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

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Message-ID: <202312081027.BA44B7B3@keescook>
Date:   Fri, 8 Dec 2023 10:29:25 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
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 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