Re: [PATCH v2 26/39] x86/cet/shstk: Introduce routines modifying shstk

[Edgecombe, Rick P]

On Wed, 2022-10-05 at 02:43 +0000, Andrew Cooper wrote:
> On 29/09/2022 23:29, Rick Edgecombe wrote:
> > diff --git a/arch/x86/include/asm/special_insns.h
> > b/arch/x86/include/asm/special_insns.h
> > index 35f709f619fb..f096f52bd059 100644
> > --- a/arch/x86/include/asm/special_insns.h
> > +++ b/arch/x86/include/asm/special_insns.h
> > @@ -223,6 +223,19 @@ static inline void clwb(volatile void *__p)
> >                : [pax] "a" (p));
> >   }
> >   
> > +#ifdef CONFIG_X86_SHADOW_STACK
> > +static inline int write_user_shstk_64(u64 __user *addr, u64 val)
> > +{
> > +     asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
> > +                       _ASM_EXTABLE(1b, %l[fail])
> > +                       :: [addr] "r" (addr), [val] "r" (val)
> > +                       :: fail);
> 
> "1: wrssq %[val], %[addr]\n"
> _ASM_EXTABLE(1b, %l[fail])
> : [addr] "+m" (*addr)
> : [val] "r" (val)
> :: fail
> 
> Otherwise you've failed to tell the compiler that you wrote to *addr.
> 
> With that fixed, it's not volatile because there are no unexpressed
> side
> effects.

Ok, thanks!