linux-kernel - Re: [PATCH] perf evsel: fix NULL pointer deference when evsel->counts is NULL

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Date:	Mon, 21 Jan 2013 13:53:21 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Colin King <colin.king@...onical.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf evsel: fix NULL pointer deference when evsel->counts is NULL

Hi Colin,

On Sat, 19 Jan 2013 16:36:54 +0000, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
>
> __perf_evsel__read_on_cpu() only bails out with -ENOMEM if
> evsel->counts is NULL and perf_evsel__alloc_counts() has returned
> an error.  If perf_evsel__alloc_counts() does not return an error
> we get an NULL pointer deference on evsel->counts->cpu[cpu]
> if evsel->counts is NULL.

perf_evsel__alloc_counts() should allocate evsel->counts when it sees
evsel->counts is NULL and return negative error code if the allocation
fails.

So I don't see any problem in current code.  With your code, it won't
try to allocate if ->counts is NULL but overwrite existing ->counts?

Thanks,
Namhyung

>
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  tools/perf/util/evsel.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 1b16dd1..93acd06 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -640,7 +640,7 @@ int __perf_evsel__read_on_cpu(struct perf_evsel *evsel,
>  	if (FD(evsel, cpu, thread) < 0)
>  		return -EINVAL;
>  
> -	if (evsel->counts == NULL && perf_evsel__alloc_counts(evsel, cpu + 1) < 0)
> +	if (evsel->counts == NULL || perf_evsel__alloc_counts(evsel, cpu + 1) < 0)
>  		return -ENOMEM;
>  
>  	if (readn(FD(evsel, cpu, thread), &count, nv * sizeof(u64)) < 0)
--
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