Re: [BUG] perf stat: corrupts memory when using PMU cpumask

From: Stephane Eranian
Date: Fri Jan 17 2014 - 10:36:42 EST


I just sent the patches I wrote to fix the bugs I ran into since yesterday.

On Fri, Jan 17, 2014 at 3:09 PM, Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
> Em Fri, Jan 17, 2014 at 10:00:20AM +0100, Stephane Eranian escreveu:
>> The issue boils down to the fact that evsels have their file descriptors closed
>> twice nowadays. Once in __run_per_stat() via perf_evsel__close_fd() and
>> twice in perf_evlist__close().
>> Now, calling close() twice is okay. However the fd is then set to -1.
>> That's still okay with close(). The problem is elsewhere.
>> It comes from the ncpus argument passed to perf_evsel__close(). It is
>> DIFFERENT between the evsel and the evlist when cpumask are used.
>> Take my case, 8 CPUs machine but a 1 CPU cpumask. The evsel allocates
>> the xyarray for 1 CPU 1 thread. The fd are first close with 1 CPU, 1 thread.
>> But then evlist_close() comes in and STILL thinks the events were using
>> 8 CPUs, 1 thread and thus a xyarray of that size. And this causes writes
>> to entries that are beyond the xyarray when the fds are set to -1, thereby
>> causing memory corruption which I was lucky to catch via glibc.
>> First, why are we closing the descriptors twice?
> The idea here was to reduce the boilerplate that tools need to do when
> they are done dealing with evlists, so evlist__delete would do what the
> kernel does to resources allocated to a thread when it exits without
> explicitely deallocating them: release them all.
> So it seems, from your analysis, that bugs were left that need to be
> hammered out so that this works as intended. Can you share your patch?
>> Second, I have a fix that seems to work for me. It uses the evsel->cpus
>> if evsel->cpus exists, otherwise it defaults to evtlist->cpus. Looks like
>> a reasonable thing to do to me, but is it? I would rather avoid the double
>> close altogether.
>> Opinion?
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