linux-kernel - Re: [PATCH v2] ELF: supply userspace with available page shifts (AT_PAGE_SHIFT

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Date:   Sat, 9 Dec 2023 12:44:53 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Florian Weimer <fweimer@...hat.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2] ELF: supply userspace with available page shifts
 (AT_PAGE_SHIFT_MASK)

On Fri, Dec 08, 2023 at 10:29:25AM -0800, Kees Cook wrote:
> 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,

It will become arch-independent once all relevant archs opt-in.

I doubt anyone is writing new software for sparc or alpha.

> 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.

It is inaccurate if ARCH_AT_PAGE_SHIFT_MASK is defined as "_all_ page
shift CPU supports". Inaccurate version is called AT_PAGESZ which lists
just 1 page size, there is no need for 2 inaccurate APIs.