Re: [PATCH] perf tools: Resolve symbols against debug file first

Namhyung Kim [off-list ref] writes:

On Wed, Jan 13, 2021 at 8:43 PM Jiri Slaby [off-list ref] wrote:

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On 13. 01. 21, 11:46, Jiri Olsa wrote:

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On Wed, Jan 13, 2021 at 09:01:28AM +0100, Jiri Slaby wrote:

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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

...

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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.

It seems PPC has an opd section only in the runtime_ss and that's why
we use it for section headers.

At least on my system (Ubuntu 20.04.1) I see .opd in the debug file with
NOBITS set:

$ readelf -e vmlinux.debug | grep opd
  [37] .opd              NOBITS           c000000001c1f548  01202e14


But possibly that's not the case with older toolchains?

cheers