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Date: Thu, 04 Dec 2008 09:44:45 -0600
From: Eric Sandeen <sandeen@sandeen.net>
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Subject: Re: [PATCH] Fix off by one error in page_region_mask()
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To: lachlan@sgi.com
Cc: xfs-oss <xfs@oss.sgi.com>

Lachlan McIlroy wrote:
> final is calculated to be the last bit to set (ie inclusive) but when we
> do the mask shifting final really needs to be first bit not to set.
> 
> For example if first and final are both bit 0 (ie only first bit to be set)
> then mask is completely shifted and becomes all zeroes.
> 
> Or if first is 0 and final is 63 then the mask is shifted one bit when it
> shouldn't be shifted at all.

Lachlan, what's the end result of this bug?  What's the broken behavior?

Thanks,
-Eric

> --- xfs-fix.orig/fs/xfs/linux-2.6/xfs_buf.c
> +++ xfs-fix/fs/xfs/linux-2.6/xfs_buf.c
> @@ -129,15 +129,17 @@ page_region_mask(
>   	int		first, final;
> 
>   	first = BTOPR(offset);
> -	final = BTOPRT(offset + length - 1);
> -	first = min(first, final);
> +	final = BTOPRT(offset + length);
> +
> +	if (first >= final)
> +		return 0UL;
> 
>   	mask = ~0UL;
>   	mask <<= BITS_PER_LONG - (final - first);
>   	mask >>= BITS_PER_LONG - (final);
> 
>   	ASSERT(offset + length <= PAGE_CACHE_SIZE);
> -	ASSERT((final - first) < BITS_PER_LONG && (final - first) >= 0);
> +	ASSERT((final - first) <= BITS_PER_LONG && (final - first) > 0);
> 
>   	return mask;
>   }
> 
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