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

[Jiri Olsa]

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@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 for explanation, could you please put this comment in the code?

thanks,
jirka