Re: [PATCH] perf tools: Avoid unaligned pointer operations

[Ian Rogers]

On Wed, Nov 27, 2024 at 1:26 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> The sample data is 64-bit aligned basically but raw data starts with
> 32-bit length field and data follows.  In perf_event__synthesize_sample
> it treats the sample data as a 64-bit array.  And it needs some trick
> to update the raw data properly.
>
> But it seems some compilers are not happy with this and the program dies
> siliently.  I found the sample parsing test failed without any messages
> on affected systems.
>
> Let's update the code to use a 32-bit pointer directly and make sure the
> result is 64-bit aligned again.  No functional changes intended.
>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks,
Ian

> ---
>  tools/perf/util/synthetic-events.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
> index a58444c4aed1f1ea..385383ef6cf1edaf 100644
> --- a/tools/perf/util/synthetic-events.c
> +++ b/tools/perf/util/synthetic-events.c
> @@ -1686,12 +1686,16 @@ int perf_event__synthesize_sample(union perf_event *event, u64 type, u64 read_fo
>         }
>
>         if (type & PERF_SAMPLE_RAW) {
> -               u.val32[0] = sample->raw_size;
> -               *array = u.val64;
> -               array = (void *)array + sizeof(u32);
> +               u32 *array32 = (void *)array;
> +
> +               *array32 = sample->raw_size;
> +               array32++;
> +
> +               memcpy(array32, sample->raw_data, sample->raw_size);
> +               array = (void *)(array32 + (sample->raw_size / sizeof(u32)));
>
> -               memcpy(array, sample->raw_data, sample->raw_size);
> -               array = (void *)array + sample->raw_size;
> +               /* make sure the array is 64-bit aligned */
> +               BUG_ON(((long)array) / sizeof(u64));
>         }
>
>         if (type & PERF_SAMPLE_BRANCH_STACK) {
> --
> 2.47.0.338.g60cca15819-goog
>