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

[Jiri Slaby]

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@xxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> > Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> > Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> > ---
> >   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,
--
js
suse labs