Date: Thu, 17 Nov 2016 09:18:48 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Vince Weaver <vincent.weaver@maine.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        "davej@codemonkey.org.uk" <davej@codemonkey.org.uk>,
        "dvyukov@google.com" <dvyukov@google.com>,
        Stephane Eranian <eranian@gmail.com>
Subject: Re: perf: fuzzer KASAN unwind_get_return_address
Message-ID: <20161117151848.7sdss3g4waanxfsk@treble>
References: <alpine.DEB.2.20.1611151239060.31288@macbook-air>
 <20161115185756.GL3142@twins.programming.kicks-ass.net>
 <alpine.DEB.2.20.1611151404130.31288@macbook-air>
 <20161115205748.xtroftp55igs55bz@treble>
 <20161116130337.GT3142@twins.programming.kicks-ass.net>
 <20161116143746.zoxdxrfqvmx35wln@treble>
 <20161116144943.GB3117@twins.programming.kicks-ass.net>
 <20161116145849.GR3157@twins.programming.kicks-ass.net>
 <20161117044828.vedc3whqkuki624r@treble>
 <20161117090446.GC3142@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20161117090446.GC3142@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.6.0.1 (2016-04-01)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 980
Lines: 24

On Thu, Nov 17, 2016 at 10:04:46AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 16, 2016 at 10:48:28PM -0600, Josh Poimboeuf wrote:
> > Peter or Vince, can you try to recreate with this patch?  It dumps the
> > raw stack contents during a stack dump.  Hopefully that would give a
> > clue about what's going wrong.
> 
> 
> Here goes... I'll do another run and get you the results of that as
> well.

Thanks, I just waded through this and it turned up some good clues.  And
according to 'git blame', you might be able to help :-)

It's not stack corruption.  Instead it looks like
__intel_pmu_pebs_event() is creating a bad or stale pt_regs which gets
passed to the unwinder.  Specifically, regs->bp points to a seemingly
random address on the NMI stack.  Which seems odd, considering the code
itself is running on the same NMI stack.

I don't know much about the PEBS code but it seems like it's passing
some stale data.  Either that or there's some NMI nesting going on.

-- 
Josh