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

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Message-ID: <20241128010325.946897-1-namhyung@kernel.org>
Date: Wed, 27 Nov 2024 17:03:25 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.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: [PATCH v2] perf tools: Avoid unaligned pointer operations

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>
---
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) {
-- 
2.47.0.338.g60cca15819-goog