[PATCH 3/3] arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

[Christopher Covington]

On 03/17/2015 06:11 AM, Ard Biesheuvel wrote:
> According to the arm64 boot protocol, registers x1 to x3 should be
> zero upon kernel entry, and non-zero values are reserved for future
> use. This future use is going to be problematic if we never enforce
> the current rules, so start enforcing them now, by emitting a warning
> if non-zero values are detected.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/kernel/head.S  |  8 ++++++++
>  arch/arm64/kernel/setup.c | 13 +++++++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 1651c0fd50e6..fe5354eae069 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -231,6 +231,10 @@ section_table:
>  #endif
>  
>  ENTRY(stext)
> +	adr	x8, boot_regs			// record the contents of
> +	stp	x0, x1, [x8]			// x0 .. x3 at kernel entry
> +	stp	x2, x3, [x8, #16]
> +
>  	mov	x21, x0				// x21=FDT
>  	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
>  	adrp	x24, KERNEL_START - TEXT_OFFSET	// x24=PHYS_OFFSET
> @@ -251,6 +255,10 @@ ENTRY(stext)
>  	b	__cpu_setup			// initialise processor
>  ENDPROC(stext)
>  
> +	.align	3
> +ENTRY(boot_regs)
> +	.skip	4 * 8				// x0 .. x3
> +
>  /*
>   * Determine validity of the x21 FDT pointer.
>   * The dtb must be 8-byte aligned and live in the first 512M of memory.
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 6c5fb5aff325..2b81d0a907ce 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -532,3 +532,16 @@ const struct seq_operations cpuinfo_op = {
>  	.stop	= c_stop,
>  	.show	= c_show
>  };
> +
> +static int verify_boot_protocol(void)
> +{
> +	extern u64 boot_regs[];
> +
> +	if (boot_regs[1] || boot_regs[2] || boot_regs[3]) {
> +		pr_err("WARNING: boot protocol violation detected (x1 == %llx, x2 == %llx, x3 == %llx)\n",
> +			boot_regs[1], boot_regs[2], boot_regs[3]);
> +		pr_err("WARNING: your bootloader may fail to load newer kernels\n");

pr_warn?

> +	}
> +	return 0;
> +}
> +late_initcall(verify_boot_protocol);
> 

Chris

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