linux-kernel - Re: [PATCH v2 1/3] x86/KASLR: Update document about KERNEL_IMAGE

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Message-ID: <20181003075223.GA3029@gmail.com>
Date:   Wed, 3 Oct 2018 09:52:23 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-doc@...r.kernel.org, tglx@...utronix.de,
        kirill.shutemov@...ux.intel.com, thgarnie@...gle.com,
        corbet@....net
Subject: Re: [PATCH v2 1/3] x86/KASLR: Update document about KERNEL_IMAGE_SIZE


* Baoquan He <bhe@...hat.com> wrote:

> Currently CONFIG_RANDOMIZE_BASE=y is default set, update the relevant
> document about KERNEL_IMAGE_SIZE.

Suggested wording:

  x86/KASLR: Update KERNEL_IMAGE_SIZE description
  
  Currently CONFIG_RANDOMIZE_BASE=y is set by default, which makes some of the
  old comments above the KERNEL_IMAGE_SIZE definition out of date. Update them
  to the current state of affairs.

> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  arch/x86/include/asm/page_64_types.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
> index 6afac386a434..2288ceabdb9c 100644
> --- a/arch/x86/include/asm/page_64_types.h
> +++ b/arch/x86/include/asm/page_64_types.h
> @@ -61,9 +61,10 @@
>  /*
>   * Kernel image size is limited to 1GiB due to the fixmap living in the
>   * next 1GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S). Use
> - * 512MiB by default, leaving 1.5GiB for modules once the page tables
> - * are fully set up. If kernel ASLR is configured, it can extend the
> - * kernel page table mapping, reducing the size of the modules area.
> + * 1 GiB by default, leaving 1 GiB for modules once the page tables are
> + * fully set up. If kernel ASLR is not configured, it can shrink the
> + * kernel page table mapping to decrease the size of kernel area to 512
> + * MiB, increase the size of the modules area to 1.5 GiB.
>   */

I've prettified that comment some more:

 /*
  * Maximum kernel image size is limited to 1 GiB, due to the fixmap living
  * in the next 1 GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S).
  *
  * On KASLR use 1 GiB by default, leaving 1 GiB for modules once the
  * page tables are fully set up.
  *
  * If KASLR is disabled we can shrink it to 0.5 GiB and increase the size
  * of the modules area to 1.5 GiB.
  */

>  #if defined(CONFIG_RANDOMIZE_BASE)
>  #define KERNEL_IMAGE_SIZE	(1024 * 1024 * 1024)

BTW., while at it, shouldn't we make that:

   #ifdef CONFIG_RANDOMIZE_BASE

?

Thanks,

	Ingo