linux-kernel - Re: [PATCH v9 13/18] arm64: kexec: add expandable argument to relocation function

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Date:   Thu, 7 May 2020 17:22:17 +0100
From:   James Morse <james.morse@....com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     jmorris@...ei.org, sashal@...nel.org, ebiederm@...ssion.com,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        corbet@....net, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, maz@...nel.org,
        vladimir.murzin@....com, matthias.bgg@...il.com,
        bhsharma@...hat.com, linux-mm@...ck.org, mark.rutland@....com,
        steve.capper@....com, rfontana@...hat.com, tglx@...utronix.de,
        selindag@...il.com
Subject: Re: [PATCH v9 13/18] arm64: kexec: add expandable argument to
 relocation function

Hi Pavel,

On 26/03/2020 03:24, Pavel Tatashin wrote:
> Currently, kexec relocation function (arm64_relocate_new_kernel) accepts
> the following arguments:
> 
> head:		start of array that contains relocation information.
> entry:		entry point for new kernel or purgatory.
> dtb_mem:	first and only argument to entry.

> The number of arguments cannot be easily expended, because this
> function is also called from HVC_SOFT_RESTART, which preserves only
> three arguments. And, also arm64_relocate_new_kernel is written in
> assembly but called without stack, thus no place to move extra
> arguments to free registers.
> 
> Soon, we will need to pass more arguments: once we enable MMU we
> will need to pass information about page tables.


> Another benefit of allowing this function to accept more arguments, is that
> kernel can actually accept up to 4 arguments (x0-x3), however currently
> only one is used, but if in the future we will need for more (for example,
> pass information about when previous kernel exited to have a precise
> measurement in time spent in purgatory), we won't be easilty do that
> if arm64_relocate_new_kernel can't accept more arguments.

This is a niche debug hack.
We really don't want an ABI with purgatory. I think the register values it gets were added
early for compatibility with kexec_file_load().


> So, add a new struct: kern_reloc_arg, and place it in kexec safe page (i.e
> memory that is not overwritten during relocation).
> Thus, make arm64_relocate_new_kernel to only take one argument, that
> contains all the needed information.

Do we really not have enough registers?

The PCS[0] gives you 8 arguments. In this patch you use 6.


If this is really about the hyp-stub abi, please state that.


> diff --git a/arch/arm64/kernel/machine_kexec.c b/arch/arm64/kernel/machine_kexec.c
> index cee3be586384..b1122eea627e 100644
> --- a/arch/arm64/kernel/machine_kexec.c
> +++ b/arch/arm64/kernel/machine_kexec.c
> @@ -59,13 +60,35 @@ void machine_kexec_cleanup(struct kimage *kimage)

>  int machine_kexec_post_load(struct kimage *kimage)
>  {
>  	void *reloc_code = page_to_virt(kimage->control_code_page);
> +	struct kern_reloc_arg *kern_reloc_arg = kexec_page_alloc(kimage);
> +
> +	if (!kern_reloc_arg)
> +		return -ENOMEM;
>  
>  	memcpy(reloc_code, arm64_relocate_new_kernel,
>  	       arm64_relocate_new_kernel_size);
>  	kimage->arch.kern_reloc = __pa(reloc_code);
> +	kimage->arch.kern_reloc_arg = __pa(kern_reloc_arg);
> +	kern_reloc_arg->head = kimage->head;
> +	kern_reloc_arg->entry_addr = kimage->start;
> +	kern_reloc_arg->kern_arg0 = kimage->arch.dtb_mem;

These kern_reloc_arg values are written via the cacheable linear map.
They are read in arm64_relocate_new_kernel() where the MMU is disabled an all memory
access are non-cacheable.

To ensure you read the values you wrote, you must clean kern_reloc_arg to the PoC.


>  	kexec_image_info(kimage);
>  
>  	return 0;Thanks,

James

[0]
https://developer.arm.com/docs/ihi0055/d/procedure-call-standard-for-the-arm-64-bit-architecture