[4/6] batman-adv: mcast: shrink tracker packet after scrubbing

diff --git a/net/batman-adv/multicast_forw.c b/net/batman-adv/multicast_forw.c
index a2c5c587b8fb..fafd6ba8c056 100644
--- a/net/batman-adv/multicast_forw.c
+++ b/net/batman-adv/multicast_forw.c
@@ -697,6 +697,212 @@  batadv_mcast_forw_scrub_dests(struct batadv_priv *bat_priv,
 	}
 }
 
+/**
+ * batadv_mcast_forw_shrink_fill() - swap slot with next non-zero destination
+ * @slot: the to be filled zero-MAC destination entry in a tracker TVLV
+ * @num_dests_slot: remaining entries in tracker TVLV from/including slot
+ *
+ * Searches for the next non-zero-MAC destination entry in a tracker TVLV after
+ * the given slot pointer. And if found, swaps it with the zero-MAC destination
+ * entry which the slot points to.
+ *
+ * Return: true if slot was swapped/filled successfully, false otherwise.
+ */
+static bool batadv_mcast_forw_shrink_fill(u8 *slot, u16 num_dests_slot)
+{
+	u16 num_dests_filler;
+	u8 *filler;
+
+	/* sanity check, should not happen */
+	if (!num_dests_slot)
+		return false;
+
+	num_dests_filler = num_dests_slot - 1;
+	filler = slot + ETH_ALEN;
+
+	/* find a candidate to fill the empty slot */
+	batadv_mcast_forw_tracker_for_each_dest(filler, num_dests_filler) {
+		if (is_zero_ether_addr(filler))
+			continue;
+
+		ether_addr_copy(slot, filler);
+		eth_zero_addr(filler);
+		return true;
+	}
+
+	return false;
+}
+
+/**
+ * batadv_mcast_forw_shrink_pack_dests() - pack destinations of a tracker TVLV
+ * @skb: the batman-adv multicast packet to compact destinations in
+ *
+ * Compacts the originator destination MAC addresses in the multicast tracker
+ * TVLV of the given multicast packet. This is done by moving all non-zero
+ * MAC addresses in direction of the skb head and all zero MAC addresses in skb
+ * tail direction, within the multicast tracker TVLV.
+ *
+ * Return: The number of consecutive zero MAC address destinations which are
+ * now at the end of the multicast tracker TVLV.
+ */
+static int batadv_mcast_forw_shrink_pack_dests(struct sk_buff *skb)
+{
+	struct batadv_tvlv_mcast_tracker *mcast_tracker;
+	unsigned char *skb_net_hdr;
+	u16 num_dests_slot;
+	u8 *slot;
+
+	skb_net_hdr = skb_network_header(skb);
+	mcast_tracker = (struct batadv_tvlv_mcast_tracker *)skb_net_hdr;
+	num_dests_slot = ntohs(mcast_tracker->num_dests);
+
+	slot = (u8 *)mcast_tracker + sizeof(*mcast_tracker);
+
+	batadv_mcast_forw_tracker_for_each_dest(slot, num_dests_slot) {
+		/* find an empty slot */
+		if (!is_zero_ether_addr(slot))
+			continue;
+
+		if (!batadv_mcast_forw_shrink_fill(slot, num_dests_slot))
+			/* could not find a filler, so we successfully packed
+			 * and can stop - and must not reduce num_dests_slot!
+			 */
+			break;
+	}
+
+	/* num_dests_slot is now the amount of reduced, zeroed
+	 * destinations at the end of the tracker TVLV
+	 */
+	return num_dests_slot;
+}
+
+/**
+ * batadv_mcast_forw_shrink_align_offset() - get new alignment offset
+ * @num_dests_old: the old, to be updated amount of destination nodes
+ * @num_dests_reduce: the number of destinations that were removed
+ *
+ * Calculates the amount of potential extra alignment offset that is needed to
+ * adjust the TVLV padding after the change in destination nodes.
+ *
+ * Return:
+ *	0: If no change to padding is needed.
+ *	2: If padding needs to be removed.
+ *	-2: If padding needs to be added.
+ */
+static short
+batadv_mcast_forw_shrink_align_offset(unsigned int num_dests_old,
+				      unsigned int num_dests_reduce)
+{
+	/* even amount of removed destinations -> no alignment change */
+	if (!(num_dests_reduce % 2))
+		return 0;
+
+	/* even to odd amount of destinations -> remove padding */
+	if (!(num_dests_old % 2))
+		return 2;
+
+	/* odd to even amount of destinations -> add padding */
+	return -2;
+}
+
+/**
+ * batadv_mcast_forw_shrink_update_headers() - update shrunk mc packet headers
+ * @skb: the batman-adv multicast packet to update headers of
+ * @num_dests_reduce: the number of destinations that were removed
+ *
+ * This updates any fields of a batman-adv multicast packet that are affected
+ * by the reduced number of destinations in the multicast tracket TVLV. In
+ * particular this updates:
+ *
+ * The num_dest field of the multicast tracker TVLV.
+ * The TVLV length field of the according generic TVLV header.
+ * The batman-adv multicast packet's total TVLV length field.
+ *
+ * Return: The offset in skb's tail direction at which the new batman-adv
+ * multicast packet header needs to start.
+ */
+static unsigned int
+batadv_mcast_forw_shrink_update_headers(struct sk_buff *skb,
+					unsigned int num_dests_reduce)
+{
+	struct batadv_tvlv_mcast_tracker *mcast_tracker;
+	struct batadv_mcast_packet *mcast_packet;
+	struct batadv_tvlv_hdr *tvlv_hdr;
+	unsigned char *skb_net_hdr;
+	unsigned int offset;
+	short align_offset;
+	u16 num_dests;
+
+	skb_net_hdr = skb_network_header(skb);
+	mcast_tracker = (struct batadv_tvlv_mcast_tracker *)skb_net_hdr;
+	num_dests = ntohs(mcast_tracker->num_dests);
+
+	align_offset = batadv_mcast_forw_shrink_align_offset(num_dests,
+							     num_dests_reduce);
+	offset = ETH_ALEN * num_dests_reduce + align_offset;
+	num_dests -= num_dests_reduce;
+
+	/* update tracker header */
+	mcast_tracker->num_dests = htons(num_dests);
+
+	/* update tracker's tvlv header's length field */
+	tvlv_hdr = (struct batadv_tvlv_hdr *)(skb_network_header(skb) -
+					      sizeof(*tvlv_hdr));
+	tvlv_hdr->len = htons(ntohs(tvlv_hdr->len) - offset);
+
+	/* update multicast packet header's tvlv length field */
+	mcast_packet = (struct batadv_mcast_packet *)skb->data;
+	mcast_packet->tvlv_len = htons(ntohs(mcast_packet->tvlv_len) - offset);
+
+	return offset;
+}
+
+/**
+ * batadv_mcast_forw_shrink_move_headers() - move multicast headers by offset
+ * @skb: the batman-adv multicast packet to move headers for
+ * @offset: a non-negative offset to move headers by, towards the skb tail
+ *
+ * Moves the batman-adv multicast packet header, its multicast tracker TVLV and
+ * any TVLVs in between by the given offset in direction towards the tail.
+ */
+static void
+batadv_mcast_forw_shrink_move_headers(struct sk_buff *skb, unsigned int offset)
+{
+	struct batadv_tvlv_mcast_tracker *mcast_tracker;
+	unsigned char *skb_net_hdr;
+	unsigned int len;
+	u16 num_dests;
+
+	skb_net_hdr = skb_network_header(skb);
+	mcast_tracker = (struct batadv_tvlv_mcast_tracker *)skb_net_hdr;
+	num_dests = ntohs(mcast_tracker->num_dests);
+	len = skb_network_offset(skb) + sizeof(*mcast_tracker);
+	len += num_dests * ETH_ALEN;
+
+	batadv_mcast_forw_scrape(skb, len, offset);
+}
+
+/**
+ * batadv_mcast_forw_shrink_tracker() - remove zero addresses in a tracker tvlv
+ * @skb: the batman-adv multicast packet to (potentially) shrink
+ *
+ * Removes all destinations with a zero MAC addresses (00:00:00:00:00:00) from
+ * the given batman-adv multicast packet's tracker TVLV and updates headers
+ * accordingly to maintain a valid batman-adv multicast packet.
+ */
+static void batadv_mcast_forw_shrink_tracker(struct sk_buff *skb)
+{
+	unsigned int offset;
+	u16 dests_reduced;
+
+	dests_reduced = batadv_mcast_forw_shrink_pack_dests(skb);
+	if (!dests_reduced)
+		return;
+
+	offset = batadv_mcast_forw_shrink_update_headers(skb, dests_reduced);
+	batadv_mcast_forw_shrink_move_headers(skb, offset);
+}
+
 /**
  * batadv_mcast_forw_packet() - forward a batman-adv multicast packet
  * @bat_priv: the bat priv with all the soft interface information
@@ -786,6 +992,7 @@  static int batadv_mcast_forw_packet(struct batadv_priv *bat_priv,
 
 		batadv_mcast_forw_scrub_dests(bat_priv, neigh_node, dest,
 					      next_dest, num_dests);
+		batadv_mcast_forw_shrink_tracker(nexthop_skb);
 
 		batadv_inc_counter(bat_priv, BATADV_CNT_MCAST_TX);
 		batadv_add_counter(bat_priv, BATADV_CNT_MCAST_TX_BYTES,