Re: [PATCH -tip v4 6/6] [RFC] kprobes/x86: Call exception handlersdirectly from do_int3/do_debug

[Jiri Kosina]

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@xxxxxxx>
	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?

-- 
Jiri Kosina
SUSE Labs

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