Re: mmotm 2019-04-19-14-53 uploaded (objtool)

[Peter Zijlstra]

On Tue, Apr 23, 2019 at 09:07:01AM -0700, Andy Lutomirski wrote:
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 22ba683afdc2..c82abd6e4ca3 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -427,10 +427,11 @@ do {                                    \
> > ({                                \
> >    __label__ __pu_label;                    \
> >    int __pu_err = -EFAULT;                    \
> > -    __typeof__(*(ptr)) __pu_val;                \
> > -    __pu_val = x;                        \
> > +    __typeof__(*(ptr)) __pu_val = (x);            \
> > +    __typeof__(ptr) __pu_ptr = (ptr);            \
> 
> Hmm.  I wonder if this forces the address calculation to be done
> before STAC, which means that gcc canât use mov ..., %gs:(fancy
> stuff).  It probably depends on how clever the optimizer is. Have you
> looked at the generated code?

I have not; will do before posting the real patch.