[PATCH] arm64: mm: move zero page from .bss to right before swapper_pg_dir

[mark.rutland@arm.com (Mark Rutland)]

Hi,

On Sun, Sep 11, 2016 at 03:38:34PM +0100, Ard Biesheuvel wrote:
> Move the statically allocated zero page from the .bss section to right
> before swapper_pg_dir. This allows us to refer to its physical address
> by simply reading TTBR1_EL1 (which always points to swapper_pg_dir and
> always has its ASID field cleared), and subtracting PAGE_SIZE.

On a conflicting note, I was hoping to move the zero page into .rodata
so as to catch any erroneous modification.

Given that we can't rely on TTBR1 poiting at the swapper_pg_dir, that
leaves us with Image size reduction vs RO-ification.

Any thoughts/preference?

Thanks,
Mark,

> Inspired-by: http://marc.info/?l=linux-arm-kernel&m=147282867511801
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  arch/arm64/include/asm/mmu_context.h | 10 ++++++----
>  arch/arm64/kernel/head.S             |  1 -
>  arch/arm64/kernel/vmlinux.lds.S      |  2 ++
>  arch/arm64/mm/mmu.c                  |  1 -
>  4 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> index b1892a0dbcb0..94461ba5febd 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -49,13 +49,15 @@ static inline void contextidr_thread_switch(struct task_struct *next)
>   */
>  static inline void cpu_set_reserved_ttbr0(void)
>  {
> -	unsigned long ttbr = virt_to_phys(empty_zero_page);
> +	unsigned long dummy;
>  
> -	asm(
> +	asm volatile(
> +	"	mrs	%0, ttbr1_el1			// get TTBR1\n"
> +	"	sub	%0, %0, %1			// subtract PAGE_SIZE\n"
>  	"	msr	ttbr0_el1, %0			// set TTBR0\n"
>  	"	isb"
> -	:
> -	: "r" (ttbr));
> +	: "=&r" (dummy)
> +	: "I" (PAGE_SIZE));
>  }
>  
>  /*
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 8bc9458f9add..6020b884b076 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -449,7 +449,6 @@ __primary_switched:
>  	adr_l	x2, __bss_stop
>  	sub	x2, x2, x0
>  	bl	__pi_memset
> -	dsb	ishst				// Make zero page visible to PTW
>  
>  #ifdef CONFIG_KASAN
>  	bl	kasan_early_init
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 659963d40bb4..a14eb8ff5144 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -193,6 +193,8 @@ SECTIONS
>  	. = ALIGN(PAGE_SIZE);
>  	idmap_pg_dir = .;
>  	. += IDMAP_DIR_SIZE;
> +	empty_zero_page = .;
> +	. += PAGE_SIZE;
>  	swapper_pg_dir = .;
>  	. += SWAPPER_DIR_SIZE;
>  
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 4989948d1feb..539ce9d11325 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -53,7 +53,6 @@ EXPORT_SYMBOL(kimage_voffset);
>   * Empty_zero_page is a special page that is used for zero-initialized data
>   * and COW.
>   */
> -unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
>  EXPORT_SYMBOL(empty_zero_page);
>  
>  static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
> -- 
> 2.7.4
> 
> 
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