Messages in this thread | | | Date | Fri, 15 Apr 2022 14:00:15 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/5] sched,ptrace: Fix ptrace_check_attach() vs PREEMPT_RT |
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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.
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