Messages in this thread | | | From | Jiri Slaby <> | Subject | Re: [PATCH] perf tools: Resolve symbols against debug file first | Date | Wed, 13 Jan 2021 12:43:31 +0100 |
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On 13. 01. 21, 11:46, Jiri Olsa wrote: > On Wed, Jan 13, 2021 at 09:01:28AM +0100, Jiri Slaby wrote: >> 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> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Ingo Molnar <mingo@redhat.com> >> Cc: Arnaldo Carvalho de Melo <acme@kernel.org> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> >> Cc: Jiri Olsa <jolsa@redhat.com> >> Cc: Namhyung Kim <namhyung@kernel.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; > > we iterate symbols from syms_ss, so the fix seems to be correct > to call elf_getscn on syms_ss, not on runtime_ss as we do now > > I'd think this worked only when runtime_ss == syms_ss
No, because the headers are copied 1:1 from runtime_ss to syms_ss. And runtime_ss is then stripped, so only .debug* sections are removed there. (And syms_ss's are set as NOBITS.)
We iterated .debug* sections in syms_ss and used runtime_ss section _headers_ only to adjust symbols (sometimes). That worked.
>> 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); >> + } > > is that fallback necessary? the symbol is from syms_ss
Provided the above, we don't need the section data here, only headers, so the NOBITS test is superfluous and the fallback shouldn't be needed. Let me test it.
thanks, -- js suse labs
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