linux-kernel - Re: [PATCH v2 2/2] perf symbol: Skip recording symbols in '.gnu.warning.*' sections

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Message-ID: <CAFP8O3LJq1rt6i0_rAW5pShUiA=FJ38ceb4Ps0KmLdCNzC9xbQ@mail.gmail.com>
Date:   Sat, 23 Jul 2022 20:42:20 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] perf symbol: Skip recording symbols in
 '.gnu.warning.*' sections

On Sat, Jul 23, 2022 at 7:29 PM Leo Yan <leo.yan@...aro.org> wrote:
>
> Some symbols are observed their 'st_value' field are zeros.  E.g.
> libc.so.6 in Ubuntu contains a symbol '__evoke_link_warning_getwd' which
> resides in the '.gnu.warning.getwd' section, unlike normal symbols, this
> kind of symbols are only used for linker warning.
>
> This patch skips to record symbols from '.gnu.warning.*' sections by
> detecting the sub string '.gnu.warning' is contained in section name.
>
> Signed-off-by: Leo Yan <leo.yan@...aro.org>

Presumably __evoke_link_warning_getwd is due to `clang -fuse-ld=lld
-static ...` on a file calling the deprecated getwd.
GNU ld and gold implement a .gnu.warning.* feature which removes the
section. ld.lld just ignores this section as the usefulness of the
functionality is unclear.

The section .gnu.warning.getwd does not have the SHF_ALLOC flag. Such
sections are not part of memory images and I think it is more generic
ignoring all symbols residing in a non-SHF_ALLOC section.

> ---
>  tools/perf/util/symbol-elf.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index ef6ced5c5746..4b621e355c0e 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -1277,6 +1277,14 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
>
>                 section_name = elf_sec__name(&shdr, secstrs);
>
> +               /*
> +                * A symbol coming from ".gnu.warning.*" sections is used to
> +                * generate linker warnings, its 'sym.st_value' field usually
> +                * is zero, skip to record it.
> +                */
> +               if (strstr(section_name, ".gnu.warning"))
> +                       continue;
> +
>                 /* On ARM, symbols for thumb functions have 1 added to
>                  * the symbol address as a flag - remove it */
>                 if ((ehdr.e_machine == EM_ARM) &&
> --
> 2.25.1
>


-- 
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