batman-adv: Calculate extra tail size based on queued fragments

The fragmentation code was replaced in 610bfc6bc99bc83680d190ebc69359a05fc7f605
("batman-adv: Receive fragmented packets and merge"). The new code provided a
mostly unused parameter skb for the merging function. It is used inside the
function to calculate the additionally needed skb tailroom. But instead of
increasing its own tailroom, it is only increasing the tailroom of the first
queued skb. This is not correct in some situations because the first queued
entry can be a different one than the parameter.

An observed problem was:

1. packet with size 104, total_size 1464, fragno 1 was received
   - packet is queued
2. packet with size 1400, total_size 1464, fragno 0 was received
   - packet is queued at the end of the list
3. enough data was received and can be given to the merge function
   (1464 == (1400 - 20) + (104 - 20))
   - merge functions gets 1400 byte large packet as skb argument
4. merge function gets first entry in queue (104 byte)
   - stored as skb_out
5. merge function calculates the required extra tail as total_size - skb->len
   - pskb_expand_head tail of skb_out with 64 bytes
6. merge function tries to squeeze the extra 1380 bytes from the second queued
   skb (1400 byte aka skb parameter) in the 64 extra tail bytes of skb_out

Instead calculate the extra required tail bytes for skb_out also using skb_out
instead of using the parameter skb. The skb parameter is only used to get the
total_size from the last received packet. This is also the total_size used to
decide that all fragments were received.

Reported-by: Philipp Psurek <philipp.psurek@gmail.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
batadv_frag_merge_packets(struct hlist_head *chain, struct sk_buff *skb)
{
	struct batadv_frag_packet *packet;
	struct batadv_frag_list_entry *entry;
	struct sk_buff *skb_out = NULL;
	int size, hdr_size = sizeof(struct batadv_frag_packet);

	/* Make sure incoming skb has non-bogus data. */
	packet = (struct batadv_frag_packet *)skb->data;
	size = ntohs(packet->total_size);
	if (size > batadv_frag_size_limit())
		goto free;

	/* Remove first entry, as this is the destination for the rest of the
	 * fragments.
	 */
	entry = hlist_entry(chain->first, struct batadv_frag_list_entry, list);
	hlist_del(&entry->list);
	skb_out = entry->skb;
 	kfree(entry);
 
 	/* Make room for the rest of the fragments. */
	if (pskb_expand_head(skb_out, 0, size - skb_out->len, GFP_ATOMIC) < 0) {
 		kfree_skb(skb_out);
 		skb_out = NULL;
 		goto free;
	}

	/* Move the existing MAC header to just before the payload. (Override
	 * the fragment header.)
	 */
	skb_pull_rcsum(skb_out, hdr_size);
	memmove(skb_out->data - ETH_HLEN, skb_mac_header(skb_out), ETH_HLEN);
	skb_set_mac_header(skb_out, -ETH_HLEN);
	skb_reset_network_header(skb_out);
	skb_reset_transport_header(skb_out);

	/* Copy the payload of the each fragment into the last skb */
	hlist_for_each_entry(entry, chain, list) {
		size = entry->skb->len - hdr_size;
		memcpy(skb_put(skb_out, size), entry->skb->data + hdr_size,
		       size);
	}

free:
	/* Locking is not needed, because 'chain' is not part of any orig. */
	batadv_frag_clear_chain(chain);
	return skb_out;
}

batadv_frag_merge_packets(struct hlist_head *chain, struct sk_buff *skb)
{
	struct batadv_frag_packet *packet;
	struct batadv_frag_list_entry *entry;
	struct sk_buff *skb_out = NULL;
	int size, hdr_size = sizeof(struct batadv_frag_packet);

	/* Make sure incoming skb has non-bogus data. */
	packet = (struct batadv_frag_packet *)skb->data;
	size = ntohs(packet->total_size);
	if (size > batadv_frag_size_limit())
		goto free;

	/* Remove first entry, as this is the destination for the rest of the
	 * fragments.
	 */
	entry = hlist_entry(chain->first, struct batadv_frag_list_entry, list);
	hlist_del(&entry->list);
	skb_out = entry->skb;
 	kfree(entry);
 
 	/* Make room for the rest of the fragments. */
	if (pskb_expand_head(skb_out, 0, size - skb->len, GFP_ATOMIC) < 0) {
 		kfree_skb(skb_out);
 		skb_out = NULL;
 		goto free;
	}

	/* Move the existing MAC header to just before the payload. (Override
	 * the fragment header.)
	 */
	skb_pull_rcsum(skb_out, hdr_size);
	memmove(skb_out->data - ETH_HLEN, skb_mac_header(skb_out), ETH_HLEN);
	skb_set_mac_header(skb_out, -ETH_HLEN);
	skb_reset_network_header(skb_out);
	skb_reset_transport_header(skb_out);

	/* Copy the payload of the each fragment into the last skb */
	hlist_for_each_entry(entry, chain, list) {
		size = entry->skb->len - hdr_size;
		memcpy(skb_put(skb_out, size), entry->skb->data + hdr_size,
		       size);
	}

free:
	/* Locking is not needed, because 'chain' is not part of any orig. */
	batadv_frag_clear_chain(chain);
	return skb_out;
}
