kprobe fault handling

[Martin Hunt <hunt@redhat.com>]

I've been trying to understand how kprobes fault handling is supposed to
work and why it isn't doing what I thought it did.

When page faults happen, do_page_fault() almost immediately calls
notify_die(DIE_PAGE_FAULT,...) This calls the notifier chain which calls
kprobe_exceptions_notify(). This calls kprobe_fault_handler().

kprobe_fault_handler() checkes to see if there is a specific fault
fandler for that kprobe, and if there is, it calls it.  Question: What
do we imagine a probe-specific page fault handler would do?  Why is it
useful?

Then there is this code, which I don't understand
	if (kcb->kprobe_status & KPROBE_HIT_SS) {
		resume_execution(cur, regs, kcb);
		regs->eflags |= kcb->kprobe_old_eflags;

		reset_current_kprobe();
		preempt_enable_no_resched();
	}

And that's it. kprobe_fault_handler returns 0.  No call to
fixup_exceptions()!  So do_page_fault() will have to do the fixups, but
first it will print nasty might_sleep warnings and maybe actually sleep!

I could have sworn this was not the case previously but it has been a
very long time since I have looked at the code at this level.  Anyway,
this MUST be fixed. 

Martin