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 <jsl...@suse.cz>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Arnaldo Carvalho de Melo <a...@kernel.org>
Cc: Mark Rutland <mark.rutl...@arm.com>
Cc: Alexander Shishkin <alexander.shish...@linux.intel.com>
Cc: Jiri Olsa <jo...@redhat.com>
Cc: Namhyung Kim <namhy...@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
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
