Re: [PATCH 0/2] Better error messages for 'perf probe --add'

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Mon, Apr 15, 2024 at 09:55:09PM -0700, Dima Kogan wrote:
> 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.

Both patches look great, Masami, can I have your Acked-by?

If you find other cases where the error messages can be improved, feel
free to send us, please CC Masami, as I'm doing now.

Thanks!

- Rrnaldo
 
> 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
>