El Thu, Jul 13, 2017 at 03:34:16PM -0500 Josh Poimboeuf ha dit:
> 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); \
> })
This compiles with both gcc and clang, clang does not corrupt the
stack pointer. I wouldn't be able to tell though if it forces a stack
frame if it doesn't already exist, as the original patch intends.
