Re: perf: fuzzer triggers NULL pointer derefreence in x86_schedule_events

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 20, 2015 at 03:03:12PM +0200, Peter Zijlstra wrote:
> So new in this release is:
> 
> static void intel_put_event_constraints(struct cpu_hw_events *cpuc,
> 					struct perf_event *event)
> {
> 	...
> 
> 	/* cleanup dynamic constraint */
> 	if (c && (c->flags & PERF_X86_EVENT_DYNAMIC))
> 		event->hw.constraint = NULL;
> }
> 
> Which is the only place that value is ever cleared...
> 
> Now, I've not quite figured out how that can intersect with scheduling,
> typically we only call put_event_constraints() when we're done with the
> event.

Related to that; intel_commit_scheduling() has:

  c = event->hw.constraint;

  if (!c)
  	return;

Stephane, how can that ever be?

	x86_schedule_events() does:
		hwc->constraint = get_event_constraints()

for all events, and intel_get_event_constraints() will _always_ return a
valid constraint -- &unconstrained if nothing else.