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

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Date:   Tue, 18 Dec 2018 18:24:35 +0100
From:   Andrea Righi <righi.andrea@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Yonghong Song <yhs@...com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/3] x86: kprobes: Show correct blaclkist in debugfs

On Tue, Dec 18, 2018 at 01:50:26PM +0900, Masami Hiramatsu wrote:
...
> > 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.

OK.

> 
> > 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.

OK. Thanks for the clarification!

-Andrea