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

From: Peter Zijlstra
Date: Fri Apr 15 2022 - 08:01:06 EST


On Fri, Apr 15, 2022 at 12:16:44PM +0200, Oleg Nesterov wrote:
> 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?

The purpose of 1/5 was to not have any unique state in __state. To at
all times be able to reconstruct __state from outside information (where
needed).

Agreed that this particular piece of state isn't needed until 5/5, but
the concept is independent (also 5/5 is insanely large already).

> 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);
> }
>
> ?

This would be broken when we so signal_wake_up_state() when state
doesn't match. Does that happen? I'm thikning siglock protects us from
the most obvious races, but still.

If not broken, then it needs at least a comment explaining why not etc..
I'm sure to not remember many of these details.

Also, signal_wake_up_state() really can do with that
lockdep_assert_held() as well ;-)

> > > /*
> > > * 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().

Right.

> > > 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().

Urgh, forgot about that one ;-)

> > > 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.

Agreed.

>
> > > 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 ?

Yes, but I'm starting to feel a little paranoid here. Better safe than
sorry etc..

> 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?

Agreed on this.

> > > @@ -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 ?

We can for the sake of 2 avoid TRACED_FROZEN, but as explained at the
start, the point of 1 was to ensure there is no unique state in __state,
and I think in that respect we can keep it, hmm?

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

Yes.