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Re: [PATCH v2] xfs: fix the extent count when allocating an new indirect

To: Jeff Liu <jeff.liu@xxxxxxxxxx>
Subject: Re: [PATCH v2] xfs: fix the extent count when allocating an new indirection array entry
From: Ben Myers <bpm@xxxxxxx>
Date: Thu, 31 Oct 2013 16:36:24 -0500
Cc: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <526A153C.2090408@xxxxxxxxxx>
References: <526A153C.2090408@xxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hey Jeff,

On Fri, Oct 25, 2013 at 02:52:44PM +0800, Jeff Liu wrote:
> From: Jie Liu <jeff.liu@xxxxxxxxxx>
> 
> At xfs_iext_add(), if extent(s) are being appended to the last page in
> the indirection array and the new extent(s) don't fit in the page, the
> number of extents(erp->er_extcount) in a new allocated entry should be
> the minimum value between count and XFS_LINEAR_EXTS, instead of count.
> 
> For now, there is no existing test case can demonstrates a problem with
> the er_extcount being set incorrectly here, but it obviously like a bug.
> 
> Signed-off-by: Jie Liu <jeff.liu@xxxxxxxxxx>
> ---
> v2: * Declare count to uint as it will be decreased to 0 and XFS_LINEAR_EXTS
>       can be uint because of a case in the macro.
>     * Convert MIN() to min().
>     * Revise the commits log to indicate there is no existing test case can
>       reflect this issue for future tracking up.
> 
>  fs/xfs/xfs_inode_fork.c |    9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/xfs_inode_fork.c b/fs/xfs/xfs_inode_fork.c
> index 22c9837..cfee14a 100644
> --- a/fs/xfs/xfs_inode_fork.c
> +++ b/fs/xfs/xfs_inode_fork.c
> @@ -1021,15 +1021,14 @@ xfs_iext_add(
>                * the next index needed in the indirection array.
>                */
>               else {
> -                     int     count = ext_diff;
> +                     uint    count = ext_diff;
>  
>                       while (count) {
>                               erp = xfs_iext_irec_new(ifp, erp_idx);
> -                             erp->er_extcount = count;
> -                             count -= MIN(count, (int)XFS_LINEAR_EXTS);
> -                             if (count) {
> +                             erp->er_extcount = min(count, XFS_LINEAR_EXTS);
> +                             count -= erp->er_extcount;
> +                             if (count)
>                                       erp_idx++;
> -                             }
>                       }
>               }
>       }

Really nice find.  So there is potential for incorrect er_extcount and
er_extoff when adding > 256 extents to the end of the indirection array.  You'd
think we'd be seeing some side effects since xfs_iext_idx_to_irec uses them in
it's binary search.

Reviewed-by: Ben Myers <bpm@xxxxxxx>

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