linux-kernel - Re: [PATCH] selftests/ftrace: Extend multiple_kprobes.tc to add multiple consecutive probes in a function

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Date:   Fri, 13 Jan 2023 00:51:53 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Akanksha J N <akanksha@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, mhiramat@...nel.org,
        rostedt@...dmis.org, shuah@...nel.org
Subject: Re: [PATCH] selftests/ftrace: Extend multiple_kprobes.tc to add
 multiple consecutive probes in a function

On Thu, 12 Jan 2023 18:51:14 +0530
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:

> Akanksha J N wrote:
> > Commit 97f88a3d723162 ("powerpc/kprobes: Fix null pointer reference in
> > arch_prepare_kprobe()") fixed a recent kernel oops that was caused as
> > ftrace-based kprobe does not generate kprobe::ainsn::insn and it gets
> > set to NULL.
> > Extend multiple kprobes test to add kprobes on first 256 bytes within a
> > function, to be able to test potential issues with kprobes on
> > successive instructions.

What is the purpose of that test? If you intended to add a kprobe events
with some offset so that it becomes ftrace-based kprobe, it should be
a different test case, because

 - This is a test case for checking multiple (at least 256) kprobe events
  can be defined and enabled.

 - If you want to check the ftrace-based kprobe, it should be near the
   function entry, maybe within 16 bytes or so.

 - Also, you don't need to enable it at once (and should not for this case).

> > The '|| true' is added with the echo statement to ignore errors that are
> > caused by trying to add kprobes to non probeable lines and continue with
> > the test.

Can you add another test case for that? (and send it to the MLs which Cc'd
to this mail)
e.g. 

   for i in `seq 0 16`; do
     echo p:testprobe $FUNCTION_FORK+${i} >> kprobe_events || continue
     echo 1 > events/kprobes/testprobe/enable
     ( echo "forked" )
     echo 0 > events/kprobes/testprobe/enable
     echo > kprobe_events
   done


BTW, after we introduce the fprobe event (https://lore.kernel.org/linux-trace-kernel/166792255429.919356.14116090269057513181.stgit@devnote3/) that test case may be
update to check fprobe events.

Thank you,

> > 
> > Signed-off-by: Akanksha J N <akanksha@...ux.ibm.com>
> > ---
> >  .../selftests/ftrace/test.d/kprobe/multiple_kprobes.tc        | 4 ++++
> >  1 file changed, 4 insertions(+)
> 
> Thanks for adding this test!
> 
> > 
> > diff --git a/tools/testing/selftests/ftrace/test.d/kprobe/multiple_kprobes.tc b/tools/testing/selftests/ftrace/test.d/kprobe/multiple_kprobes.tc
> > index be754f5bcf79..f005c2542baa 100644
> > --- a/tools/testing/selftests/ftrace/test.d/kprobe/multiple_kprobes.tc
> > +++ b/tools/testing/selftests/ftrace/test.d/kprobe/multiple_kprobes.tc
> > @@ -25,6 +25,10 @@ if [ $L -ne 256 ]; then
> >    exit_fail
> >  fi
> > 
> > +for i in `seq 0 255`; do
> > +  echo p $FUNCTION_FORK+${i} >> kprobe_events || true
> > +done
> > +
> >  cat kprobe_events >> $testlog
> > 
> >  echo 1 > events/kprobes/enable
> 
> Thinking about this more, I wonder if we should add an explicit fork 
> after enabling the events, similar to kprobe_args.tc:
> 	( echo "forked" )
> 
> That will ensure we hit all the probes we added. With that change:
> Acked-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> 
> 
> - Naveen


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>