uprobes/x86: Emulate nop's using ops->emulate()

Finally we can kill the ugly (and very limited) code in __skip_sstep().
Just change branch_setup_xol_ops() to treat "nop" as jmp to the next insn.

Thanks to lib/insn.c, it is clever enough. OPCODE1() == 0x90 includes
"(rep;)+ nop;" at least, and (afaics) much more.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jim Keniston <jkenisto@us.ibm.com>

arch/x86/kernel/uprobes.c

@@ -478,6 +478,7 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 	switch (OPCODE1(insn)) {
 	case 0xeb:	/* jmp 8 */
 	case 0xe9:	/* jmp 32 */
+	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
 		break;
 	default:
 		return -ENOSYS;
@@ -710,29 +711,10 @@ void arch_uprobe_abort_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
 		regs->flags &= ~X86_EFLAGS_TF;
 }
 
-/*
- * Skip these instructions as per the currently known x86 ISA.
- * rep=0x66*; nop=0x90
- */
 static bool __skip_sstep(struct arch_uprobe *auprobe, struct pt_regs *regs)
 {
-	int i;
-
 	if (auprobe->ops->emulate)
 		return auprobe->ops->emulate(auprobe, regs);
-
-	/* TODO: move this code into ->emulate() hook */
-	for (i = 0; i < MAX_UINSN_BYTES; i++) {
-		if (auprobe->insn[i] == 0x66)
-			continue;
-
-		if (auprobe->insn[i] == 0x90) {
-			regs->ip += i + 1;
-			return true;
-		}
-
-		break;
-	}
 	return false;
 }