[PATCH v2 8/9] arm64: head.S: use ordinary stack frame for __primary_switched()

[Mark Rutland]

On Wed, Aug 24, 2016 at 04:36:05PM +0200, Ard Biesheuvel wrote:
> Instead of stashing the value of the link register in x28 before setting
> up the stack and calling into C code, create an ordinary PCS compatible
> stack frame so that we can push the return address onto the stack.
> 
> Since exception handlers require a stack as well, assign the stach pointer
> register before installing the vector table.

Nit: s/stach/stack/

> Note that this accounts for the difference between THREAD_START_SP and
> THREAD_SIZE, given that the stack pointer is always decremented before
> calling into any C code.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

Reviewed-by: Mark Rutland <mark.rutland at arm.com>

Mark.

> ---
>  arch/arm64/kernel/head.S | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 27f51272de68..ad1dc61d67ac 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -415,13 +415,18 @@ ENDPROC(__create_page_tables)
>   *
>   *   x0 = __PHYS_OFFSET
>   */
> -	.set	initial_sp, init_thread_union + THREAD_START_SP
>  __primary_switched:
> -	mov	x28, lr				// preserve LR
> +	adrp	x4, init_thread_union
> +	add	sp, x4, #THREAD_SIZE
> +	msr	sp_el0, x4			// Save thread_info
> +
>  	adr_l	x8, vectors			// load VBAR_EL1 with virtual
>  	msr	vbar_el1, x8			// vector table address
>  	isb
>  
> +	stp	xzr, x30, [sp, #-16]!
> +	mov	x29, sp
> +
>  	str_l	x21, __fdt_pointer, x5		// Save FDT pointer
>  
>  	ldr_l	x4, kimage_vaddr		// Save the offset between
> @@ -436,11 +441,6 @@ __primary_switched:
>  	bl	__pi_memset
>  	dsb	ishst				// Make zero page visible to PTW
>  
> -	adr_l	sp, initial_sp, x4
> -	mov	x4, sp
> -	and	x4, x4, #~(THREAD_SIZE - 1)
> -	msr	sp_el0, x4			// Save thread_info
> -	mov	x29, #0
>  #ifdef CONFIG_KASAN
>  	bl	kasan_early_init
>  #endif
> @@ -452,8 +452,8 @@ __primary_switched:
>  	bl	kaslr_early_init		// parse FDT for KASLR options
>  	cbz	x0, 0f				// KASLR disabled? just proceed
>  	orr	x23, x23, x0			// record KASLR offset
> -	ret	x28				// we must enable KASLR, return
> -						// to __primary_switch()
> +	ldp	x29, x30, [sp], #16		// we must enable KASLR, return
> +	ret					// to __primary_switch()
>  0:
>  #endif
>  	b	start_kernel
> -- 
> 2.7.4
>