perf probe dwarf on shared libraries

perf probe dwarf on shared libraries

[david lerner]

Has perf probe for userspace shared libraries been modified to use dwarf info?
The first commit to provide this useful functionality...
   fb7345bbf7fad9  perf probe: Support basic dwarf-based operations on
uprobe events,
shows examples only with the executable perf, but not with a shared library.

With version 3.14 I'm able to set probes in application executable,
for example for the test application 'libunwind-test.out' I see the
usual args to main():
   root@qemu0:~# perf probe -x ./libunwind-test.out --vars main
   Available variables at main
        @<main+0>
                char**  a
                int     c
But I can't access the variables in a library that has embedded (not
split out) dwarf debug info...
   root@qemu0:~# perf probe -x /usr/lib64/libunwind.so --vars unw_backtrace
   Failed to find symbol at 0x33c38039cd
   Failed to find variables at unw_backtrace (-2)
   Error: Failed to show vars. (-2)
I've used gdb to confirm that perf found the correct function address
for unw_backtrace(), and that gdb knows the symbolic names of the
arguments.

Is using dwarf debuginfo with shared libraries for perf uprobes on the
to-do list?
Or is this capability incomplete in version 3.14 and I need to pick up
other patches?

Thanks,
Dave

Re: perf probe dwarf on shared libraries

[Arnaldo Carvalho de Melo]

Em Thu, Sep 04, 2014 at 03:11:31PM -0500, david lerner escreveu:
> Has perf probe for userspace shared libraries been modified to use dwarf info?
> The first commit to provide this useful functionality...
>    fb7345bbf7fad9  perf probe: Support basic dwarf-based operations on
> uprobe events,
> shows examples only with the executable perf, but not with a shared library.
 
> With version 3.14 I'm able to set probes in application executable,
> for example for the test application 'libunwind-test.out' I see the
> usual args to main():
>    root@qemu0:~# perf probe -x ./libunwind-test.out --vars main
>    Available variables at main
>         @<main+0>
>                 char**  a
>                 int     c
> But I can't access the variables in a library that has embedded (not
> split out) dwarf debug info...
>    root@qemu0:~# perf probe -x /usr/lib64/libunwind.so --vars unw_backtrace
>    Failed to find symbol at 0x33c38039cd
>    Failed to find variables at unw_backtrace (-2)
>    Error: Failed to show vars. (-2)
> I've used gdb to confirm that perf found the correct function address
> for unw_backtrace(), and that gdb knows the symbolic names of the
> arguments.
 
> Is using dwarf debuginfo with shared libraries for perf uprobes on the
> to-do list?
> Or is this capability incomplete in version 3.14 and I need to pick up
> other patches?

Have to check this, but perhaps Masami has this working already?

- Arnaldo

Re: perf probe dwarf on shared libraries

[Masami Hiramatsu]

(2014/09/11 23:51), Arnaldo Carvalho de Melo wrote:
> Em Thu, Sep 04, 2014 at 03:11:31PM -0500, david lerner escreveu:
>> Has perf probe for userspace shared libraries been modified to use dwarf info?
>> The first commit to provide this useful functionality...
>>    fb7345bbf7fad9  perf probe: Support basic dwarf-based operations on
>> uprobe events,
>> shows examples only with the executable perf, but not with a shared library.
>  
>> With version 3.14 I'm able to set probes in application executable,
>> for example for the test application 'libunwind-test.out' I see the
>> usual args to main():
>>    root@qemu0:~# perf probe -x ./libunwind-test.out --vars main
>>    Available variables at main
>>         @<main+0>
>>                 char**  a
>>                 int     c
>> But I can't access the variables in a library that has embedded (not
>> split out) dwarf debug info...
>>    root@qemu0:~# perf probe -x /usr/lib64/libunwind.so --vars unw_backtrace
>>    Failed to find symbol at 0x33c38039cd
>>    Failed to find variables at unw_backtrace (-2)
>>    Error: Failed to show vars. (-2)
>> I've used gdb to confirm that perf found the correct function address
>> for unw_backtrace(), and that gdb knows the symbolic names of the
>> arguments.
>  
>> Is using dwarf debuginfo with shared libraries for perf uprobes on the
>> to-do list?
>> Or is this capability incomplete in version 3.14 and I need to pick up
>> other patches?
> 
> Have to check this, but perhaps Masami has this working already?

Hmm, I thought it should work. But no, it doesn't.
I've checked it and have found it doesn't work as below.

----
 $ ./perf probe -x /usr/lib64/libunwind.so -vvv -V access_reg
 probe-definition(0): access_reg
 symbol:access_reg file:(null) line:0 offset:0 return:0 lazy:(null)
 0 arguments
 Open Debuginfo file: /usr/lib/debug/usr/lib64/libunwind.so.8.0.1.debug
 Searching variables at access_reg
 Failed to find symbol at 0x2f40
 Failed to find the address of access_reg
   Error: Failed to show vars. Reason: No such file or directory (Code: -2)
----

I guess it might fail to verify the symbol address. Because
it is a shared library, dwarf information will be a bit
different. Anyway I'll try to fix that.

Thank you for reporting! :)


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com