[PATCH v2 20/31] arm64: User access library function

On Tuesday 14 August 2012, Catalin Marinas wrote:

+/*
+ * Single-value transfer routines.  They automatically use the right
+ * size if we just have the right pointer type.  Note that the functions
+ * which read from user space (*get_*) need to take care not to leak
+ * kernel data even if the calling code is buggy and fails to check
+ * the return value.  This means zeroing out the destination variable
+ * or buffer on error.  Normally this is done out of line by the
+ * fixup code, but there are a few places where it intrudes on the
+ * main code path.  When we only write to user space, there is no
+ * problem.
+ */
+extern long __get_user_1(void *);
+extern long __get_user_2(void *);
+extern long __get_user_4(void *);
+extern long __get_user_8(void *);
+
+#define __get_user_x(__r2,__p,__e,__s,__i...)				\
+	   asm volatile(						\
+		__asmeq("%0", "x0") __asmeq("%1", "x2")			\
+		"bl	__get_user_" #__s				\
+		: "=&r" (__e), "=r" (__r2)				\
+		: "0" (__p)						\
+		: __i, "cc")
+
+#define get_user(x,p)							\
+	({								\
+		register const typeof(*(p)) __user *__p asm("x0") = (p);\
+		register unsigned long __r2 asm("x2");			\
+		register long __e asm("x0");				\
+		switch (sizeof(*(__p))) {				\
+		case 1:							\
+			__get_user_x(__r2, __p, __e, 1, "x30");		\
+			break;						\
+		case 2:							\
+			__get_user_x(__r2, __p, __e, 2, "x3", "x30");	\
+			break;						\
+		case 4:							\
+			__get_user_x(__r2, __p, __e, 4, "x30");		\
+			break;						\
+		case 8:							\
+			__get_user_x(__r2, __p, __e, 8, "x30");		\
+			break;						\
+		default: __e = __get_user_bad(); break;			\
+		}							\
+		x = (typeof(*(p))) __r2;				\
+		__e;							\
+	})

It's fairly unusual to have out of line get_user/put_user functions.
What is the reason for this, other than copying from ARM?

+
+__get_user_bad:
+	mov	x2, #0
+	mov	x0, #-EFAULT
+	ret
+ENDPROC(__get_user_bad)

+__put_user_bad:
+	mov	x0, #-EFAULT
+	ret
+ENDPROC(__put_user_bad)
+

The purpose of these symbols is to provoke a link error when you
pass the wrong data into get_user/put_user. Actually defining them
completely breaks this logic, so you should remove these!

	Arnd