Re: [PATCH 05/10] efi/libstub: distinguish between native/mixed not 32/64 bit

[Arvind Sankar]

On Sat, Dec 14, 2019 at 02:46:27PM -0500, Arvind Sankar wrote:
> On Sat, Dec 14, 2019 at 06:57:30PM +0100, Ard Biesheuvel wrote:
> > +
> > +#define efi_table_attr(table, attr, instance) ({			\
> > +	__typeof__(((table##_t *)0)->attr) __ret;			\
> > +	if (efi_is_native()) {						\
> > +		__ret = ((table##_t *)instance)->attr;			\
> > +	} else {							\
> > +		__typeof__(((table##_32_t *)0)->attr) at;		\
> > +		at = (((table##_32_t *)(unsigned long)instance)->attr);	\
> > +		__ret = (__typeof__(__ret))(unsigned long)at;		\
> > +	}								\
> > +	__ret;								\
> > +})
> 
> The casting of `at' is appropriate if the attr is a pointer type which
> needs to be zero-extended to 64-bit, but for other fields it is
> unnecessary at best and possibly dangerous.  There are probably no
> instances currently where it is called for a non-pointer field, but is
> it possible to detect if the type is pointer and avoid the cast if not?

To clarify, I mean the casting via `unsigned long' -- casting to type of
__ret should be ok. We could also use uintptr_t for cleanliness when the
cast is required?