Re: [PATCH] perf tests: Fix flaky test 'Object code reading'

From: Arnaldo Carvalho de Melo
Date: Fri Sep 24 2021 - 14:53:42 EST


Em Mon, Sep 06, 2021 at 04:22:38PM +0100, James Clark escreveu:
> 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@plt+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.

Thanks, applied.

- Arnaldo


> Signed-off-by: James Clark <james.clark@xxxxxxx>
> ---
> 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

--

- Arnaldo