Date: Wed, 16 Nov 2016 08:58:40 -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: <20161116145840.gvh5wwycmdkluhuy@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>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20161116144943.GB3117@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.6.0.1 (2016-04-01)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1703
Lines: 42

On Wed, Nov 16, 2016 at 03:49:43PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 16, 2016 at 08:37:46AM -0600, Josh Poimboeuf wrote:
> > On Wed, Nov 16, 2016 at 02:03:37PM +0100, Peter Zijlstra wrote:
> > > On Tue, Nov 15, 2016 at 02:57:48PM -0600, Josh Poimboeuf wrote:
> > > > Would you mind posting a disassembly of unwind_get_return_address()?
> > > > Any idea how recreatable it is?  (In particular I'd be interested in
> > > > seeing this dump with the latest unwinder improvements in the -tip tree,
> > > > which dump the pt_regs associated with an interrupt.)
> > > 
> > > Fairly reproducable it seems, doesn't seem to include pt_regs dumps
> > > though :/
> > > 
> > > tip/master as of this morning.
> > 
> > Thanks.  This is actually a different issue than the one reported by
> > Vince.  In this case FRAME_POINTER is disabled, so it uses the "guess"
> > unwinder which scans every address on the stack, looking for text
> > addresses.  So the kasan errors are expected.
> > 
> > (The missing pt_regs are also expected: the guess unwinder doesn't show
> > them.)
> > 
> > I'll work up a patch to fix this.  I still have no idea what's causing
> > Vince's bug in the frame pointer unwinder.
> 
> Hurm,.. by the number of '?' entries in Vince's backtrace I was assuming
> it was without frame pointers.

When frame pointers are disabled, *all* the addresses are prefixed with
'?'.

When frame pointers are enabled, and there are a lot of '?' addresses,
it usually means the containing functions reserved a lot of stack space
and the printed addresses are mostly leftovers from previous runs.

> Let me enable those and run again, it didn't insta-trigger like it does
> without.

Thanks!

-- 
Josh