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

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Message-ID: <427d39c1-ab02-4a66-a0ba-d4fb98ab72d6@arm.com>
Date: Tue, 27 Jan 2026 10:03:56 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb@...nel.org>
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 27/01/2026 09:49, Ard Biesheuvel wrote:
> 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.

Oh yes, sorry I completely missed that. And you've even stated it in the commit
log...

I'm struggling to see where this gets zeroed though? I assume it must be zeroed
before the old empty_zero_page would have been so everything works fine?

Assuming yes, then:

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> 
>> /*
>>  *  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.