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

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Message-ID: <202312081037.895E558@keescook>
Date:   Fri, 8 Dec 2023 10:38:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 07:35:25PM +0100, Florian Weimer wrote:
> * Kees Cook:
> 
> > 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.
> 
> The downside is that as an application programmer, I have to go and
> chase for the information the legacy way if I encounter
> getauxval(AT_PAGE_SHIFT_MASK) == getpagesize() for a longer time
> because the interface does not signal the absence of any extended
> page sizes.

Are there architectures besides x86 where AT_PAGE_SHIFT_MASK isn't a
single bit? If so, let's get them added now along with x86.

-- 
Kees Cook