return false, and in this case

	signal_wake_up_state:

		if (wake_up_state(t, state | TASK_INTERRUPTIBLE))
			t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED | JOBCTL_TRACED_QUIESCE);

won't clear these flags. And this is nice too.

But. fatal_signal_pending() is true! And once we change freeze_traced()
to not abuse p->__state, schedule() won't block because it will check
signal_pending_state(TASK_TRACED == TASK_WAKEKILL | __TASK_TRACED) and
__fatal_signal_pending() == T.

In this case ptrace_stop() will leak JOBCTL_TRACED, so we simply need
to clear it before return along with LISTENING | DELAY_WAKEKILL.

> I'll go
> over it all again in the morning, perhaps I'll reach a different
> conclusion :-)

Same here ;)

Oleg.