Re: [PATCH 3/4] perf probe: Fix probing kretprobes

[Jiri Olsa]

On Sat, Jan 10, 2015 at 07:33:47PM +0900, Namhyung Kim wrote:
> The commit dfef99cd0b2c ("perf probe: Use ref_reloc_sym based address
> instead of the symbol name") converts kprobes to use ref_reloc_sym
> (i.e. _stext) and offset instead of using symbol's name directly.  So
> on my system, adding do_fork ends up with like below:
> 
>   $ sudo perf probe -v --add do_fork%return
>   probe-definition(0): do_fork%return
>   symbol:do_fork file:(null) line:0 offset:0 return:1 lazy:(null)
>   0 arguments
>   Looking at the vmlinux_path (7 entries long)
>   Using /lib/modules/3.17.6-1-ARCH/build/vmlinux for symbols
>   Could not open debuginfo. Try to use symbols.
>   Opening /sys/kernel/debug/tracing/kprobe_events write=1
>   Added new event:
>   Writing event: r:probe/do_fork _stext+456136
>   Failed to write event: Invalid argument
>   Error: Failed to add events. Reason: Operation not permitted (Code: -1)
> 
> As you can see, the do_fork was translated to _stext+456136.  This was
> because to support (local) symbols that have same name.  But the
> problem is that kretprobe requires to be inserted at function start
> point so it simply checks whether it's called with offset 0.  And if
> not, it'll return with -EINVAL.  You can see it with dmesg.
> 
>   $ dmesg | tail -1
>     [125621.764103] Return probe must be used without offset.
> 
> So we need to use the symbol name instead of ref_reloc_sym in case of
> return probes.

with your patchset on top of current Arnaldo's perf/core,
I'm still getting the same error:


[root@krava perf]# ./perf probe -a fork_exit=do_fork%return
Added new event:
Failed to write event: Invalid argument
  Error: Failed to add events.
[root@krava perf]# ./perf probe -v --add do_fork%return
probe-definition(0): do_fork%return
symbol:do_fork file:(null) line:0 offset:0 return:1 lazy:(null)
0 arguments
Using /root/.debug/.build-id/60/4c5fd8a6c9202acac3a9e82ecd34b22b8caef6 for symbols
Open Debuginfo file: /root/.debug/.build-id/60/4c5fd8a6c9202acac3a9e82ecd34b22b8caef6
Try to find probe point from debuginfo.
Probe point found: do_fork+0
Found 1 probe_trace_events.
Opening /sys/kernel/debug/tracing/kprobe_events write=1
Added new event:
Writing event: r:probe/do_fork _text+597840
Failed to write event: Invalid argument
  Error: Failed to add events. Reason: Invalid argument (Code: -22)
[root@krava perf]# dmesg | tail -1
[113205.738833] Return probe must be used without offset.


thanks,
jirka
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