linux-kernel - Re: [PATCH 2/2] perf stat: fix memory corruption of xyarray when cpumask is used

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Date:	Mon, 20 Jan 2014 17:06:52 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
	jolsa@...hat.com, dsahern@...il.com
Subject: Re: [PATCH 2/2] perf stat: fix memory corruption of xyarray when
 cpumask is used

Em Fri, Jan 17, 2014 at 04:34:06PM +0100, Stephane Eranian escreveu:
> This patch fixes a memory corruption problem with the xyarray when
> the evsel fds get closed at the end of the run_perf_stat() call.
> 
> It could be triggered with:
>  # perf stat -a -e power/energy-cores/ ls
 
> When cpumask are used by events (.e.g, RAPL or uncores) then the evsel
> fds are allocated based on the actual number of CPUs monitored. That
> number can be smaller than the total number of CPUs on the system. The
> problem arises at the end by perf stat closes the fds twice. When fds
> are closed, their entry in the xyarray are set to -1. The first
> close() on the evsel is made from __run_perf_stat() and it uses the
> actual number of CPUS for the event which is how the xyarray was
> allocated for. The second is from perf_evlist_close() but that one is
> on the total number of CPUs in the system, so it assume the xyarray
> was allocated to cover it. However it was not, and some writes corrupt
> memory.
 
> The fix is in perf_evlist_close() is to first try with the
> evsel->cpus if present, if not use the evlist->cpus. That
> fixes the problem.

Humm, if there is an evsel->cpus, why does perf_evsel__close needs to
get that ncpus parameter?

My first reaction is that evsel->cpus needs to point to what is being
used for its evsel->fd member, that way we will never need to pass a
ncpus, because evsel->cpus->nr will be the size of ots evsel->fd
xyarray.

Now to figure out why is that we pass ncpus when we have evsel->cpus,
bet ->cpus came after ->fd, i.e. the assumption was that we don't ever
need to have a per evsel cpu map, as it would use the one in the evlist
that contains said evsel, but for some reason we grew evsel->cpus anyway
abd forgot to use its cpus->nr to ditch the ncpus evsel__close() parm.

I'll apply your patch, as it fixes the issue, but the above analysis
probably will led us to remove that.

But just a moment, why have you removed the evsel->fd zeroing at the end
of perf_evsel__close()? Since we called perf_evsel__free_fd(), ok, that
is because perf_evsel__free_fd() does that already, no need to zero it
again, ok, moving that to a separate, prep patch.

Thanks for looking into this, I'll get this to Ingo soon.

- Arnaldo
 
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> ---
>  tools/perf/util/evlist.c |    7 +++++--
>  tools/perf/util/evsel.c  |    2 --
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 40bd2c0..59ef280 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -1003,9 +1003,12 @@ void perf_evlist__close(struct perf_evlist *evlist)
>  	struct perf_evsel *evsel;
>  	int ncpus = cpu_map__nr(evlist->cpus);
>  	int nthreads = thread_map__nr(evlist->threads);
> +	int n;
>  
> -	evlist__for_each_reverse(evlist, evsel)
> -		perf_evsel__close(evsel, ncpus, nthreads);
> +	evlist__for_each_reverse(evlist, evsel) {
> +		n = evsel->cpus ? evsel->cpus->nr : ncpus;
> +		perf_evsel__close(evsel, n, nthreads);
> +	}
>  }
>  
>  int perf_evlist__open(struct perf_evlist *evlist)
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 22e18a2..0a878db 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -667,7 +667,6 @@ int perf_evsel__alloc_fd(struct perf_evsel *evsel, int ncpus, int nthreads)
>  {
>  	int cpu, thread;
>  	evsel->fd = xyarray__new(ncpus, nthreads, sizeof(int));
> -
>  	if (evsel->fd) {
>  		for (cpu = 0; cpu < ncpus; cpu++) {
>  			for (thread = 0; thread < nthreads; thread++) {
> @@ -1081,7 +1080,6 @@ void perf_evsel__close(struct perf_evsel *evsel, int ncpus, int nthreads)
>  
>  	perf_evsel__close_fd(evsel, ncpus, nthreads);
>  	perf_evsel__free_fd(evsel);
> -	evsel->fd = NULL;
>  }
>  
>  static struct {
> -- 
> 1.7.9.5
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