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

[Jiri Olsa <jolsa@redhat.com>]

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

Namhyung, any idea?

thanks,
jirka