Re: [PATCH 3/4] x86/uaccess: rework user access speculative harden guards

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Nov 19, 2024 at 03:29:58PM +0100, Jan Beulich wrote:
> On 19.11.2024 11:34, Roger Pau Monne wrote:
> > The current guards to select whether user accesses should be speculative
> > hardened violate Misra rule 20.7, as the UA_KEEP() macro doesn't (and can't)
> > parenthesize the 'args' argument.
> > 
> > Change the logic so the guard is implemented inside the assembly block using
> > the .if assembly directive.
> 
> Hmm, interesting idea. I don't overly like emitting stuff to pre-processed
> and even assembly files, but doing so is probably warranted here. Nevertheless:
> Did we consider at all to deviate these macros instead?

I think the proposal is not overly ugly, as I would otherwise simply
suggest to deviate.  I'm assuming the preference is to attempt to fix
when possible rather than deviate.

> > --- a/xen/arch/x86/usercopy.c
> > +++ b/xen/arch/x86/usercopy.c
> > @@ -11,23 +11,23 @@
> >  #include <asm/uaccess.h>
> >  
> >  #ifndef GUARD
> > -# define GUARD UA_KEEP
> > +# define GUARD 1
> >  #endif
> 
> At least in cases like this one I think a comment is necessary, perhaps as
> terse as /* Keep */ (and /* Drop */ further down).

Right, can adjust if we agree this is the way forward.

Thanks, Roger.