[PATCH] perf symbol: Do not fixup end address of labels

[PATCH] perf symbol: Do not fixup end address of labels

[Namhyung Kim]

When it loads symbols from an ELF file, it loads label symbols which is
0 size.  Sometimes it has the same address with other symbols and might
shadow the original symbols because it fixes up the size of the symbol.

For example, in my system __do_softirq is shadowed and only accepts the
__softirqentry_text_start instead.  But it should accept __do_softirq.

  $ readelf -sW vmlinux | grep -e __do_softirq -e __softirqentry_text_start
  105089: ffffffff82000000   814 FUNC    GLOBAL DEFAULT    1 __do_softirq
  111954: ffffffff82000000     0 NOTYPE  GLOBAL DEFAULT    1 __softirqentry_text_start

  $ perf annotate --stdio __do_softirq
  Error:
  The perf.data data has no samples!

  $ perf annotate --stdio __softirqentry_text_start | head
   Percent |	Source code & Disassembly of vmlinux for cycles (26 samples, percent: local period)
  ---------------------------------------------------------------------------------------------------
           : 0                0xffffffff82000000 <__softirqentry_text_start>:
      0.00 :   ffffffff82000000:        nopl    (%rax,%rax)
     30.77 :   ffffffff82000005:        pushq   %rbp
      3.85 :   ffffffff82000006:        movq    %rsp, %rbp
      0.00 :   ffffffff82000009:        pushq   %r15
      3.85 :   ffffffff8200000b:        pushq   %r14
      3.85 :   ffffffff8200000d:        pushq   %r13
      0.00 :   ffffffff8200000f:        pushq   %r12

We can ignore NOTYPE symbols in the symbols__fixup_end() so that it can
pick the __do_softirq() in choose_best_symbol().  This should be fine
since most symbols have either STT_FUNC or STT_OBJECT.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/symbol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index ea0da3f46f10b756..a445035f7dd7ed88 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -262,7 +262,7 @@ void symbols__fixup_end(struct rb_root_cached *symbols, bool is_kallsyms)
 		 * like in:
 		 *   ffffffffc1937000 T hdmi_driver_init  [snd_hda_codec_hdmi]
 		 */
-		if (prev->end == prev->start) {
+		if (prev->end == prev->start && prev->type != STT_NOTYPE) {
 			const char *prev_mod;
 			const char *curr_mod;
 
-- 
2.46.0.662.g92d0881bb0-goog

Re: [PATCH] perf symbol: Do not fixup end address of labels

[Namhyung Kim]

On Thu, 12 Sep 2024 15:42:08 -0700, Namhyung Kim wrote:

> When it loads symbols from an ELF file, it loads label symbols which is
> 0 size.  Sometimes it has the same address with other symbols and might
> shadow the original symbols because it fixes up the size of the symbol.
> 
> For example, in my system __do_softirq is shadowed and only accepts the
> __softirqentry_text_start instead.  But it should accept __do_softirq.
> 
> [...]

Applied to perf-tools-next, thanks!

Best regards,
Namhyung