On 07/13/2017 09:47 PM, Matthias Kaehlcke wrote:
> Thanks for your analysis!
>
>> What happens if you try the below patch instead of the revert? Any
>> chance the offending instruction goes away?
>>
>> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
>> index 11433f9..beac907 100644
>> --- a/arch/x86/include/asm/uaccess.h
>> +++ b/arch/x86/include/asm/uaccess.h
>> @@ -171,7 +171,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) >
>> sizeof(0UL), 0ULL, 0UL))
>> might_fault(); \
>> asm volatile("call __get_user_%P4" \
>> : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \
>> - : "0" (ptr), "i" (sizeof(*(ptr)))); \
>> + : "0" (ptr), "i" (sizeof(*(ptr))), "r" (__sp)); \
>> (x) = (__force __typeof__(*(ptr))) __val_gu; \
>> __builtin_expect(__ret_gu, 0); \
>> })
>
> The generated code is basically the same, only that now the value from
> the stack is stored in a register and written twice to RSP:
>
AFAIR clang works much better with global named registers.
Could you check if the patch bellow helps?
---
arch/x86/include/asm/uaccess.h | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a059aac9e937..121204387978 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -157,15 +157,18 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL),
0ULL, 0UL))
* Clang/LLVM cares about the size of the register, but still wants
* the base register for something that ends up being a pair.
*/
+
+register unsigned long __current_sp asm(_ASM_SP);
+
#define get_user(x, ptr) \
({ \
int __ret_gu; \
register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX); \
- register void *__sp asm(_ASM_SP); \
__chk_user_ptr(ptr); \
might_fault(); \
asm volatile("call __get_user_%P4" \
- : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \
+ : "=a" (__ret_gu), "=r" (__val_gu), \
+ "+r" (__current_sp) \
: "0" (ptr), "i" (sizeof(*(ptr)))); \
(x) = (__force __typeof__(*(ptr))) __val_gu; \
__builtin_expect(__ret_gu, 0); \
--
2.13.0
