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Date: Fri, 22 May 2015 11:46:19 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Stephane Eranian <eranian@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Vince Weaver <vincent.weaver@maine.edu>, Jiri Olsa <jolsa@redhat.com>,
        "Liang, Kan" <kan.liang@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, Andrew Hunter <ahh@google.com>,
        Maria Dimakopoulou <maria.n.dimakopoulou@gmail.com>
Subject: Re: [PATCH 01/10] perf,x86: Fix event/group validation
Message-ID: <20150522094619.GA18829@gmail.com>
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* Stephane Eranian <eranian@google.com> wrote:

> On Thu, May 21, 2015 at 11:49 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Stephane Eranian <eranian@google.com> wrote:
> >
> > > On Thu, May 21, 2015 at 7:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > > > 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.
> > >
> > > Ok, I think I get it now. It is not related to fake_cpuc vs. cpuc,
> > > it is related to the fact that the constraint is cached in the event
> > > struct itself and that one is shared between validate_group() and
> > > x86_schedule_events() because cpu_hw_event->event_list[] is an array
> > > of pointers to events and not an array of events.
> >
> > Btw., comments and the code structure should be greatly enhanced 
> > to make all that very clear and hard to mess up.
> >
> Peter and I will clean this up.

Great, thanks!

	Ingo
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