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    SubjectRe: [PATCH -tip v4 6/6] [RFC] kprobes/x86: Call exception handlers directly from do_int3/do_debug
    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?

    --
    Jiri Kosina
    SUSE Labs



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