On Thu, Jul 13, 2017 at 11:20:04PM +0300, Andrey Rybainin wrote:
> 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?
And yet another one to try (clobbering sp) :-)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 11433f9..21f0c39 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -166,12 +166,12 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL),
0ULL, 0UL))
({ \
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) \
- : "0" (ptr), "i" (sizeof(*(ptr)))); \
+ asm volatile("call __get_user_%P3" \
+ : "=a" (__ret_gu), "=r" (__val_gu) \
+ : "0" (ptr), "i" (sizeof(*(ptr))) \
+ : "sp"); \
(x) = (__force __typeof__(*(ptr))) __val_gu; \
__builtin_expect(__ret_gu, 0); \
})
