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perf symbols: Resolve symbols against debug file first

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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@suse.cz>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210217122125.26416-1-jslaby@suse.cz
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Jiri Slaby 2021-02-17 13:21:25 +01:00 committed by Arnaldo Carvalho de Melo
parent 37b3fa0ef3
commit 6833e0b81a
1 changed files with 15 additions and 1 deletions
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16
tools/perf/util/symbol-elf.c
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@ -1226,12 +1226,26 @@ 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);
/*
* We have to fallback to runtime when syms' section header has
* NOBITS set. NOBITS results in file offset (sh_offset) not
* being incremented. So sh_offset used below has different
* values for syms (invalid) and runtime (valid).
*/
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;