Re: [PATCH 01/10] perf,x86: Fix event/group validation

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 21, 2015 at 06:07:20AM -0700, Stephane Eranian wrote:
> On Thu, May 21, 2015 at 5:56 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, May 21, 2015 at 05:35:02AM -0700, Stephane Eranian wrote:
> >> > Commit e979121b1b15 ("perf/x86/intel: Implement cross-HT corruption
> >> > bug workaround") made the situation much worse by actually setting the
> >> > event->hw.constraint value to NULL, so when validation and actual
> >> > scheduling interact we get NULL ptr derefs.
> >> >
> >>
> >> But  x86_schedule_events() does reset the hw.constraint for each invocation:
> >>
> >>            c = x86_pmu.get_event_constraints(cpuc, i, cpuc->event_list[i]);
> >>            hwc->constraint = c;
> >
> > Yes, so if you have:
> >
> >         validate_group()
> >
> >                 hwc->constraint = c;
> >
> Ok, you get that because validate_group() invokes x6_schedule_events() but
> on the fake_cpuc. This on fake_cpuc->event_list[]->hwc.
> 
> >         <context switch>
> >
> >                 c = hwc->constraint;
> >
> > The second c might not be the first.
> And where does this assignment come from?

That's a read. The <context switch> can include a call to
x86_schedule_events().

> For actual scheduling, we are using the actual cpuc, not fake_cpuc.
> Validate_group() does not modify global cpuc state. Or am I missing
> something?

No, but x86_schedule_event() can modify event state, which is the fail.