linux-kernel - Re: [RFC][PATCH 1/2] Fix event.c misaligned address error

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Date:   Wed, 11 Sep 2019 02:08:32 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Numfor Mbiziwo-Tiapo <nums@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Song Liu <songliubraving@...com>, mbd@...com,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC][PATCH 1/2] Fix event.c misaligned address error

An idea here is that, if this breaks backward compatibility, we
introduce an aligned variant and work to deprecate the unaligned
variant. I will look into making a patch set.

Thanks,
Ian

On Wed, Jul 24, 2019 at 3:50 PM Numfor Mbiziwo-Tiapo <nums@...gle.com> wrote:
>
> The ubsan (undefined behavior sanitizer) build of perf throws an
> error when the synthesize "Synthesize cpu map" function from
> perf test is run.
>
> This can be reproduced by running (from the tip directory):
> make -C tools/perf USE_CLANG=1 EXTRA_CFLAGS="-fsanitize=undefined"
>
> (see cover letter for why perf may not build)
>
> then running: tools/perf/perf test 44 -v
>
> This bug occurs because the cpu_map_data__synthesize function in
> event.c calls synthesize_mask, casting the 'data' variable
> (of type cpu_map_data*) to a cpu_map_mask*. Since struct
> cpu_map_data is 2 byte aligned and struct cpu_map_mask is 8 byte
> aligned this causes memory alignment issues.
>
> This is fixed by adding 6 bytes of padding to the struct cpu_map_data,
> however, this will break compatibility between perf data files - a file
> written by an old perf wouldn't work with a perf with this patch due
> to event data alignment changing.
>
> Comments?
>
> Not-Quite-Signed-off-by: Numfor Mbiziwo-Tiapo <nums@...gle.com>
> ---
>  tools/perf/util/event.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
> index eb95f3384958..82eaf06c2604 100644
> --- a/tools/perf/util/event.h
> +++ b/tools/perf/util/event.h
> @@ -433,6 +433,7 @@ struct cpu_map_mask {
>
>  struct cpu_map_data {
>         u16     type;
> +       u8 pad[6];
>         char    data[];
>  };
>
> --
> 2.22.0.657.g960e92d24f-goog
>