Re: [PATCH 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT

[Oleg Nesterov]

On 04/15, Peter Zijlstra wrote:
>
> On Thu, Apr 14, 2022 at 08:34:33PM +0200, Oleg Nesterov wrote:
>
> > If it can work, then 1/5 needs some changes, I think. In particular,
> > it should not introduce JOBCTL_TRACED_FROZEN until 5/5, and perhaps
>
> That TRACED_FROZEN was to distinguish the TASK_TRACED and __TASK_TRACED
> state, and isn't related to the freezer.

Lets forget about 3-5 which I didn't read carefully yet. So why do we
need TRACED_FROZEN?

>From 1/5:

	 static inline void signal_wake_up(struct task_struct *t, bool resume)
	 {
	+	lockdep_assert_held(&t->sighand->siglock);
	+
	+	if (resume && !(t->jobctl & JOBCTL_TRACED_FROZEN))
	+		t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
	+
		signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
	 }
	+
	 static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
	 {
	+	lockdep_assert_held(&t->sighand->siglock);
	+
	+	if (resume)
	+		t->jobctl &= ~JOBCTL_TRACED;
	+
		signal_wake_up_state(t, resume ? __TASK_TRACED : 0);
	 }

Can't we simply change signal_wake_up_state(),

	void signal_wake_up_state(struct task_struct *t, unsigned int state)
	{
		set_tsk_thread_flag(t, TIF_SIGPENDING);
		/*
		 * TASK_WAKEKILL also means wake it up in the stopped/traced/killable
		 * case. We don't check t->state here because there is a race with it
		 * executing another processor and just now entering stopped state.
		 * By using wake_up_state, we ensure the process will wake up and
		 * handle its death signal.
		 */
		if (wake_up_state(t, state | TASK_INTERRUPTIBLE))
			t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
		else
			kick_process(t);
	}

?

> > 		/*
> > 		 * We take the read lock around doing both checks to close a
> > 		 * possible race where someone else attaches or detaches our
> > 		 * natural child.
> > 		 */
> > 		read_lock(&tasklist_lock);
> > 		traced = child->ptrace && child->parent == current;
> > 		read_unlock(&tasklist_lock);
> >
> > 		if (!traced)
> > 			return -ESRCH;
>
> The thing being, that if it is our ptrace child, it won't be going away
> since we're running this code and not ptrace_detach().  Right?

Yes. and nobody else can detach it.

Another tracer can't attach until child->ptrace is cleared, but this can
only happen if a) this child is killed and b) another thread does wait()
and reaps it; but after that attach() is obviously impossible.

But since this child can go away, the patch changes ptrace_freeze_traced()
to use lock_task_sighand().

> > 		for (;;) {
> > 			if (fatal_signal_pending(current))
> > 				return -EINTR;
>
> What if signal_wake_up(.resume=true) happens here? In that case we miss
> the fatal pending, and task state isn't changed yet so we'll happily go
> sleep.

No, it won't sleep, see the signal_pending_state() check in schedule().

> > 			set_current_state(TASK_KILLABLE);

And let me explain TASK_KILLABLE just in case... We could just use
TASK_UNINTERRUPTIBLE and avoid the signal_pending() check, but KILLABLE
looks "safer" to me. If the tracer hangs because of some bug, at least
it can be killed from userspace.


> > 			if (!(READ_ONCE(child->jobctl) & JOBCTL_TRACED)) {
>
>   TRACED_XXX ?

oops ;)

> > -	spin_lock_irq(&task->sighand->siglock);
> >  	if (task_is_traced(task) && !looks_like_a_spurious_pid(task) &&
> >  	    !__fatal_signal_pending(task)) {
> >  		task->jobctl |= JOBCTL_TRACED_FROZEN;
> >  		WRITE_ONCE(task->__state, __TASK_TRACED);
> >  		ret = true;
> >  	}
>
> I would feel much better if this were still a task_func_call()
> validating !->on_rq && !->on_cpu.

Well, but "on_rq || on_cpu" would mean that wait_task_inactive() is buggy ?

But! I forgot to make anothet change in this code. I do not think it should
rely on task_is_traced(). We are going to abuse task->__state, so I think
it should check task->__state == TASK_TRACED directly. Say,

	if (READ_ONCE(task->__state) == TASK_TRACED && ...) {
		WRITE_ONCE(task->__state, __TASK_TRACED);
		WARN_ON_ONCE(!task_is_traced(task));
		ret = true;
	}

looks more clean to me. What do you think?

> > @@ -2307,13 +2313,14 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
> >  		 */
> >  		if (gstop_done)
> >  			do_notify_parent_cldstop(current, false, why);
> > +		clear_traced_xxx();
> > +		read_unlock(&tasklist_lock);
> >
> > -		/* tasklist protects us from ptrace_freeze_traced() */
> > +		/* JOBCTL_TRACED_XXX protects us from ptrace_freeze_traced() */
>
> But... TRACED_XXX has just been cleared ?!

Cough ;) OK, I'll move __set_current_state() back under tasklist.

And in this case we do not need wake_up(parent), so we can shift it from
clear_traced_xxx() into another branch.

OK, so far it seems that this patch needs a couple of simple fixes you
pointed out, but before I send V2:

	- do you agree we can avoid JOBCTL_TRACED_FROZEN in 1-2 ?

	- will you agree if I change ptrace_freeze_traced() to rely
	  on __state == TASK_TRACED rather than task_is_traced() ?

Thanks,

Oleg.