linux-kernel - Re: [perf] more perf_fuzzer memory corruption

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Message-ID: <alpine.DEB.2.02.1405011433180.6261@ionos.tec.linutronix.de>
Date:	Thu, 1 May 2014 14:35:02 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [perf] more perf_fuzzer memory corruption

On Thu, 1 May 2014, Peter Zijlstra wrote:
> On Thu, May 01, 2014 at 12:26:02PM +0200, Peter Zijlstra wrote:
> > On Thu, May 01, 2014 at 12:51:33AM +0200, Thomas Gleixner wrote:
> > > And that's the issue which puzzles us. Let's look at what we expect:
> > > 
> > > Now the trace shows a different story:
> > > 
> > >      perf_fuzzer-4387  [001]  1802.628659: sys_enter:            NR 298 (69bb58, 0, ffffffff, 12, 0, 0)
> > 
> > That's a per-cpu event (.pid = -1, .cpu = 12), they don't get inherited,
> > so the only thing keeping it alive is the fd the child got. So
> > exit_files() killing this thing makes perfect sense.

Duh, right. Should have noticed :(

> 
> grep ptr=0xffff880118fda000 bug.out | less
> 
> We find lovely bits such as:
> 
>      perf_fuzzer-4387  [001]  1773.427175: kmalloc:              (perf_event_alloc+0x5a) call_site=ffffffff8113a8fa ptr=0xffff880118fda000 bytes_req=1272 bytes_alloc=2048 gfp_flags=GFP_KERNEL|GFP_ZERO
>      ksoftirqd/6-38    [006]  1773.457770: kfree:                (free_event_rcu+0x2f) call_site=ffffffff8113177f ptr=0xffff880118fda000
>           <idle>-0     [007]  1774.020378: kfree:                (free_event_rcu+0x2f) call_site=ffffffff8113177f ptr=0xffff880118fda000
>      perf_fuzzer-4387  [000]  1774.096354: kmalloc:              (perf_event_alloc+0x5a) call_site=ffffffff8113a8fa ptr=0xffff880118fda000 bytes_req=1272 bytes_alloc=2048 gfp_flags=GFP_KERNEL|GFP_ZERO
> 
> 
> That's almost half a second between the double free, Vince, is your TSC
> solid?

grep DROPPED bug.out

Now align that with the double malloc/free sites and you have an explanation ...



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