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

From: Arnaldo Carvalho de Melo
Date: Fri Jan 17 2014 - 09:06:18 EST

Em Fri, Jan 17, 2014 at 10:00:20AM +0100, Stephane Eranian escreveu:
> Hi,
> I have been debugging a NULL pointer issue with perf stat unit/scale code
> and in the process I ran into what appeared like a double-free issue reported
> by glibc. It took me a while to realize that it was because of memory corruption
> caused by a recent change in how evsel are freed.
> My test case is simple. I used RAPL but I think any event with a suggested
> cpumask in /sys/devices/XXX/cpumask will do:
> # perf stat -a -e power/energy-cores/ ls
> 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.

Oops, at some point I knew that set of globals and mixup of evlists in
builtin-stat would bite :-\

I guess it was introduced in:

commit 7ae92e744e3fb389afb1e24920ecda331d360c61
Author: Yan, Zheng <zheng.z.yan@xxxxxxxxx>
Date: Mon Sep 10 15:53:50 2012 +0800

perf stat: Check PMU cpumask file

I need to untangle that direct usage of the target, and global evlist to
properly fix this, but in the meantime I'll take a look at your patch,
thanks for doing this work.

> 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?
> 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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