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

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Message-ID: <d40c6d0e-3755-4cfa-bb9e-e2aa520edb0f@linux.intel.com>
Date: Thu, 28 Nov 2024 09:59:51 -0500
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Namhyung Kim <namhyung@...nel.org>,
 Arnaldo Carvalho de Melo <acme@...nel.org>, Ian Rogers <irogers@...gle.com>
Cc: Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2] perf tools: Avoid unaligned pointer operations



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@...gle.com>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>

Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>

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) {