Re: perf: perf_fuzzer triggers GPF in perf_prepare_sample

[Vince Weaver]

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