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

[Josh Poimboeuf]

On Thu, Jul 13, 2017 at 11:47:48AM -0700, Matthias Kaehlcke wrote:
> > 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:
> 
> ffffffff813676ba:       31 c0                   xor    %eax,%eax
> ffffffff813676bc:       48 89 45 c8             mov    %rax,-0x38(%rbp)
> ffffffff813676c0:       45 31 ff                xor    %r15d,%r15d
> ffffffff813676c3:       48 89 45 a8             mov    %rax,-0x58(%rbp)
> ...
> ffffffff81367918:       48 8b 4d a8             mov    -0x58(%rbp),%rcx
> ffffffff8136791c:       48 89 cc                mov    %rcx,%rsp
> ffffffff8136791f:       48 89 cc                mov    %rcx,%rsp
> ffffffff81367922:       e8 69 26 f1 ff          callq  ffffffff81279f90 <__get_user_4>

LOL.  Why corrupt the stack pointer with a single instruction (reading a
zero from memory, no less) when you can instead do it with three
instructions, including two duplicates?

Anyway this seems like a clang bug to me.  If I specify RSP as an input
register then the compiler shouldn't overwrite it first.  For that
matter it has no reason to overwrite it if it's an output register
either.

-- 
Josh