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From: Dima Kogan <dima@secretsauce.net>
To: linux-perf-users@vger.kernel.org
Cc: Dima Kogan <dima@secretsauce.net>
Subject: [PATCH 0/2] Better error messages for 'perf probe --add'
Date: Mon, 15 Apr 2024 21:55:09 -0700	[thread overview]
Message-ID: <20240416045533.162692-1-dima@secretsauce.net> (raw)

Hi.

I tried to add some userspace probes to some C++ code, and got this:

  $ sudo perf probe -x tst --add _ZN.....
  Error: Failed to add events.

Note the completely non-useful error message. The issue is that the C++ symbol
mangling can create very loooong symbol names, causing perf to try to use very
long probe names by default, causing this problem. It took a little while to
figure this out, and a better error message would have helped, hence these
patches.

To reproduce, here's a tst.cc:

  #include <stdio.h>

  namespace n1111111111111111111111111111111111111111111111111111111111111111 {
      namespace n2222222222222222222222222222222222222222222222222222222222222222 {

          void f(void)
          {
              printf("f()\n");
          }
      }
  }

  int main(void)
  {
      n1111111111111111111111111111111111111111111111111111111111111111::n2222222222222222222222222222222222222222222222222222222222222222::f();
      n1111111111111111111111111111111111111111111111111111111111111111::n2222222222222222222222222222222222222222222222222222222222222222::f();
      return 0;
  }

I then

  g++ -g -o tst tst.cc

and

  sudo perf probe -x ~/tmp/tst --add _ZN65n111111111111111111111111111111111111111111111111111111111111111165n22222222222222222222222222222222222222222222222222222222222222221fEv

After the patch a better error message results, and the workaround is clear:

  sudo perf probe -x ~/tmp/tst --add probe=_ZN65n111111111111111111111111111111111111111111111111111111111111111165n22222222222222222222222222222222222222222222222222222222222222221fEv

Dima Kogan (2):
  perf probe-event: un-hardcoded sizeof(buf)
  perf probe-event: better error message for a too-long probe name

 tools/perf/util/probe-event.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

-- 
2.42.0

next             reply	other threads:[~2024-04-16  4:55 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-04-16  4:55 Dima Kogan [this message]
2024-04-16  4:55 ` [PATCH 1/2] perf probe-event: un-hardcoded sizeof(buf) Dima Kogan
2024-04-16  4:55 ` [PATCH 2/2] perf probe-event: better error message for a too-long probe name Dima Kogan
2024-04-16 15:46 ` [PATCH 0/2] Better error messages for 'perf probe --add' Arnaldo Carvalho de Melo
2024-04-17 20:35   ` Masami Hiramatsu
2024-04-18 14:24     ` Arnaldo Carvalho de Melo

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