Messages in this thread | | | Date | Wed, 3 Feb 2021 20:32:10 +0100 | From | Jiri Olsa <> | Subject | Re: [PATCH] perf tools: Resolve symbols against debug file first |
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On Thu, Jan 28, 2021 at 11:43:07AM +0100, Jiri Slaby wrote: > 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 > > > > > 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 > > To resume this and answer: > > Yes, the fallback is necessary. > > It's because syms_ss section header has NOBITS set for the sections, so file > offset is not incremented. So shdr.sh_offset (the file offset) used further > in dso__load_sym has different values for syms and runtime. The syms_ss (the > NOBITS) one is invalid as it has 0x1000 here. The runtime one contains good > values (like 000509d0 here): > > .text 00082560 00000000000509d0 00000000000509d0 [-00001000-] > {+000509d0+} 2**4 > > That is, without the fallback, the computed symbol address is wrong.
thanks for explanation, could you please put this comment in the code?
thanks, jirka
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