linux-kernel - [PATCH v3 2/2] perf symbol: Skip symbols if SHF

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Message-Id: <20220724060013.171050-3-leo.yan@linaro.org>
Date:   Sun, 24 Jul 2022 14:00:13 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     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>,
        Fangrui Song <maskray@...gle.com>,
        Chang Rui <changruinj@...il.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Leo Yan <leo.yan@...aro.org>
Subject: [PATCH v3 2/2] perf symbol: Skip symbols if SHF_ALLOC flag is not set

Some symbols are observed the '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 sections, such kind of sections are used for linker
warning when a file calls deprecated functions, but they are not part of
memory images, the symbols in these sections should be dropped.

This patch checks the section attribute SHF_ALLOC bit, if the bit is not
set, it skips symbols to avoid spurious ones.

Suggested-by: Fangrui Song <maskray@...gle.com>
Signed-off-by: Leo Yan <leo.yan@...aro.org>
---
 tools/perf/util/symbol-elf.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index ef6ced5c5746..b3be5b1d9dbb 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -1255,6 +1255,17 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
 
 		gelf_getshdr(sec, &shdr);
 
+		/*
+		 * If the attribute bit SHF_ALLOC is not set, the section
+		 * doesn't occupy memory during process execution.
+		 * E.g. ".gnu.warning.*" section is used by linker to generate
+		 * warnings when calling deprecated functions, the symbols in
+		 * the section aren't loaded to memory during process execution,
+		 * so skip them.
+		 */
+		if (!(shdr.sh_flags & SHF_ALLOC))
+			continue;
+
 		secstrs = secstrs_sym;
 
 		/*
-- 
2.25.1