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Message-ID: <1432217038.30671.7.camel@twins>
Date: Thu, 21 May 2015 16:03:58 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Stephane Eranian <eranian@...gle.com>
Cc: Ingo Molnar <mingo@...nel.org>,
Vince Weaver <vincent.weaver@...ne.edu>,
Jiri Olsa <jolsa@...hat.com>,
"Liang, Kan" <kan.liang@...el.com>,
LKML <linux-kernel@...r.kernel.org>,
Andrew Hunter <ahh@...gle.com>,
Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
Subject: Re: [PATCH 01/10] perf,x86: Fix event/group validation
On Thu, 2015-05-21 at 06:36 -0700, Stephane Eranian wrote:
> On Thu, May 21, 2015 at 6:29 AM, Peter Zijlstra <peterz@...radead.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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