Re: [PATCH v4] ELF: AT_PAGE_SHIFT_MASK -- supply userspace with available page shifts

Kees Cook
2024-02-10T00:41:42Z

On Fri, Feb 09, 2024 at 03:30:37PM +0300, Alexey Dobriyan wrote:
> On Mon, Feb 05, 2024 at 04:48:08AM -0800, Kees Cook wrote:
> > On Mon, Feb 05, 2024 at 12:51:43PM +0300, Alexey Dobriyan wrote:
> > > +#define ARCH_AT_PAGE_SHIFT_MASK \
> > > + do { \
> > > + u32 val = 1 << 12; \
> > > + if (boot_cpu_has(X86_FEATURE_PSE)) { \
> > > + val |= 1 << 21; \
> > > + } \
> > > + if (boot_cpu_has(X86_FEATURE_GBPAGES)) { \
> > > + val |= 1 << 30; \
> > > + } \
> >
> > Can we use something besides literal "12", "21", and "30" values here?
>
> Ehh, no, why? Inside x86_64 the page shifts are very specific numbers,
> they won't change.

Well, it's nicer to have meaningful words to describe these things. In
fact, PAGE_SHIFT already exists for 12, and HPAGE_SHIFT already exists
for 21. Please use those, and add another, perhaps GBPAGE_SHIFT, for 30.

-Kees

--
Kees Cook