powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

unsafe_put_user() is designed to take benefit of 'asm goto'.

Instead of using the standard __put_user() approach and branch
based on the returned error, use 'asm goto' and make the
exception code branch directly to the error label. There is
no code anymore in the fixup section.

This change significantly simplifies functions using
unsafe_put_user()

Small exemple of the benefit with the following code:

struct test {
	u32 item1;
	u16 item2;
	u8 item3;
	u64 item4;
};

int set_test_to_user(struct test __user *test, u32 item1, u16 item2, u8 item3, u64 item4)
{
	unsafe_put_user(item1, &test->item1, failed);
	unsafe_put_user(item2, &test->item2, failed);
	unsafe_put_user(item3, &test->item3, failed);
	unsafe_put_user(item4, &test->item4, failed);
	return 0;
failed:
	return -EFAULT;
}

Before the patch:

00000be8 <set_test_to_user>:
 be8:	39 20 00 00 	li      r9,0
 bec:	90 83 00 00 	stw     r4,0(r3)
 bf0:	2f 89 00 00 	cmpwi   cr7,r9,0
 bf4:	40 9e 00 38 	bne     cr7,c2c <set_test_to_user+0x44>
 bf8:	b0 a3 00 04 	sth     r5,4(r3)
 bfc:	2f 89 00 00 	cmpwi   cr7,r9,0
 c00:	40 9e 00 2c 	bne     cr7,c2c <set_test_to_user+0x44>
 c04:	98 c3 00 06 	stb     r6,6(r3)
 c08:	2f 89 00 00 	cmpwi   cr7,r9,0
 c0c:	40 9e 00 20 	bne     cr7,c2c <set_test_to_user+0x44>
 c10:	90 e3 00 08 	stw     r7,8(r3)
 c14:	91 03 00 0c 	stw     r8,12(r3)
 c18:	21 29 00 00 	subfic  r9,r9,0
 c1c:	7d 29 49 10 	subfe   r9,r9,r9
 c20:	38 60 ff f2 	li      r3,-14
 c24:	7d 23 18 38 	and     r3,r9,r3
 c28:	4e 80 00 20 	blr
 c2c:	38 60 ff f2 	li      r3,-14
 c30:	4e 80 00 20 	blr

00000000 <.fixup>:
	...
  b8:	39 20 ff f2 	li      r9,-14
  bc:	48 00 00 00 	b       bc <.fixup+0xbc>
			bc: R_PPC_REL24	.text+0xbf0
  c0:	39 20 ff f2 	li      r9,-14
  c4:	48 00 00 00 	b       c4 <.fixup+0xc4>
			c4: R_PPC_REL24	.text+0xbfc
  c8:	39 20 ff f2 	li      r9,-14
  cc:	48 00 00 00 	b       cc <.fixup+0xcc>
  d0:	39 20 ff f2 	li      r9,-14
  d4:	48 00 00 00 	b       d4 <.fixup+0xd4>
			d4: R_PPC_REL24	.text+0xc18

00000000 <__ex_table>:
	...
			a0: R_PPC_REL32	.text+0xbec
			a4: R_PPC_REL32	.fixup+0xb8
			a8: R_PPC_REL32	.text+0xbf8
			ac: R_PPC_REL32	.fixup+0xc0
			b0: R_PPC_REL32	.text+0xc04
			b4: R_PPC_REL32	.fixup+0xc8
			b8: R_PPC_REL32	.text+0xc10
			bc: R_PPC_REL32	.fixup+0xd0
			c0: R_PPC_REL32	.text+0xc14
			c4: R_PPC_REL32	.fixup+0xd0

After the patch:

00000be8 <set_test_to_user>:
 be8:	90 83 00 00 	stw     r4,0(r3)
 bec:	b0 a3 00 04 	sth     r5,4(r3)
 bf0:	98 c3 00 06 	stb     r6,6(r3)
 bf4:	90 e3 00 08 	stw     r7,8(r3)
 bf8:	91 03 00 0c 	stw     r8,12(r3)
 bfc:	38 60 00 00 	li      r3,0
 c00:	4e 80 00 20 	blr
 c04:	38 60 ff f2 	li      r3,-14
 c08:	4e 80 00 20 	blr

00000000 <__ex_table>:
	...
			a0: R_PPC_REL32	.text+0xbe8
			a4: R_PPC_REL32	.text+0xc04
			a8: R_PPC_REL32	.text+0xbec
			ac: R_PPC_REL32	.text+0xc04
			b0: R_PPC_REL32	.text+0xbf0
			b4: R_PPC_REL32	.text+0xc04
			b8: R_PPC_REL32	.text+0xbf4
			bc: R_PPC_REL32	.text+0xc04
			c0: R_PPC_REL32	.text+0xbf8
			c4: R_PPC_REL32	.text+0xc04
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/23e680624680a9a5405f4b88740d2596d4b17c26.1587143308.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/uaccess.h

@@ -93,12 +93,12 @@ static inline int __access_ok(unsigned long addr, unsigned long size,
 #define __get_user(x, ptr) \
 	__get_user_nocheck((x), (ptr), sizeof(*(ptr)), true)
 #define __put_user(x, ptr) \
-	__put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)), true)
+	__put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)))
+#define __put_user_goto(x, ptr, label) \
+	__put_user_nocheck_goto((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)), label)
 
 #define __get_user_allowed(x, ptr) \
 	__get_user_nocheck((x), (ptr), sizeof(*(ptr)), false)
-#define __put_user_allowed(x, ptr) \
-	__put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)), false)
 
 #define __get_user_inatomic(x, ptr) \
 	__get_user_nosleep((x), (ptr), sizeof(*(ptr)))
@@ -162,7 +162,7 @@ do {								\
 	prevent_write_to_user(ptr, size);			\
 } while (0)
 
-#define __put_user_nocheck(x, ptr, size, do_allow)			\
+#define __put_user_nocheck(x, ptr, size)			\
 ({								\
 	long __pu_err;						\
 	__typeof__(*(ptr)) __user *__pu_addr = (ptr);		\
@@ -172,10 +172,7 @@ do {								\
 	if (!is_kernel_addr((unsigned long)__pu_addr))		\
 		might_fault();					\
 	__chk_user_ptr(__pu_addr);				\
-	if (do_allow)								\
-		__put_user_size(__pu_val, __pu_addr, __pu_size, __pu_err);	\
-	else									\
-		__put_user_size_allowed(__pu_val, __pu_addr, __pu_size, __pu_err); \
+	__put_user_size(__pu_val, __pu_addr, __pu_size, __pu_err);	\
 								\
 	__pu_err;						\
 })
@@ -208,6 +205,52 @@ do {								\
 })
 
 
+#define __put_user_asm_goto(x, addr, label, op)			\
+	asm volatile goto(					\
+		"1:	" op "%U1%X1 %0,%1	# put_user\n"	\
+		EX_TABLE(1b, %l2)				\
+		:						\
+		: "r" (x), "m<>" (*addr)				\
+		:						\
+		: label)
+
+#ifdef __powerpc64__
+#define __put_user_asm2_goto(x, ptr, label)			\
+	__put_user_asm_goto(x, ptr, label, "std")
+#else /* __powerpc64__ */
+#define __put_user_asm2_goto(x, addr, label)			\
+	asm volatile goto(					\
+		"1:	stw%X1 %0, %1\n"			\
+		"2:	stw%X1 %L0, %L1\n"			\
+		EX_TABLE(1b, %l2)				\
+		EX_TABLE(2b, %l2)				\
+		:						\
+		: "r" (x), "m" (*addr)				\
+		:						\
+		: label)
+#endif /* __powerpc64__ */
+
+#define __put_user_size_goto(x, ptr, size, label)		\
+do {								\
+	switch (size) {						\
+	case 1: __put_user_asm_goto(x, ptr, label, "stb"); break;	\
+	case 2: __put_user_asm_goto(x, ptr, label, "sth"); break;	\
+	case 4: __put_user_asm_goto(x, ptr, label, "stw"); break;	\
+	case 8: __put_user_asm2_goto(x, ptr, label); break;	\
+	default: __put_user_bad();				\
+	}							\
+} while (0)
+
+#define __put_user_nocheck_goto(x, ptr, size, label)		\
+do {								\
+	__typeof__(*(ptr)) __user *__pu_addr = (ptr);		\
+	if (!is_kernel_addr((unsigned long)__pu_addr))		\
+		might_fault();					\
+	__chk_user_ptr(ptr);					\
+	__put_user_size_goto((x), __pu_addr, (size), label);	\
+} while (0)
+
+
 extern long __get_user_bad(void);
 
 /*
@@ -491,7 +534,7 @@ static __must_check inline bool user_access_begin(const void __user *ptr, size_t
 
 #define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
 #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
-#define unsafe_put_user(x, p, e) unsafe_op_wrap(__put_user_allowed(x, p), e)
+#define unsafe_put_user(x, p, e) __put_user_goto(x, p, e)
 #define unsafe_copy_to_user(d, s, l, e) \
 	unsafe_op_wrap(raw_copy_to_user_allowed(d, s, l), e)