linux-kernel - Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get

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Date:   Thu, 13 Jul 2017 23:20:04 +0300
From:   Andrey Rybainin <ryabinin.a.a@...il.com>
To:     Matthias Kaehlcke <mka@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Chris J Arges <chris.j.arges@...onical.com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Bernhard.Rosenkranzer@...aro.org
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in
 get_user() inline asm"

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