powerpc/kprobes: Optimize kprobe in kretprobe_trampoline()

Kprobe placed on the  kretprobe_trampoline() during boot time can be
optimized, since the instruction at probe point is a 'nop'.
Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/kprobes.c

@@ -285,6 +285,7 @@ asm(".global kretprobe_trampoline\n"
 	".type kretprobe_trampoline, @function\n"
 	"kretprobe_trampoline:\n"
 	"nop\n"
+	"blr\n"
 	".size kretprobe_trampoline, .-kretprobe_trampoline\n");
 
 /*
@@ -337,6 +338,13 @@ static int __kprobes trampoline_probe_handler(struct kprobe *p,
 
 	kretprobe_assert(ri, orig_ret_address, trampoline_address);
 	regs->nip = orig_ret_address;
+	/*
+	 * Make LR point to the orig_ret_address.
+	 * When the 'nop' inside the kretprobe_trampoline
+	 * is optimized, we can do a 'blr' after executing the
+	 * detour buffer code.
+	 */
+	regs->link = orig_ret_address;
 
 	reset_current_kprobe();
 	kretprobe_hash_unlock(current, &flags);

arch/powerpc/kernel/optprobes.c

@@ -72,12 +72,11 @@ static unsigned long can_optimize(struct kprobe *p)
 
 	/*
 	 * kprobe placed for kretprobe during boot time
-	 * is not optimizing now.
-	 *
-	 * TODO: Optimize kprobe in kretprobe_trampoline
+	 * has a 'nop' instruction, which can be emulated.
+	 * So further checks can be skipped.
 	 */
 	if (p->addr == (kprobe_opcode_t *)&kretprobe_trampoline)
-		return 0;
+		return (unsigned long)p->addr + sizeof(kprobe_opcode_t);
 
 	/*
 	 * We only support optimizing kernel addresses, but not