linux-kernel - [PATCH] perf tools: Resolve symbols against debug file first

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Date:   Wed, 13 Jan 2021 09:01:28 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     acme@...nel.org
Cc:     linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: [PATCH] perf tools: Resolve symbols against debug file first

With LTO, there are symbols like these:
/usr/lib/debug/usr/lib64/libantlr4-runtime.so.4.8-4.8-1.4.x86_64.debug
 10305: 0000000000955fa4     0 NOTYPE  LOCAL  DEFAULT   29 Predicate.cpp.2bc410e7

This comes from a runtime/debug split done by the standard way:
objcopy --only-keep-debug $runtime $debug
objcopy --add-gnu-debuglink=$debugfn -R .comment -R .GCC.command.line --strip-all $runtime

perf currently cannot resolve such symbols (relicts of LTO), as section
29 exists only in the debug file (29 is .debug_info). And perf resolves
symbols only against runtime file. This results in all symbols from such
a library being unresolved:
     0.38%  main2    libantlr4-runtime.so.4.8  [.] 0x00000000000671e0

So try resolving against the debug file first. And only if it fails (the
section has NOBITS set), try runtime file. We can do this, as "objcopy
--only-keep-debug" per documentation preserves all sections, but clears
data of some of them (the runtime ones) and marks them as NOBITS.

The correct result is now:
     0.38%  main2    libantlr4-runtime.so.4.8  [.] antlr4::IntStream::~IntStream

Note that these LTO symbols are properly skipped anyway as they belong
neither to *text* nor to *data* (is_label && !elf_sec__filter(&shdr,
secstrs) is true).

Signed-off-by: Jiri Slaby <jslaby@...e.cz>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/util/symbol-elf.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index f3577f7d72fe..a31b716fa61c 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -1226,12 +1226,20 @@ int dso__load_sym(struct dso *dso, struct map *map, struct symsrc *syms_ss,
 		if (sym.st_shndx == SHN_ABS)
 			continue;
 
-		sec = elf_getscn(runtime_ss->elf, sym.st_shndx);
+		sec = elf_getscn(syms_ss->elf, sym.st_shndx);
 		if (!sec)
 			goto out_elf_end;
 
 		gelf_getshdr(sec, &shdr);
 
+		if (shdr.sh_type == SHT_NOBITS) {
+			sec = elf_getscn(runtime_ss->elf, sym.st_shndx);
+			if (!sec)
+				goto out_elf_end;
+
+			gelf_getshdr(sec, &shdr);
+		}
+
 		if (is_label && !elf_sec__filter(&shdr, secstrs))
 			continue;
 
-- 
2.30.0