Re: [PATCH v2] perf tools: Avoid unaligned pointer operations

From: Liang, Kan
Date: Thu Nov 28 2024 - 10:00:01 EST




On 2024-11-27 8:03 p.m., Namhyung Kim 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.
>
> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

Thanks,
Kan

> ---
> v2) use '%' instead of '/' to check alignment
>
> 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..6923b0d5efede4a7 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) {