linux-kernel - Re: [PATCH v2 6/8] x86/kaslr: Simplify process_gb_huge

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Date:   Tue, 28 Jul 2020 12:27:17 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/8] x86/kaslr: Simplify process_gb_huge_pages

On Mon, Jul 27, 2020 at 07:07:59PM -0400, Arvind Sankar wrote:
> Short-circuit the whole function on 32-bit.
> 
> Replace the loop to determine the number of 1Gb pages with arithmetic.
> 
> Fix one minor bug: if the end of the region is aligned on a 1Gb
> boundary, the current code will not use the last available 1Gb page due
> to an off-by-one error.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>

Can you add some KUnit tests could be written to do validation of the
refactorings? Touching this code is so painful. :)

-Kees

> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -546,49 +546,43 @@ static void store_slot_info(struct mem_vector *region, unsigned long image_size)
>  static void
>  process_gb_huge_pages(struct mem_vector *region, unsigned long image_size)
>  {
> -	unsigned long addr, size = 0;
> +	unsigned long pud_start, pud_end, gb_huge_pages;
>  	struct mem_vector tmp;
> -	int i = 0;
>  
> -	if (!max_gb_huge_pages) {
> +	if (IS_ENABLED(CONFIG_X86_32) || !max_gb_huge_pages) {
>  		store_slot_info(region, image_size);
>  		return;
>  	}

Won't max_gb_huge_pages always be false for 32-bit?

-- 
Kees Cook