Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm"

[Andrey Rybainin]

On 07/14/2017 12:14 AM, Matthias Kaehlcke wrote:
> El Thu, Jul 13, 2017 at 11:20:04PM +0300 Andrey Rybainin ha dit:
> 
>> 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); \
> 
> Thanks for the suggestion, however it fails to build with both gcc and clang:
> 
> fs/ioctl.c:585:6: error: use of undeclared identifier '__current_sp'
>         if (get_user(count, &argp->dest_count)) {
>             ^
> arch/x86/include/asm/uaccess.h:168:16: note: expanded from macro 'get_user'
>                        "+r" (__current_sp)
>       \
> 
> The references I found refer to __current_sp as an intrinsic function
> for ARM32.

What? __current_sp declared right above get_user() as "register unsigned long __current_sp asm(_ASM_SP);" 
Did you actually applied my patch or you just modified the code yourself but have missed
"register unsigned long __current_sp asm(_ASM_SP);" ?

FWIW patch works (builds) for me with gcc.