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

[Naveen N. Rao]

Masami Hiramatsu wrote:
> On Thu, 12 Jan 2023 18:51:14 +0530
> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> 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 follow up to:
http://lore.kernel.org/1664530538.ke6dp49pwh.naveen@xxxxxxxxxxxxx

The intent is to add consecutive probes covering KPROBES_ON_FTRACE, 
vanilla trap-based kprobes as well as optprobes to ensure all of those 
and their interactions are good.

>  - 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

The current test to add multiple kprobes within a function also falls 
under the purview of multiple_kprobes.tc, but it can be split into a 
separate multiple_kprobes_func.tc if you think that will be better.

> 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.

Indeed, I suppose that can be a separate test.


Thanks,
Naveen

> Thank you,
>
> > > 
> > > Signed-off-by: Akanksha J N <akanksha@xxxxxxxxxxxxx>
> > > ---
> > >  .../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@xxxxxxxxxxxxxxxxxx>
> >
> >
> > - Naveen
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>