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SubjectRe: perf_fuzzer compiled for x32 causes reboot
On Thu, Feb 27, 2014 at 08:00:04PM -0500, Vince Weaver wrote:
> On Thu, 27 Feb 2014, H. Peter Anvin wrote:
>
> > On 02/27/2014 03:30 PM, Steven Rostedt wrote:
> > > On Thu, 27 Feb 2014 14:52:54 -0800
> > > "H. Peter Anvin" <hpa@zytor.com> wrote:
> > >
> > >> On 02/27/2014 02:31 PM, Steven Rostedt wrote:
> > >>>
> > >>> Yeah, something is getting mesed up.
> > >>>
> > >>
> > >> What it *looks* like to me is that we try to nest the cr2 save/restore,
> > >> which doesn't nest because it is a percpu variable.
> > >>
> > >> ... except in the x86-64 case, we *ALSO* save/restore cr2 inside
> > >> entry_64.S, which makes the stuff in do_nmi completely redundant and
> > >> there for no good reason.
> > >
> > > Peter, look at the code. That percpu cr2 is in a #ifdef CONFIG_X86_32
> > > section. That is, it isn't even executed. That's i386 code. The only
> > > place the cr2 is saved for x86_64 is in entry_64.S.
> > >
> >
> > Right, egg on my face. However, I still think it would make more sense
> > for it to nest the way entry_64.S does if at all possible.
> >
> > That makes this even more confusing, though. I would still like to see
> > what happens with the patch I sent Vince.
>
> I'll try your patch momentarily, first I had some other changes I started
> running before I left work (for some reason it recompiled the whole
> kernel).
>
> 8: function: perf_output_begin
> 8: bprint: perf_output_begin: VMW: event type 2 config 2a st: 2c3e
> 8: bputs: perf_output_begin: VMW: before rcu_dereference
> 9: function: __do_page_fault
> 9: function: down_read_trylock
> 9: function: _cond_resched
> 9: function: find_vma
>
> so it looks like the fault happens
>
> rcu_read_lock();
>
> 116 /*
> 117 * For inherited events we send all the output towards the parent.
> 118 */
> 119 if (event->parent)
> 120 event = event->parent;
> 121
>
> somewhere between here
>
> 122 rb = rcu_dereference(event->rb);
> 123 if (unlikely(!rb))
> 124 goto out;
>
> and here
>
> 125
> 126 if (unlikely(!rb->nr_pages))
> 127 goto out;
>
> although if rcu locks do anything to turn off tracing then this could be
> suspect.

The most likely suspect is of course event->rb in the rcu_dereference.
I have to defer to Steven on how rcu_read_lock() and rcu_read_unlock()
currently interact with tracing. ;-)

Thanx, Paul



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