linux-kernel - Re: [PATCH 08/11] x86: make TASK_SIZE

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Date:   Tue, 18 Aug 2020 12:44:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 08/11] x86: make TASK_SIZE_MAX usable from assembly code

On Mon, Aug 17, 2020 at 09:32:09AM +0200, Christoph Hellwig wrote:
> For 64-bit the only hing missing was a strategic _AC, and for 32-bit we

typo: thing

> need to use __PAGE_OFFSET instead of PAGE_OFFSET in the TASK_SIZE
> definition to escape the explicit unsigned long cast.  This just works
> because __PAGE_OFFSET is defined using _AC itself and thus never needs
> the cast anyway.

Shouldn't this be folded into the prior patch so there's no bisection
problem?

-Kees

> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  arch/x86/include/asm/page_32_types.h | 4 ++--
>  arch/x86/include/asm/page_64_types.h | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/page_32_types.h b/arch/x86/include/asm/page_32_types.h
> index 26236925fb2c36..f462895a33e452 100644
> --- a/arch/x86/include/asm/page_32_types.h
> +++ b/arch/x86/include/asm/page_32_types.h
> @@ -44,8 +44,8 @@
>  /*
>   * User space process size: 3GB (default).
>   */
> -#define IA32_PAGE_OFFSET	PAGE_OFFSET
> -#define TASK_SIZE		PAGE_OFFSET
> +#define IA32_PAGE_OFFSET	__PAGE_OFFSET
> +#define TASK_SIZE		__PAGE_OFFSET
>  #define TASK_SIZE_LOW		TASK_SIZE
>  #define TASK_SIZE_MAX		TASK_SIZE
>  #define DEFAULT_MAP_WINDOW	TASK_SIZE
> diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
> index 996595c9897e0a..838515daf87b36 100644
> --- a/arch/x86/include/asm/page_64_types.h
> +++ b/arch/x86/include/asm/page_64_types.h
> @@ -76,7 +76,7 @@
>   *
>   * With page table isolation enabled, we map the LDT in ... [stay tuned]
>   */
> -#define TASK_SIZE_MAX	((1UL << __VIRTUAL_MASK_SHIFT) - PAGE_SIZE)
> +#define TASK_SIZE_MAX	((_AC(1,UL) << __VIRTUAL_MASK_SHIFT) - PAGE_SIZE)
>  
>  #define DEFAULT_MAP_WINDOW	((1UL << 47) - PAGE_SIZE)
>  
> -- 
> 2.28.0
> 

-- 
Kees Cook