linux-kernel - Re: [PATCH v1] libperf mmap: In user mmap rdpmc avoid undefined behavior

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Message-ID: <aN6-xfp4pcnxbnaq@x1>
Date: Thu, 2 Oct 2025 15:04:53 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] libperf mmap: In user mmap rdpmc avoid undefined
 behavior

On Sun, Sep 14, 2025 at 11:24:37AM -0700, Ian Rogers wrote:
> A shift left of a signed 64-bit s64 may overflow and result in
> undefined behavior caught by ubsan. Switch to a u64 instead.

Thanks, applied to perf-tools-next,

- Arnaldo
 
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
>  tools/lib/perf/mmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
> index c1a51d925e0e..ec124eb0ec0a 100644
> --- a/tools/lib/perf/mmap.c
> +++ b/tools/lib/perf/mmap.c
> @@ -508,7 +508,7 @@ int perf_mmap__read_self(struct perf_mmap *map, struct perf_counts_values *count
>  		idx = READ_ONCE(pc->index);
>  		cnt = READ_ONCE(pc->offset);
>  		if (pc->cap_user_rdpmc && idx) {
> -			s64 evcnt = read_perf_counter(idx - 1);
> +			u64 evcnt = read_perf_counter(idx - 1);
>  			u16 width = READ_ONCE(pc->pmc_width);
>  
>  			evcnt <<= 64 - width;
> -- 
> 2.51.0.384.g4c02a37b29-goog
>