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

From: Andrea Righi <righi.andrea@gmail.com>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	"Naveen N . Rao" <naveen.n.rao@linux.vnet.ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
	"David S . Miller" <davem@davemloft.net>,
	Yonghong Song <yhs@fb.com>, Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/3] x86: kprobes: Show correct blaclkist in debugfs
Date: Tue, 1 Jan 2019 14:37:16 +0100	[thread overview]
Message-ID: <20190101133716.GA1811@xps-13> (raw)
In-Reply-To: <20190101221654.fedda794776540d6b81f6167@kernel.org>

On Tue, Jan 01, 2019 at 10:16:54PM +0900, Masami Hiramatsu wrote:
...
> > > > > 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.
> > 
> > I'm planning to spend a little bit more time on this and see if I can
> > identify the problematic ftrace functions and eventually drop
> > CONFIG_KPROBE_EVENTS_ON_NOTRACE, following the sane solution.
> > 
> > However, in the meantime, with the following patch I've been able to get
> > a more reliable kprobes blacklist and show also the notrace functions in
> > debugfs when CONFIG_KPROBE_EVENTS_ON_NOTRACE is off.
> 
> Hmm, if CONFIG_KPROBE_EVENTS_ON_NOTRACE=n, we already have a whitelist of
> functions in /sys/kernel/debug/tracing/available_filter_functions,
> so I don't think we need a blacklist.

OK.

> 
> > It's probably ugly and inefficient, because it's iterating over all
> > symbols in x86's arch_populate_kprobe_blacklist(), but it seems to work
> > for my specific use case, so I thought it shouldn't be bad to share it,
> > just in case (maybe someone else is also interested).
> 
> Hmm, but in that case, it limits other native kprobes users like systemtap
> to disable probing on notrace functions with no reasons. That may not be acceptable.

True...

> 
> OK, I'll retry to find which notrace function combination tracing with
> kprobes are problematic. Let me do it...

OK. Thanks tons for looking into this!

-Andrea