ebt_vnat -- Slapping on a VLAN tag on the bridge

Hi,
This question is related to "Headroom for bridging question". I wrote a new target to add a VLAN tag to specified packets. This is how you set the rule:
# ebtables -t nat -A POSTROUTING -s 00:11:22:33:44:55 -j vnat --to-vlan 6
 
Most of the code was simply copied from ebt_s/dnat both in user and kernel code.
 
This is the problem: 
br_netfilter: Argh!! br_nf_post_routing: bad mac.raw pointer.[eth0][br0]
 
Looks like br_netfilter did not like the fact I did skb_push():
from br_nf_postrouting():
     /* Be very paranoid. Must be a device driver bug. */
     if (skb->mac.raw < skb->head || skb->mac.raw + ETH_HLEN > skb->data) {
         printk(KERN_CRIT "br_netfilter: Argh!! br_nf_post_routing: "
                  "bad mac.raw pointer.");

Inside ebt_target_vnat() I am doing the following operations on pskb and returning the info->target if things go well :
 
if (skb_headroom(*pskb) < VLAN_HLEN) {
         struct sk_buff *sk_tmp = *pskb;
         printk("%s: In if skb_headroom\n", __FUNCTION__);
         *pskb = skb_realloc_headroom(sk_tmp, VLAN_HLEN);
         kfree_skb(sk_tmp);
         if (*pskb == NULL) {
             return EBT_DROP;
         }
     } else {
         /* This causes a panic
         printk("%s: In else skb_headroom\n", __FUNCTION__);
         if (!(*pskb = skb_unshare(*pskb, GFP_ATOMIC))) {
             printk(KERN_ERR "vlan: failed to unshare skbuff\n");
             return EBT_DROP;
         }
         */
     }
     // This actually pushes skb->data, returns new skb->data
     printk("%s: before: mac.raw: %x skb->data: %x\n",
                     __FUNCTION__, (*pskb)->mac.raw, (*pskb)->data);
     veth = (struct vlan_ethhdr *)skb_push(*pskb, VLAN_HLEN);
     printk("%s: after: mac.raw: %x skb->data: %x\n",
                     __FUNCTION__, (*pskb)->mac.raw, (*pskb)->data);
 
     /* Move the mac addresses to the beginning of the new header. */
     memmove((*pskb)->data, (*pskb)->data + VLAN_HLEN, 12);
 
     /* first, the ethernet type */
     /* put_unaligned(__constant_htons(ETH_P_8021Q), &veth->h_vlan_proto); */
     veth->h_vlan_proto = __constant_htons(ETH_P_8021Q);
 
     /* Now, construct the second two bytes. This field looks something
      * like:
      * usr_priority: 3 bits  (high bits)
      * CFI       1 bit
      * VLAN ID   12 bits (low bits)
      */
     veth_TCI = info->vid;
     veth_TCI |= 0;
     veth->h_vlan_TCI = htons(veth_TCI);
 
     return info->target;
 
How do I fix the mismatch between skb->mac.raw and skb->data? I was thinking of putting this in the OUTPUT chain instead of postrouting...
 
Thanks,
Ashwin
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~            \|/ ____ \|/ 

Ashwin Kashyap                                   @~/ ,. \~@ 

Intern                                          /_( \__/ )_\ 

Thomson - Corporate Research                       \__U_/ 

(609) 987-7334                                  Don't Panic!