Re: [PATCH v3 25/60] arm64: head: Clear BSS and the kernel page tables in one go

Ryan Roberts
2023-04-17T14:02:00Z

On 07/03/2023 14:04, Ard Biesheuvel wrote:
> We will move the CPU feature overrides into BSS in a subsequent patch,
> and this requires that BSS is zeroed before the feature override
> detection code runs. So let's map BSS read-write in the ID map, and zero
> it via this mapping.
>
> Since the kernel page tables are right next to it, and also zeroed via
> the ID map, let's drop the separate clear_page_tables() function, and
> just zero everything in one go.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/arm64/kernel/head.S | 33 +++++++-------------
> 1 file changed, 11 insertions(+), 22 deletions(-)
>
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 0fa44b3188c1e204..ade0cb99c8a83a3d 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -177,17 +177,6 @@ SYM_CODE_START_LOCAL(preserve_boot_args)
> ret
> SYM_CODE_END(preserve_boot_args)
>
> -SYM_FUNC_START_LOCAL(clear_page_tables)
> - /*
> - * Clear the init page tables.
> - */
> - adrp x0, init_pg_dir
> - adrp x1, init_pg_end
> - sub x2, x1, x0
> - mov x1, xzr
> - b __pi_memset // tail call
> -SYM_FUNC_END(clear_page_tables)
> -
> /*
> * Macro to populate page table entries, these entries can be pointers to the next level
> * or last level entries pointing to physical memory.
> @@ -386,9 +375,9 @@ SYM_FUNC_START_LOCAL(create_idmap)
>
> map_memory x0, x1, x3, x6, x7, x3, IDMAP_PGD_ORDER, x10, x11, x12, x13, x14, EXTRA_SHIFT
>
> - /* Remap the kernel page tables r/w in the ID map */
> + /* Remap BSS and the kernel page tables r/w in the ID map */
> adrp x1, _text
> - adrp x2, init_pg_dir
> + adrp x2, __bss_start
> adrp x3, _end
> bic x4, x2, #SWAPPER_BLOCK_SIZE - 1
> mov x5, SWAPPER_RW_MMUFLAGS
> @@ -489,14 +478,6 @@ SYM_FUNC_START_LOCAL(__primary_switched)
> mov x0, x20
> bl set_cpu_boot_mode_flag
>
> - // Clear BSS
> - adr_l x0, __bss_start
> - mov x1, xzr
> - adr_l x2, __bss_stop
> - sub x2, x2, x0
> - bl __pi_memset
> - dsb ishst // Make zero page visible to PTW
> -
> #if VA_BITS > 48
> adr_l x8, vabits_actual // Set this early so KASAN early init
> str x25, [x8] // ... observes the correct value
> @@ -780,6 +761,15 @@ SYM_FUNC_START_LOCAL(__primary_switch)
> adrp x1, reserved_pg_dir
> adrp x2, init_idmap_pg_dir
> bl __enable_mmu
> +
> + // Clear BSS
> + adrp x0, __bss_start
> + mov x1, xzr
> + adrp x2, init_pg_end
> + sub x2, x2, x0
> + bl __pi_memset
> + dsb ishst // Make zero page visible to PTW

Is it possible to add an assert somewhere (or at the very least a comment in
vmlinux.lds.S) to ensure that nothing gets inserted between the BSS and the page
tables? It feels a bit fragile otherwise.

I also wonder what's the point in calling __pi_memset() from here? Why not just
do it all in C?

> +
> #ifdef CONFIG_RELOCATABLE
> adrp x23, KERNEL_START
> and x23, x23, MIN_KIMG_ALIGN - 1
> @@ -794,7 +784,6 @@ SYM_FUNC_START_LOCAL(__primary_switch)
> orr x23, x23, x0 // record kernel offset
> #endif
> #endif
> - bl clear_page_tables
> bl create_kernel_mapping
>
> adrp x1, init_pg_dir