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Date:	Thu, 12 Dec 2013 13:40:26 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>, x86@...nel.org,
	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, systemtap@...rceware.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Seiji Aguchi <seiji.aguchi@....com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH -tip v4 6/6] [RFC] kprobes/x86: Call exception handlers
 directly from do_int3/do_debug

(2013/12/11 22:31), Jiri Kosina wrote:
> On Tue, 3 Dec 2013, Steven Rostedt wrote:
> 
>>> To avoid a kernel crash by probing on lockdep code, call
>>> kprobe_int3_handler and kprobe_debug_handler directly
>>> from do_int3 and do_debug. Since there is a locking code
>>> in notify_die, lockdep code can be invoked. And because
>>> the lockdep involves printk() related things, theoretically,
>>> we need to prohibit probing on much more code...
>>>
>>> Anyway, most of the int3 handlers in the kernel are already
>>> called from do_int3 directly, e.g. ftrace_int3_handler,
>>> poke_int3_handler, kgdb_ll_trap. Actually only
>>> kprobe_exceptions_notify is on the notifier_call_chain.
>>>
>>> So I think this is not a crazy thing.
>>
>> What? Oh, yeah. No, using notifiers in int3 handler is the crazy
>> thing ;-)
> 
> Yeah, it's broken. Obviously, if you happen to trigger int3 before the 
> notifier has been registered, it'd cause int3 exception to be unhandled. 
> See
> 
> 	commit 17f41571bb2c4a398785452ac2718a6c5d77180e
> 	Author: Jiri Kosina <jkosina@...e.cz>
> 	Date:   Tue Jul 23 10:09:28 2013 +0200
> 
> 	    kprobes/x86: Call out into INT3 handler directly instead of using notifier
> 
> for one such issue that happened with jump labels.
> 
>> Hmm, if there's no users of the int3 notifier, should we just remove it?
> 
> Hmm, there are still uprobes, right?

Right, uprobes still use it, however, since it only handles user-space
breakpoint, there is no problem.

Thank you!


-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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