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

[Josh Poimboeuf]

On Thu, Jul 13, 2017 at 02:12:45PM -0700, Matthias Kaehlcke wrote:
> El Thu, Jul 13, 2017 at 03:34:16PM -0500 Josh Poimboeuf ha dit:
> > 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.

Whether it forces the stack frame on clang is a very minor issue
compared to the double fault.  That really only matters when you want to
use CONFIG_STACK_VALIDATION to get 100% reliable stacktraces with frame
pointers.  And that feature is currently very GCC-specific.  So you
probably don't need to worry about that for now, at least until you want
to do live patching with a clang-compiled kernel.

IIRC, clobbering SP does at least force the stack frame on GCC, though I
need to double check that.  I can try to work up an official patch in
the next week or so (need to do some testing first).

-- 
Josh