[v2] netfilter: Correct calculation using skb->tail and skb-network_header

This corrects an regression introduced by "net: Use 16bits for *_headers
fields of struct skbuff" when NET_SKBUFF_DATA_USES_OFFSET is not set. In
that case skb->tail will be a pointer whereas skb->network_header
will be an offset from head. This is corrected by using wrappers that
ensure that calculations are always made using pointers.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Simon Horman <horms@verge.net.au>

--
v2
* Use skb_tail_pointer() to ensure the tail portion of the calculation is
  always a pointer - it is not if NET_SKBUFF_DATA_USES_OFFSET is not set.
---
 net/netfilter/nf_nat_helper.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Fri, May 31, 2013 at 09:05:46AM +0900, Simon Horman wrote:
> This corrects an regression introduced by "net: Use 16bits for *_headers
> fields of struct skbuff" when NET_SKBUFF_DATA_USES_OFFSET is not set. In
> that case skb->tail will be a pointer whereas skb->network_header
> will be an offset from head. This is corrected by using wrappers that
> ensure that calculations are always made using pointers.
> 
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Reported-by: Chen Gang <gang.chen@asianux.com>
> Signed-off-by: Simon Horman <horms@verge.net.au>

I forgot to note in the subject that this patch is for net-next.
I will re-post accordingly.

> 
> --
> v2
> * Use skb_tail_pointer() to ensure the tail portion of the calculation is
>   always a pointer - it is not if NET_SKBUFF_DATA_USES_OFFSET is not set.
> ---
>  net/netfilter/nf_nat_helper.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/nf_nat_helper.c b/net/netfilter/nf_nat_helper.c
> index 5fea563..85e20a9 100644
> --- a/net/netfilter/nf_nat_helper.c
> +++ b/net/netfilter/nf_nat_helper.c
> @@ -104,7 +104,7 @@ static void mangle_contents(struct sk_buff *skb,
>  	/* move post-replacement */
>  	memmove(data + match_offset + rep_len,
>  		data + match_offset + match_len,
> -		skb->tail - (skb->network_header + dataoff +
> +		skb_tail_pointer(skb) - (skb_network_header(skb) + dataoff +
>  			     match_offset + match_len));
>  
>  	/* insert data from buffer */
> -- 
> 1.7.10.4
> 
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