Messages in this thread | | | Subject | Re: [PATCH 01/10] perf,x86: Fix event/group validation | From | Peter Zijlstra <> | Date | Thu, 21 May 2015 16:03:58 +0200 |
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On Thu, 2015-05-21 at 06:36 -0700, Stephane Eranian wrote: > On Thu, May 21, 2015 at 6:29 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Thu, 2015-05-21 at 06:27 -0700, Stephane Eranian wrote: > >> Or are you talking about a preemption while executing x86_schedule_events()? > > > > That. > > > > And we can of course cure that by an earlier patch I send; but I find it > > a much simpler rule to just never allow modifying global state for > > validation. > > I can see validation being preempted, but not the context switch code path. > Is that what you are talking about? > > You are saying validate_group() is in the middle of x86_schedule_events() > using fake_cpuc, when it gets preempted. The context switch code when it loads > the new thread's PMU state calls x86_schedule_events() which modifies the > cpuc->event_list[]->hwc. But this is cpuc vs. fake_cpuc again. So yes, the calls > nest but they do not touch the same state.
They both touch event->hw->constraint.
> And when you eventually come back > to validate_group() you are back to using the fake_cpuc. So I am still not clear > on how the corruption can happen.
validate_group() x86_schedule_events() event->hw.constraint = c; # store
<context switch> perf_task_event_sched_in() ... x86_schedule_events(); event->hw.constraint = c2; # store
...
put_event_constraints(event); # assume failure to schedule intel_put_event_constraints() event->hw.constraint = NULL;
<context switch end>
c = event->hw.constraint; # read -> NULL
if (!test_bit(hwc->idx, c->idxmsk)) # <- *BOOM* NULL deref
This in particular is possible when the event in question is a cpu-wide event and group-leader, where the validate_group() tries to add an event to the group.
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