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Commit 3578baae authored by H. Peter Anvin's avatar H. Peter Anvin
Browse files

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: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent 16640165

arch/x86/include/asm/uaccess.h

+14 −43
Original line number Diff line number Diff line
@@ -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;							\

})