Re: [PATCH v2] ELF: supply userspace with available page shifts (AT_PAGE_SHIFT_MASK)

Kees Cook
2023-12-08T18:29:27Z

On Thu, Dec 07, 2023 at 05:57:05PM +0300, Alexey Dobriyan wrote:
> On Wed, Dec 06, 2023 at 12:47:27PM -0800, Kees Cook wrote:
> > Can't we have a generic ARCH_AT_PAGE_SHIFT_MASK too? Something like:
> >
> > #ifndef ARCH_AT_PAGE_SHIFT_MASK
> > #define ARCH_AT_PAGE_SHIFT_MASK
> > NEW_AUX_ENT(AT_PAGE_SHIFT_MASK, 1 << PAGE_SHIFT)
> > #endif
> >
> > Or am I misunderstanding something here?
>
> 1) Arch maintainers can opt into this new way to report information at
> their own pace.
>
> 2) AT_PAGE_SHIFT_MASK is about _all_ pagesizes supported by CPU.
> Reporting just one is missing the point.
>
> I'll clarify comment: mmap() support require many things including
> tests for hugetlbfs being mounted, this is about CPU support.

I significantly prefer APIs not being arch-specific, so I'd prefer we
always include AT_PAGE_SHIFT_MASK. For an architecture that doesn't
define its own ARCH_AT_PAGE_SHIFT_MASK, it's not _inaccurate_ to report
1 << PAGE_SHIFT, but it might be incomplete.

--
Kees Cook