[PATCH v2] perf tools: Resolve symbols against debug file first

[PATCH v2] perf tools: Resolve symbols against debug file first

[Jiri Slaby]

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>
Acked-by: Namhyung Kim <namhyung@kernel.org>
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>
---
[v2] added a comment

 tools/perf/util/symbol-elf.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index f3577f7d72fe..ecc05aa8399d 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -1226,12 +1226,26 @@ 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;
 
 		gelf_getshdr(sec, &shdr);
 
+		/*
+		 * We have to fallback to runtime when syms' section header has
+		 * NOBITS set. NOBITS results in file offset (sh_offset) not
+		 * being incremented. So sh_offset used below has different
+		 * values for syms (invalid) and runtime (valid).
+		 */
+		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);
+		}
+
 		if (is_label && !elf_sec__filter(&shdr, secstrs))
 			continue;
 
-- 
2.30.1

Re: [PATCH v2] perf tools: Resolve symbols against debug file first

[Arnaldo Carvalho de Melo]

Em Wed, Feb 17, 2021 at 01:21:25PM +0100, Jiri Slaby escreveu:
> 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>
> Acked-by: Namhyung Kim <namhyung@kernel.org>

Thanks, applied.

- Arnaldo

> 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>
> ---
> [v2] added a comment
> 
>  tools/perf/util/symbol-elf.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index f3577f7d72fe..ecc05aa8399d 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -1226,12 +1226,26 @@ 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;
>  
>  		gelf_getshdr(sec, &shdr);
>  
> +		/*
> +		 * We have to fallback to runtime when syms' section header has
> +		 * NOBITS set. NOBITS results in file offset (sh_offset) not
> +		 * being incremented. So sh_offset used below has different
> +		 * values for syms (invalid) and runtime (valid).
> +		 */
> +		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);
> +		}
> +
>  		if (is_label && !elf_sec__filter(&shdr, secstrs))
>  			continue;
>  
> -- 
> 2.30.1
> 

-- 

- Arnaldo