ftrace: x86 use copy to and from user functions

The modification of code is performed either by kstop_machine, before
SMP starts, or on module code before the module is executed. There is
no reason to do the modifications from assembly. The copy to and from
user functions are sufficient and produces cleaner and easier to read
code.

Thanks to Benjamin Herrenschmidt for suggesting the idea.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/ftrace.c

@@ -11,6 +11,7 @@
 
 #include <linux/spinlock.h>
 #include <linux/hardirq.h>
+#include <linux/uaccess.h>
 #include <linux/ftrace.h>
 #include <linux/percpu.h>
 #include <linux/init.h>
@@ -60,11 +61,7 @@ notrace int
 ftrace_modify_code(unsigned long ip, unsigned char *old_code,
 		   unsigned char *new_code)
 {
-	unsigned replaced;
+	unsigned char replaced[MCOUNT_INSN_SIZE];
-	unsigned old = *(unsigned *)old_code; /* 4 bytes */
-	unsigned new = *(unsigned *)new_code; /* 4 bytes */
-	unsigned char newch = new_code[4];
-	int faulted = 0;
 
 	/*
 	 * Note: Due to modules and __init, code can
@@ -72,29 +69,20 @@ ftrace_modify_code(unsigned long ip, unsigned char *old_code,
 	 *  as well as code changing.
 	 *
 	 * No real locking needed, this code is run through
-	 * kstop_machine.
+	 * kstop_machine, or before SMP starts.
 	 */
-	asm volatile (
+	if (__copy_from_user(replaced, (char __user *)ip, MCOUNT_INSN_SIZE))
-		"1: lock\n"
+		return 1;
-		"   cmpxchg %3, (%2)\n"
+
-		"   jnz 2f\n"
+	if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
-		"   movb %b4, 4(%2)\n"
+		return 2;
-		"2:\n"
+
-		".section .fixup, \"ax\"\n"
+	WARN_ON_ONCE(__copy_to_user((char __user *)ip, new_code,
-		"3:	movl $1, %0\n"
+				    MCOUNT_INSN_SIZE));
-		"	jmp 2b\n"
+
-		".previous\n"
-		_ASM_EXTABLE(1b, 3b)
-		: "=r"(faulted), "=a"(replaced)
-		: "r"(ip), "r"(new), "c"(newch),
-		  "0"(faulted), "a"(old)
-		: "memory");
 	sync_core();
 
-	if (replaced != old && replaced != new)
+	return 0;
-		faulted = 2;
-
-	return faulted;
 }
 
 notrace int ftrace_update_ftrace_func(ftrace_func_t func)