linux-kernel - [PATCH] perf tools: Allow build-id with trailing zeros

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Date:   Fri, 10 Sep 2021 09:41:50 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>
Subject: [PATCH] perf tools: Allow build-id with trailing zeros

Current 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 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: 39be8d0115b3 ("perf tools: Pass build_id object to dso__build_id_equal()")

Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/util/dso.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
index ee15db2be2f4..0c0dd877d4e9 100644
--- a/tools/perf/util/dso.c
+++ b/tools/perf/util/dso.c
@@ -1349,6 +1349,18 @@ 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)
 {
+	static const char zeros[BUILD_ID_SIZE];
+
+	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) &&
+			!memcmp(&dso->bid.data[bid->size], zeros,
+				dso->bid.size - bid->size);
+	}
+
 	return dso->bid.size == bid->size &&
 	       memcmp(dso->bid.data, bid->data, dso->bid.size) == 0;
 }
-- 
2.33.0.309.g3052b89438-goog