[PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

Christophe Leroy
2020-04-17T17:08:57Z

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>
cc: R_PPC_REL24 .text+0xc08
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>
---
v4:
- no change

v3:
- Added <> modifier to __put_user_asm_goto()
- Removed %U1 modifier to __put_user_asm2_goto()

v2:
- Grouped most __goto() macros together
- Removed stuff in .fixup section, referencing the error label
directly from the extable
- Using more flexible addressing in asm.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
arch/powerpc/include/asm/uaccess.h | 61 +++++++++++++++++++++++++-----
1 file changed, 52 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 9cc9c106ae2a..9365b59495a2 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/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)))
@@ -162,17 +162,14 @@ 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); \
if (!is_kernel_addr((unsigned long)__pu_addr)) \
might_fault(); \
__chk_user_ptr(ptr); \
- if (do_allow) \
- __put_user_size((x), __pu_addr, (size), __pu_err); \
- else \
- __put_user_size_allowed((x), __pu_addr, (size), __pu_err); \
+ __put_user_size((x), __pu_addr, (size), __pu_err); \
__pu_err; \
})

@@ -196,6 +193,52 @@ 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);

/*
@@ -470,7 +513,7 @@ 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)

--
2.25.0