perf tools: Allow build-id with trailing zeros

Currently perf saves a build-id with size but old versions assumes the
size of 20.  In case the build-id is less than 20 (like for MD5), it'd
fill the rest with 0s.

I saw a problem when old version of perf record saved a binary in the
build-id cache and new version of perf reads the data.  The symbols
should be read from the build-id cache (as the path no longer has the
same binary) but it failed due to mismatch in the build-id.

  symsrc__init: build id mismatch for /home/namhyung/.debug/.build-id/53/e4c2f42a4c61a2d632d92a72afa08f00000000/elf.

The build-id event in the data has 20 byte build-ids, but it saw a
different size (16) when it reads the build-id of the elf file in the
build-id cache.

  $ readelf -n ~/.debug/.build-id/53/e4c2f42a4c61a2d632d92a72afa08f00000000/elf

  Displaying notes found in: .note.gnu.build-id
    Owner                Data size 	Description
    GNU                  0x00000010	NT_GNU_BUILD_ID (unique build ID bitstring)
      Build ID: 53e4c2f42a4c61a2d632d92a72afa08f

Let's fix this by allowing trailing zeros if the size is different.

Fixes: 39be8d01 ("perf tools: Pass build_id object to dso__build_id_equal()")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210910224630.1084877-1-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/dso.c

@@ -1349,6 +1349,16 @@ void dso__set_build_id(struct dso *dso, struct build_id *bid)
 
 bool dso__build_id_equal(const struct dso *dso, struct build_id *bid)
 {
+	if (dso->bid.size > bid->size && dso->bid.size == BUILD_ID_SIZE) {
+		/*
+		 * For the backward compatibility, it allows a build-id has
+		 * trailing zeros.
+		 */
+		return !memcmp(dso->bid.data, bid->data, bid->size) &&
+			!memchr_inv(&dso->bid.data[bid->size], 0,
+				    dso->bid.size - bid->size);
+	}
+
 	return dso->bid.size == bid->size &&
 	       memcmp(dso->bid.data, bid->data, dso->bid.size) == 0;
 }