Re: [PATCH] xfs: Fix xfs_attr_leafblock definition

[Eric Sandeen <sandeen@sandeen.net>]

On 8/13/15 4:26 AM, Jan Kara wrote:
> struct xfs_attr_leafblock contains 'entries' array which is declared
> with size 1 altough it can in fact contain much more entries. Since this
> array is followed by further struct members, gcc (at least in version
> 4.8.3) thinks that the array has the fixed size of 1 element and thus
> may optimize away all accesses beyond the end of array resulting in
> non-working code. This problem was only observed with userspace code in
> xfsprogs, however it's better to be safe in kernel as well and have
> matching kernel and xfsprogs definitions.
> 
> Signed-off-by: Jan Kara <jack@suse.com>
> ---
>  fs/xfs/libxfs/xfs_da_format.h | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
>  This is a kernel version of the xfsprogs patch I've sent a while ago.
> 
> diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
> index 74bcbabfa523..b14bbd6bb05f 100644
> --- a/fs/xfs/libxfs/xfs_da_format.h
> +++ b/fs/xfs/libxfs/xfs_da_format.h
> @@ -680,8 +680,15 @@ typedef struct xfs_attr_leaf_name_remote {
>  typedef struct xfs_attr_leafblock {
>  	xfs_attr_leaf_hdr_t	hdr;	/* constant-structure header block */
>  	xfs_attr_leaf_entry_t	entries[1];	/* sorted on key, not name */
> -	xfs_attr_leaf_name_local_t namelist;	/* grows from bottom of buf */
> -	xfs_attr_leaf_name_remote_t valuelist;	/* grows from bottom of buf */
> +	/*
> +	 * The rest of the block contains the following structures after the
> +	 * leaf entries, growing from the bottom up. The variables are never
> +	 * referenced and definining them can actually make gcc optimize away

Nitpick, s/definining/defining/, maybe can be fixed on commit :)

Don't we have other similar constructs elsewhere?  Do they suffer the same
fate?

... looks ...

I guess not; the other ones have the variable length array as the last member.

-Eric

> +	 * accesses to the 'entries' array above index 0 so don't do that.
> +	 *
> +	 * xfs_attr_leaf_name_local_t namelist;
> +	 * xfs_attr_leaf_name_remote_t valuelist;
> +	 */
>  } xfs_attr_leafblock_t;
>  
>  /*
> 

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