Subject: Re: [PATCH v6 09/15] arm64: Move fixmap and kasan page tables to end of kernel image

From: Kevin Brodsky <[email-masked]>
Date: Fri, 29 May 2026 16:42:55 +0200

On 29/05/2026 13:19, Ard Biesheuvel wrote:
> On Fri, 29 May 2026, at 10:27, Kevin Brodsky wrote:
>> On 26/05/2026 19:58, Ard Biesheuvel wrote:
>>> From: Ard Biesheuvel <[email-masked]>
>>>
>>> Move the fixmap and kasan page tables out of the BSS section, and place
>>> them at the end of the image, right before the init_pg_dir section where
>>> some of the other statically allocated page tables live.
>>>
>>> These page tables are currently the only data objects in vmlinux that
>>> are meant to be accessed via the kernel image's linear alias, and so
>>> placing them together allows the remainder of the data/bss section to be
>>> remapped read-only or unmapped entirely.
>>>
>>> Reviewed-by: Kevin Brodsky <[email-masked]>
>>> Signed-off-by: Ard Biesheuvel <[email-masked]>
>>> ---
>>>  arch/arm64/include/asm/mmu.h    | 2 ++
>>>  arch/arm64/kernel/vmlinux.lds.S | 8 +++++++-
>>>  arch/arm64/mm/fixmap.c          | 6 +++---
>>>  arch/arm64/mm/kasan_init.c      | 2 +-
>>>  4 files changed, 13 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
>>> index 5e1211c540ab..fb95754f2876 100644
>>> --- a/arch/arm64/include/asm/mmu.h
>>> +++ b/arch/arm64/include/asm/mmu.h
>>> @@ -13,6 +13,8 @@
>>>  
>>>  #ifndef __ASSEMBLER__
>>>  
>>> +#define __pgtbl_bss __section(".pgdir.bss") __aligned(PAGE_SIZE)
>>> +
>>>  #include <linux/refcount.h>
>>>  #include <asm/cpufeature.h>
>>>  
>>> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
>>> index e1ac876200a3..2b0ebfb30c63 100644
>>> --- a/arch/arm64/kernel/vmlinux.lds.S
>>> +++ b/arch/arm64/kernel/vmlinux.lds.S
>>> @@ -349,9 +349,15 @@ SECTIONS
>>>  	_edata = .;
>>>  
>>>  	/* start of zero-init region */
>>> -	BSS_SECTION(SBSS_ALIGN, 0, 0)
>>> +	BSS_SECTION(SBSS_ALIGN, 0, PAGE_SIZE)
>>>  	__pi___bss_start = __bss_start;
>>>  
>>> +	/* fixmap BSS starts here - preceding data/BSS is omitted from the linear map */
>>> +	.pgdir.bss (NOLOAD) : ALIGN(PAGE_SIZE) {
>> Do we actually need the NOLOAD type here?
> Yes, otherwise it is emitted as PROGBITS, resulting in all of BSS to be
> emitted into Image.

That's rather strange, aren't the .pgdir.bss input sections already
NOBITS since __pgtbl_bss is only used on default-initialised globals?
Also AFAIU NOLOAD does not prevent the output section from being emitted
into the ELF file.

- Kevin