perf tools: Fix build-id event recording

The build-id events written at the end of the record session are broken
due to unexpected data.  The write_buildid() writes the fixed length
event first and then variable length filename.

But a recent change made it write more data in the padding area
accidentally.  So readers of the event see zero-filled data for the
next entry and treat it incorrectly.  This resulted in wrong kernel
symbols because the kernel DSO loaded a random vmlinux image in the
path as it didn't have a valid build-id.

Fixes: ae39ba1 ("perf inject: Fix build ID injection")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/Z0aRFFW9xMh3mqKB@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/util/build-id.c

@@ -277,7 +277,7 @@ static int write_buildid(const char *name, size_t name_len, struct build_id *bid
 	struct perf_record_header_build_id b;
 	size_t len;
 
-	len = sizeof(b) + name_len + 1;
+	len = name_len + 1;
 	len = PERF_ALIGN(len, sizeof(u64));
 
 	memset(&b, 0, sizeof(b));
@@ -286,7 +286,7 @@ static int write_buildid(const char *name, size_t name_len, struct build_id *bid
 	misc |= PERF_RECORD_MISC_BUILD_ID_SIZE;
 	b.pid = pid;
 	b.header.misc = misc;
-	b.header.size = len;
+	b.header.size = sizeof(b) + len;
 
 	err = do_write(fd, &b, sizeof(b));
 	if (err < 0)