x86, mm: Redesign get_user with a __builtin_choose_expr hack

Instead of using a bitfield, use an odd little trick using typeof,
__builtin_choose_expr, and sizeof.  __builtin_choose_expr is
explicitly defined to not convert its type (its argument is required
to be a constant expression) so this should be well-defined.

The code is still not 100% preturbation-free versus the baseline
before 64-bit get_user(), but the differences seem to be very small,
mostly related to padding and to gcc deciding when to spill registers.

Cc: Jamie Lokier <jamie@shareable.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. J. Lu <hjl.tools@gmail.com>
Link: http://lkml.kernel.org/r/511A8922.6050908@zytor.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 14 insertions, 43 deletions

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a8d12653f304..d710a2555fd6 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -125,13 +125,12 @@ extern int __get_user_4(void);
 extern int __get_user_8(void);
 extern int __get_user_bad(void);
 
-#define __get_user_x(size, ret, x, ptr)               \
-        asm volatile("call __get_user_" #size         \
-                     : "=a" (ret), "=d" (x)           \
-                     : "0" (ptr))                     \
-
-/* Careful: we have to cast the result to the type of the pointer
- * for sign reasons */
+/*
+ * This is a type: either unsigned long, if the argument fits into
+ * that type, or otherwise unsigned long long.
+ */
+#define __inttype(x) \
+__typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
 
 /**
  * get_user: - Get a simple variable from user space.
@@ -149,48 +148,20 @@ extern int __get_user_bad(void);
  *
  * Returns zero on success, or -EFAULT on error.
  * On error, the variable @x is set to zero.
+ *
+ * Careful: we have to cast the result to the type of the pointer
+ * for sign reasons.
  */
-#ifdef CONFIG_X86_32
-#define __get_user_8(ret, x, ptr)                     \
-do {                                                  \
-        register unsigned long long __xx asm("%edx"); \
-        asm volatile("call __get_user_8"              \
-                     : "=a" (ret), "=r" (__xx)        \
-                     : "0" (ptr));                    \
-        (x) = __xx;                                   \
-} while (0)
-
-#else
-#define __get_user_8(__ret_gu, __val_gu, ptr)                           \
-                __get_user_x(8, __ret_gu, __val_gu, ptr)
-#endif
-
 #define get_user(x, ptr)                                                \
 ({                                                                      \
         int __ret_gu;                                                   \
-        struct {                                                        \
-                unsigned long long __val_n : 8*sizeof(*(ptr));          \
-        } __val_gu;                                                     \
+        register __inttype(*(ptr)) __val_gu asm("%edx");                \
         __chk_user_ptr(ptr);                                            \
         might_fault();                                                  \
-        switch (sizeof(*(ptr))) {                                       \
-        case 1:                                                         \
-                __get_user_x(1, __ret_gu, __val_gu.__val_n, ptr);       \
-                break;                                                  \
-        case 2:                                                         \
-                __get_user_x(2, __ret_gu, __val_gu.__val_n, ptr);       \
-                break;                                                  \
-        case 4:                                                         \
-                __get_user_x(4, __ret_gu, __val_gu.__val_n, ptr);       \
-                break;                                                  \
-        case 8:                                                         \
-                __get_user_8(__ret_gu, __val_gu.__val_n, ptr);          \
-                break;                                                  \
-        default:                                                        \
-                __get_user_x(X, __ret_gu, __val_gu.__val_n, ptr);       \
-                break;                                                  \
-        }                                                               \
-        (x) = (__typeof__(*(ptr)))__val_gu.__val_n;                     \
+        asm volatile("call __get_user_%P3"                              \
+                     : "=a" (__ret_gu), "=r" (__val_gu)                 \
+                     : "0" (ptr), "i" (sizeof(*(ptr))));                \
+        (x) = (__typeof__(*(ptr))) __val_gu;                            \
         __ret_gu;                                                       \
 })