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

[Kevin Brodsky]

On 26/05/2026 19:58, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> 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 <kevin.brodsky@xxxxxxx>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  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?

Aside from that:

Reviewed-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>

> +		*(.pgdir.bss)
> +	}
> +	ASSERT(ADDR(.pgdir.bss) == __bss_stop, ".pgdir.bss must follow BSS")
> +
>  	. = ALIGN(PAGE_SIZE);
>  	__pi_init_pg_dir = .;
>  	. += INIT_DIR_SIZE;
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index c5c5425791da..1a3bbd67dd76 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -31,9 +31,9 @@ static_assert(NR_BM_PMD_TABLES == 1);
>  
>  #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
>  
> -static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
> -static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
> -static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
> +static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __pgtbl_bss;
> +static pmd_t bm_pmd[PTRS_PER_PMD] __pgtbl_bss __maybe_unused;
> +static pud_t bm_pud[PTRS_PER_PUD] __pgtbl_bss __maybe_unused;
>  
>  static inline pte_t *fixmap_pte(unsigned long addr)
>  {
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index abeb81bf6ebd..dbf22cae82ee 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -214,7 +214,7 @@ asmlinkage void __init kasan_early_init(void)
>  		 * shadow pud_t[]/p4d_t[], which could end up getting corrupted
>  		 * when the linear region is mapped.
>  		 */
> -		static pte_t tbl[PTRS_PER_PTE] __page_aligned_bss;
> +		static pte_t tbl[PTRS_PER_PTE] __pgtbl_bss;
>  		pgd_t *pgdp = pgd_offset_k(KASAN_SHADOW_START);
>  
>  		set_pgd(pgdp, __pgd(__pa_symbol(tbl) | PGD_TYPE_TABLE));