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

On Fri, 29 May 2026, at 16:42, Kevin Brodsky wrote:
> 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 protected]>
>>>>
>>>> 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 protected]>
>>>> Signed-off-by: Ard Biesheuvel <[email protected]>
>>>> ---
>>>>  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?

Not sure why, but the section was PROGBITS not NOBITS before I added the (NOLOAD)

> Also AFAIU NOLOAD does not prevent the output section from being emitted
> into the ELF file.
>

NOLOAD marks it as NOBITS, which means there is no data in the file that should
be used to populate the section in memory.