linux-kernel - [PATCH] perf tests: Fix flaky test 'Object code reading'

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Date:   Mon,  6 Sep 2021 16:22:38 +0100
From:   James Clark <james.clark@....com>
To:     acme@...nel.org, linux-perf-users@...r.kernel.org
Cc:     James Clark <james.clark@....com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] perf tests: Fix flaky test 'Object code reading'

This test occasionally fails on aarch64 when a sample is taken in
free@plt and it fails with "Bytes read differ from those read by
objdump". This is because that symbol is near a section boundary in the
elf file. Despite the -z option to always output zeros, objdump uses
bfd_map_over_sections() to iterate through the elf file so it doesn't
see outside of the sections where these zeros are and can't print them.

For example this boundary proceeds free@plt in libc with a gap of 48
bytes between .plt and .text:

  objdump -d -z --start-address=0x23cc8 --stop-address=0x23d08 libc-2.30.so

  libc-2.30.so:     file format elf64-littleaarch64

  Disassembly of section .plt:

  0000000000023cc8 <*ABS*+0x7fd00@...+0x8>:
     23cc8:	91018210 	add	x16, x16, #0x60
     23ccc:	d61f0220 	br	x17

  Disassembly of section .text:

  0000000000023d00 <abort@@GLIBC_2.17-0x98>:
     23d00:	a9bf7bfd 	stp	x29, x30, [sp, #-16]!
     23d04:	910003fd 	mov	x29, sp

Taking a sample in free@plt is very rare because it is so small, but the
test can be forced to fail almost every time on any platform by linking
the test with a shared library that has a single empty function and
calling it in a loop.

The fix is to zero the buffers so that when there is a jump in the
addresses output by objdump, zeros are already filled in between.

Signed-off-by: James Clark <james.clark@....com>
---
 tools/perf/tests/code-reading.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/tests/code-reading.c b/tools/perf/tests/code-reading.c
index 9866cddebf23..9b4a765e4b73 100644
--- a/tools/perf/tests/code-reading.c
+++ b/tools/perf/tests/code-reading.c
@@ -229,8 +229,8 @@ static int read_object_code(u64 addr, size_t len, u8 cpumode,
 			    struct thread *thread, struct state *state)
 {
 	struct addr_location al;
-	unsigned char buf1[BUFSZ];
-	unsigned char buf2[BUFSZ];
+	unsigned char buf1[BUFSZ] = {0};
+	unsigned char buf2[BUFSZ] = {0};
 	size_t ret_len;
 	u64 objdump_addr;
 	const char *objdump_name;
-- 
2.28.0