Re: [PATCH v2 12/13] arm64: add support for relocatable kernel

James Morse
2016-01-08T10:18:11Z

Hi Ard!

On 30/12/15 15:26, Ard Biesheuvel wrote:
> This adds support for runtime relocation of the kernel Image, by
> building it as a PIE (ET_DYN) executable and applying the dynamic
> relocations in the early boot code.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---

> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 01a33e42ed70..ab582ee58b58 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -243,6 +253,16 @@ ENDPROC(stext)
> preserve_boot_args:
> mov x21, x0 // x21=FDT
>
> +#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
> + /*
> + * Mask off the bits of the random value supplied in x1 so it can serve
> + * as a KASLR displacement value which will move the kernel image to a
> + * random offset in the lower half of the VMALLOC area.
> + */
> + mov x23, #(1 << (VA_BITS - 2)) - 1
> + and x23, x23, x1, lsl #SWAPPER_BLOCK_SHIFT
> +#endif

I've managed to make this fail to boot by providing a seed that caused
the kernel to overlap a 1G boundary on a 4K system.

(It looks like your v3 may have the same issue - but I haven't tested it.)

> +
> adr_l x0, boot_args // record the contents of
> stp x21, x1, [x0] // x0 .. x3 at kernel entry
> stp x2, x3, [x0, #16]

Thanks!

James