linux-kernel - Re: [PATCH v2 03/10] arm64: mm: Permit contiguous descriptors to be rewritten

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Message-ID: <CAMj1kXFskBM-LT_ifX5HpxgQnt1E5tt2QA_5fYuxPQBiSWSDZw@mail.gmail.com>
Date: Tue, 27 Jan 2026 16:03:10 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, will@...nel.org, 
	catalin.marinas@....com, mark.rutland@....com, 
	Anshuman Khandual <anshuman.khandual@....com>, Liz Prucka <lizprucka@...gle.com>, 
	Seth Jenkins <sethjenkins@...gle.com>, Kees Cook <kees@...nel.org>, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 03/10] arm64: mm: Permit contiguous descriptors to be rewritten

On Tue, 27 Jan 2026 at 10:45, Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 26/01/2026 09:26, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > Currently, pgattr_change_is_safe() is overly pedantic when it comes to
> > descriptors with the contiguous hint attribute set, as it rejects
> > assignments even if the old and the new value are the same.
> >
> > So relax the check to allow that.
>
> But why do we require the relaxation? Why are we re-writing a PTE in the first
> place? Either the caller already knows it's the same in which case it can be
> avoided, or it doesn't know in which case it is accidentally the same and couple
> probably just as easily been accidentally different? So it's better to warn
> regardless I would think?
>

Based on rule RJQQTC in your reply to another patch in this series, my
conclusion here is that we can drop this check entirely.