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Date: Thu, 12 Dec 2013 13:40:26 +0900
From: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Organization: Hitachi, Ltd., Japan
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To: Jiri Kosina <jkosina@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Sandeepa Prabhu <sandeepa.prabhu@linaro.org>,
        Frederic Weisbecker <fweisbec@gmail.com>, x86@kernel.org,
        lkml <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, systemtap@sourceware.org,
        "H. Peter Anvin" <hpa@zytor.com>, Sasha Levin <sasha.levin@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Seiji Aguchi <seiji.aguchi@hds.com>,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH -tip v4 6/6] [RFC] kprobes/x86: Call exception handlers
 directly from do_int3/do_debug
References: <20131204012841.22118.82992.stgit@kbuild-fedora.novalocal> <20131204012856.22118.91079.stgit@kbuild-fedora.novalocal> <20131203213904.5ff1794c@gandalf.local.home> <alpine.LRH.2.00.1312111418550.6318@twin.jikos.cz>
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(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@suse.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@hitachi.com