A special kprobe type which can be placed on function entry points, and
employs a simple mirroring principle to allow seamless access to the
arguments of a function being probed.  The probe handler routine should
have the same prototype as the function being probed.  Currently
implemented for x86.

The way it works is that when the probe is hit, the breakpoint handler
simply irets to the probe handler's eip while retaining register and stack
state corresponding to the function entry.  After it is done, the probe
handler calls jprobe_return() which traps again to restore processor state
and switch back to the probed function.  Linus noted correctly at KS that
we need to be careful as gcc assumes that the callee owns arguments.  We
save and restore enough stack bytes to cover argument space.

Sample Usage:
	static int jip_queue_xmit(struct sk_buff *skb, int ipfragok)
	{
		... whatever ...
		jprobe_return();
		return 0;
	}

	struct jprobe jp = {
		{.addr = (kprobe_opcode_t *) ip_queue_xmit},
		.entry = (kprobe_opcode_t *) jip_queue_xmit
	};
	register_jprobe(&jp);
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>