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

From: Oleg Nesterov
Date: Fri Apr 15 2022 - 06:17:07 EST


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.