linux-kernel - Re: [PATCH] x86/head_32: Fix overflow warning with 32-bit binutils

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Date:	Fri, 19 Jun 2015 10:10:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/head_32: Fix overflow warning with 32-bit binutils


* Borislav Petkov <bp@...en8.de> wrote:

> From: Borislav Petkov <bp@...e.de>
> 
> When building the kernel with 32-bit binutils built with support only
> for the i386 target, we get the following warning:
> 
>   arch/x86/kernel/head_32.S:66: Warning: shift count out of range (32 is not between 0 and 31)
> 
> The problem is that in that case, binutils' internal type representation
> is 32-bit wide and the shift range overflows.
> 
> In order to fix this, manipulate the shift expression which creates the
> 4GiB constant to not overflow the shift count.
> 
> Reported-and-tested-by: Enrico Mioso <mrkiko.rs@...il.com>
> Suggested-by: Michael Matz <matz@...e.de>
> Signed-off-by: Borislav Petkov <bp@...e.de>
> ---
>  arch/x86/kernel/head_32.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index 53eeb226657c..c601d1de5ae5 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -63,8 +63,8 @@
>  #endif
>  
>  /* Number of possible pages in the lowmem region */
> -LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT)
> -	
> +LOWMEM_PAGES = (((2<<31) - __PAGE_OFFSET) >> PAGE_SHIFT)
> +

Hm, so if internally GAS uses 64-bit types, couldn't we 'make sure' it's a 64-bit 
type, by using something like 31LL? Assuming GAS understands that?

Or if the internal representation is 32 bits, then 2<<31 is just a fancy way of 
saying '0', right?

So this could be written as:

  LOWMEM_PAGES = (-__PAGE_OFFSET >> PAGE_SHIFT)

or:

  LOWMEM_PAGES = ((0 - __PAGE_OFFSET) >> PAGE_SHIFT)

right?

Or did it get it all wrong?

Thanks,

	Ingo
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