Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Vince Weaver <vincent.weaver@maine.edu>
Date:   Thu, 6 Dec 2018 10:35:28 -0500 (EST)
To:     Jiri Olsa <jolsa@redhat.com>
cc:     Vince Weaver <vincent.weaver@maine.edu>,
        linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Andi Kleen <andi@firstfloor.org>
Subject: Re: perf: perf_fuzzer triggers GPF in perf_prepare_sample
In-Reply-To: <20181205183326.GE3836@krava>
Message-ID: <alpine.DEB.2.21.1812061033180.2368@macbook-air>
References: <alpine.DEB.2.21.1812041039220.19558@macbook-air> <20181205124538.GA19343@krava> <20181205163838.GA3836@krava> <alpine.DEB.2.21.1812051208580.29892@macbook-air> <20181205183326.GE3836@krava>
User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, 5 Dec 2018, Jiri Olsa wrote:

> On Wed, Dec 05, 2018 at 12:11:19PM -0500, Vince Weaver wrote:
> > On Wed, 5 Dec 2018, Jiri Olsa wrote:
> > 
> > > On Wed, Dec 05, 2018 at 01:45:38PM +0100, Jiri Olsa wrote:
> > > > On Tue, Dec 04, 2018 at 10:54:55AM -0500, Vince Weaver wrote:
> > > > > Hello,
> > > > > 
> > > > > I was able to trigger another oops with the perf_fuzzer with current git.
> > > > > 
> > > > > This is 4.20-rc5 after the fix for the very similar oops I previously 
> > > > > reported got committed.
> > > > > 
> > > > > It seems to be pointing to the same location in the source as 
> > > > > before, I guess maybe triggered a different way?
> > > > 
> > > > nice.. yep, looks the same
> > > > 
> > > > > 
> > > > > Unfortunately this crash is not easily reproducible like the last one was.
> > > > 
> > > > will check
> > > 
> > > what model are hitting this on?
> > 
> > Haswell.  6/60/3.
> > 
> > While I can't deterministically trigger this, the fuzzer usually hits it
> > within an hour or two.  Is there any debug or printk messages I can
> > add that would help figure out what's going on?
> 
> I can't see how we could end up with that config other than
> some corruption.. the only way I see could be that we touch
> cpu->events array without checking its active_mask bit
> 
> but that does not explain why the crash happened in the same
> place as before

Maybe it is a corruption issue.  I had applied my own debug patch that 
would dump some info if data->callchain was NULL.

But my debug code didn't trigger this time because it looks like 
data->callchain was "1" rather than "0".

[27764.840179] BUG: unable to handle kernel NULL pointer dereference at 0000000000000001
[27764.840179] PGD 0 P4D 0 
[27764.840180] Oops: 0000 [#1] SMP PTI
[27764.840180] CPU: 1 PID: 18687 Comm: perf_fuzzer Tainted: G        W         4.20.0-rc5+ #125
[27764.840180] Hardware name: LENOVO 10AM000AUS/SHARKBAY, BIOS FBKT72AUS 01/26/2014

Vince