Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))

[Will Deacon <will@kernel.org>]

On Wed, Dec 18, 2019 at 11:22:05AM +0100, Christian Borntraeger wrote:
> On 12.12.19 21:49, Linus Torvalds wrote:
> > On Thu, Dec 12, 2019 at 11:34 AM Will Deacon <will@kernel.org> wrote:
> >>
> >> The root of my concern in all of this, and what started me looking at it in
> >> the first place, is the interaction with 'typeof()'. Inheriting 'volatile'
> >> for a pointer means that local variables in macros declared using typeof()
> >> suddenly start generating *hideous* code, particularly when pointless stack
> >> spills get stackprotector all excited.
> > 
> > Yeah, removing volatile can be a bit annoying.
> > 
> > For the particular case of the bitops, though, it's not an issue.
> > Since you know the type there, you can just cast it.
> > 
> > And if we had the rule that READ_ONCE() was an arithmetic type, you could do
> > 
> >     typeof(0+(*p)) __var;
> > 
> > since you might as well get the integer promotion anyway (on the
> > non-volatile result).
> > 
> > But that doesn't work with structures or unions, of course.
> 
> We do have a READ_ONCE on the following union in s390 code.
> 
> union ipte_control {
>         unsigned long val;
>         struct {
>                 unsigned long k  : 1;
>                 unsigned long kh : 31;
>                 unsigned long kg : 32;
>         }; 
> };
> 
> 
> In fact this one was the original failure case why we change ACCESS_ONCE.
> 
> see arch/s390/kvm/gaccess.c

Thanks. I think we should be ok just using the 'val' field instead of the
whole union but, then again, when bitfields are involved who knows what the
compiler might do. I thought we usually shied away from using them to mirror
hardware structures like this?

Will