Re: [PATCH v4 10/15] arm64: Move fixmap page tables to end of kernel image

Kevin Brodsky
2026-04-29T13:52:40Z

On 27/04/2026 17:34, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
>
> Move the fixmap 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.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

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

> ---
> arch/arm64/kernel/vmlinux.lds.S | 5 +++++
> arch/arm64/mm/fixmap.c | 7 ++++---
> 2 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index e1ac876200a3..2dca18574619 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -353,6 +353,11 @@ SECTIONS
> __pi___bss_start = __bss_start;
>
> . = ALIGN(PAGE_SIZE);
> + .fixmap_pgdir : {
> + __fixmap_pgdir_start = .;
> + *(.fixmap_bss)
> + }
> +
> __pi_init_pg_dir = .;
> . += INIT_DIR_SIZE;
> __pi_init_pg_end = .;
> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
> index c5c5425791da..b649ea1a46e4 100644
> --- a/arch/arm64/mm/fixmap.c
> +++ b/arch/arm64/mm/fixmap.c
> @@ -31,9 +31,10 @@ 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;
> +#define __fixmap_bss __section(".fixmap_bss") __aligned(PAGE_SIZE)
> +static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __fixmap_bss;
> +static pmd_t bm_pmd[PTRS_PER_PMD] __fixmap_bss __maybe_unused;
> +static pud_t bm_pud[PTRS_PER_PUD] __fixmap_bss __maybe_unused;
>
> static inline pte_t *fixmap_pte(unsigned long addr)
> {