linux-kernel - [PATCH] x86_32: add support for 64 bit __get

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Date:	Wed, 9 Mar 2016 12:22:25 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Ingo Molnar <mingo@...hat.com>, Russell King <rmk@....linux.org.uk>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] x86_32: add support for 64 bit __get_user()

The existing __get_user() implementation does not support fetching
64 bit values on 32 bit x86.  Implement this in a way that does not 
generate any incorrect warnings as cautioned by Russell King.  Test 
code available at http://www.kvack.org/~bcrl/x86_32-get_user.tar .  

Signed-off-by: Benjamin LaHaise <bcrl@...ck.org>

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a4a30e4..2d0607a 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -333,7 +333,23 @@ do {									\
 } while (0)
 
 #ifdef CONFIG_X86_32
-#define __get_user_asm_u64(x, ptr, retval, errret)	(x) = __get_user_bad()
+#define __get_user_asm_u64(x, addr, err, errret)			\
+	asm volatile(ASM_STAC "\n"					\
+		     "1:	movl %2,%%eax\n"			\
+		     "2:	movl %3,%%edx\n"			\
+		     "3: " ASM_CLAC "\n"				\
+		     ".section .fixup,\"ax\"\n"				\
+		     "4:	mov %4,%0\n"				\
+		     "	xorl %%eax,%%eax\n"				\
+		     "	xorl %%edx,%%edx\n"				\
+		     "	jmp 3b\n"					\
+		     ".previous\n"					\
+		     _ASM_EXTABLE(1b, 4b)				\
+		     _ASM_EXTABLE(2b, 4b)				\
+		     : "=r" (err), "=A"(x)				\
+		     : "m" (__m(addr)), "m" __m(((u32 *)(addr)) + 1),	\
+		       "i" (errret), "0" (err))
+
 #define __get_user_asm_ex_u64(x, ptr)			(x) = __get_user_bad()
 #else
 #define __get_user_asm_u64(x, ptr, retval, errret) \
@@ -420,11 +436,20 @@ do {									\
 #define __get_user_nocheck(x, ptr, size)				\
 ({									\
 	int __gu_err;							\
-	unsigned long __gu_val;						\
-	__uaccess_begin();						\
-	__get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);	\
-	__uaccess_end();						\
-	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
+	if (size == 8) {						\
+		unsigned long __gu_val[2];				\
+		__gu_err = 0;						\
+		__uaccess_begin();					\
+		__get_user_asm_u64(__gu_val, ptr, __gu_err, -EFAULT);	\
+		__uaccess_end();					\
+		(x) = *(__force __typeof__((ptr)))__gu_val;		\
+	} else {							\
+		unsigned long __gu_val;					\
+		__uaccess_begin();					\
+		__get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT); \
+		__uaccess_end();					\
+		(x) = (__force __typeof__(*(ptr)))__gu_val;		\
+	}								\
 	__builtin_expect(__gu_err, 0);					\
 })
 
-- 
"Thought is the essence of where you are now."