Re: [PATCH v2 0/3] x86: kprobes: Show correct blaclkist in debugfs

[Masami Hiramatsu <mhiramat@kernel.org>]

On Mon, 17 Dec 2018 16:47:13 +0100
Andrea Righi <righi.andrea@gmail.com> wrote:

> On Mon, Dec 17, 2018 at 05:20:25PM +0900, Masami Hiramatsu wrote:
> > This is v2 series for showing correct kprobe blacklist in
> > debugfs.
> > 
> > v1 is here:
> > 
> >  https://lkml.org/lkml/2018/12/7/517
> > 
> > I splitted the RFC v1 patch into x86 and generic parts,
> > also added a patch to remove unneeded arch-specific
> > blacklist check function (because those have been added
> > to the generic blacklist.)
> > 
> > If this style is good, I will make another series for the
> > archs which have own arch_within_kprobe_blacklist(), and
> > eventually replace that with arch_populate_kprobe_blacklist()
> > so that user can get the correct kprobe blacklist in debugfs.
> > 
> > Thank you,
> 
> Looks good to me. Thanks!
> 
> Tested-by: Andrea Righi <righi.andrea@gmail.com>

Thank you for testing!

> 
> Side question: there are certain symbols in arch/x86/xen that should be
> blacklisted explicitly, because they're non-attachable.
> 
> More exactly, all functions defined in arch/x86/xen/spinlock.c,
> arch/x86/xen/time.c and arch/x86/xen/irq.c.
> 
> The reason is that these files are compiled without -pg to allow the
> usage of ftrace within a Xen domain apparently (from
> arch/x86/xen/Makefile):
> 
>  ifdef CONFIG_FUNCTION_TRACER
>  # Do not profile debug and lowlevel utilities
>  CFLAGS_REMOVE_spinlock.o = -pg
>  CFLAGS_REMOVE_time.o = -pg
>  CFLAGS_REMOVE_irq.o = -pg
>  endif


Actually, the reason why you can not probe those functions via
tracing/kprobe_events is just a side effect. You can probe it if you
write a kprobe module. Since the kprobe_events depends on some ftrace
tracing functions, it sometimes cause a recursive call problem. To avoid
this issue, I have introduced a CONFIG_KPROBE_EVENTS_ON_NOTRACE, see
commit 45408c4f9250 ("tracing: kprobes: Prohibit probing on notrace function").

If you set CONFIG_KPROBE_EVENTS_ON_NOTRACE=n, you can continue putting probes
on Xen spinlock functions too.

> Do you see a nice and clean way to blacklist all these functions
> (something like arch_populate_kprobe_blacklist()), or should we just
> flag all of them explicitly with NOKPROBE_SYMBOL()?

As I pointed, you can probe it via your own kprobe module. Like systemtap,
you still can probe it. The blacklist is for "kprobes", not for "kprobe_events".
(Those are used to same, but since the above commit, those are different now)

I think the most sane solution is, identifying which (combination of) functions
in ftrace (kernel/trace/*) causes a problem, marking those NOKPROBE_SYMBOL() and
removing CONFIG_KPROBE_EVENTS_ON_NOTRACE.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>