linux-kernel - Re: [PATCH v2 01/10] arm64: Move the zero page to rodata

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Message-ID: <CAMj1kXF4k9EstLkxdNje_rwTehdu2MYvyh7n+4GoMWNcjcHYcA@mail.gmail.com>
Date: Tue, 27 Jan 2026 10:49:23 +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 01/10] arm64: Move the zero page to rodata

On Tue, 27 Jan 2026 at 10:34, Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 26/01/2026 09:26, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > The zero page should contain only zero bytes, and so mapping it
> > read-write is unnecessary. Combine it with reserved_pg_dir, which lives
> > in the read-only region of the kernel, and already serves a similar
> > purpose.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  arch/arm64/kernel/vmlinux.lds.S | 1 +
> >  arch/arm64/mm/mmu.c             | 3 +--
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> > index ad6133b89e7a..b2a093f5b3fc 100644
> > --- a/arch/arm64/kernel/vmlinux.lds.S
> > +++ b/arch/arm64/kernel/vmlinux.lds.S
> > @@ -229,6 +229,7 @@ SECTIONS
> >  #endif
> >
> >       reserved_pg_dir = .;
> > +     empty_zero_page = .;
> >       . += PAGE_SIZE;
> >
> >       swapper_pg_dir = .;
>
> Isn't there a magic macro for getting from swapper to reserved? That will need
> updating?
>

Why? This just adds an alias to refer to the same allocation.

> /*
>  *  Open-coded (swapper_pg_dir - reserved_pg_dir) as this cannot be calculated
>  *  until link time.
>  */
> #define RESERVED_SWAPPER_OFFSET (PAGE_SIZE)
>
>
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 9ae7ce00a7ef..c36422a3fae2 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -66,9 +66,8 @@ long __section(".mmuoff.data.write") __early_cpu_boot_status;
> >
> >  /*
> >   * Empty_zero_page is a special page that is used for zero-initialized data
> > - * and COW.
> > + * and COW. Defined in the linker script.
> >   */
> > -unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
> >  EXPORT_SYMBOL(empty_zero_page);
>
> What's the benefit of giving it it's own place in the linker script vs just
> declaring it as const and having it placed in the rodata?
>

Because it collapses the two into one.