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

[Namhyung Kim]

On Mon, Jan 12, 2015 at 09:22:13PM +0900, Masami Hiramatsu wrote:
> (2015/01/10 19:33), 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.
> 
> Thanks, but as I pointed in previous mail, this patch fixes just one
> code path which doesn't use debuginfo. With debuginfo, perf-probe
> executes try_to_find_probe_trace_events() to analyze debuginfo.
> We can just skip calling it for retprobes, but I prefer to keep
> it works. So just skipping conversion in post_process_probe_trace_events()
> is better.

Ah, missed that.  I only tested on my laptop which has no debuginfo.
I will add the check into the post_process_probe_trace_events().

Thanks,
Namhyung
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