[PATCH v2 2/4] perf synthetic-events: Ensure features are aligned

[Ian Rogers <irogers@google.com>]

Features like hostname have arbitrary size and break the assumed
8-byte alignment of perf events. Pad all feature events until 8-byte
alignment is restored.

Rather than invent a new mechanism, reuse write_padded but pass an
empty buffer as what to write. As no alignment may be necessary,
update write_padded to be tolerant of 0 as a count and count_aligned
value.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/header.c           | 7 ++++---
 tools/perf/util/synthetic-events.c | 2 ++
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 03e43a9894d4..d10703090e83 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -168,11 +168,12 @@ int write_padded(struct feat_fd *ff, const void *bf,
 		 size_t count, size_t count_aligned)
 {
 	static const char zero_buf[NAME_ALIGN];
-	int err = do_write(ff, bf, count);
+	int err = count > 0 ? do_write(ff, bf, count) : 0;
 
-	if (!err)
+	if (!err && count_aligned > count) {
+		assert(count_aligned - count < sizeof(zero_buf));
 		err = do_write(ff, zero_buf, count_aligned - count);
-
+	}
 	return err;
 }
 
diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
index f8ac2ac2da45..ee6e9c2fb11b 100644
--- a/tools/perf/util/synthetic-events.c
+++ b/tools/perf/util/synthetic-events.c
@@ -2401,6 +2401,8 @@ int perf_event__synthesize_features(const struct perf_tool *tool, struct perf_se
 			pr_debug("Error writing feature\n");
 			continue;
 		}
+		write_padded(&ff, /*bf=*/NULL, /*count=*/0,
+			     /*count_aligned=*/PERF_ALIGN(ff.offset, sizeof(u64)) - ff.offset);
 		/* ff.buf may have changed due to realloc in do_write() */
 		fe = ff.buf;
 		memset(fe, 0, sizeof(*fe));
-- 
2.47.1.613.gc27f4b7a9f-goog