netfilter: ctnetlink: allocate right-sized ctnetlink skb

Try to allocate a Netlink skb roughly the size of the actual
message, with the help from the l3 and l4 protocol helpers.
This is all to prevent a reallocation in netlink_trim() later.

The overhead of allocating the right-sized skb is rather small, with
ctnetlink_alloc_skb() actually being inlined away on my x86_64 box.
The size of the per-proto space is determined at registration time of
the protocol helper.
Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_netlink.c

@@ -405,6 +405,69 @@ ctnetlink_fill_info(struct sk_buff *skb, u32 pid, u32 seq,
 }
 
 #ifdef CONFIG_NF_CONNTRACK_EVENTS
+/*
+ * The general structure of a ctnetlink event is
+ *
+ *  CTA_TUPLE_ORIG
+ *    <l3/l4-proto-attributes>
+ *  CTA_TUPLE_REPLY
+ *    <l3/l4-proto-attributes>
+ *  CTA_ID
+ *  ...
+ *  CTA_PROTOINFO
+ *    <l4-proto-attributes>
+ *  CTA_TUPLE_MASTER
+ *    <l3/l4-proto-attributes>
+ *
+ * Therefore the formular is
+ *
+ *   size = sizeof(headers) + sizeof(generic_nlas) + 3 * sizeof(tuple_nlas)
+ *		+ sizeof(protoinfo_nlas)
+ */
+static struct sk_buff *
+ctnetlink_alloc_skb(const struct nf_conntrack_tuple *tuple, gfp_t gfp)
+{
+	struct nf_conntrack_l3proto *l3proto;
+	struct nf_conntrack_l4proto *l4proto;
+	int len;
+
+#define NLA_TYPE_SIZE(type)		nla_total_size(sizeof(type))
+
+	/* proto independant part */
+	len = NLMSG_SPACE(sizeof(struct nfgenmsg))
+		+ 3 * nla_total_size(0)		/* CTA_TUPLE_ORIG|REPL|MASTER */
+		+ 3 * nla_total_size(0)		/* CTA_TUPLE_IP */
+		+ 3 * nla_total_size(0)		/* CTA_TUPLE_PROTO */
+		+ 3 * NLA_TYPE_SIZE(u_int8_t)	/* CTA_PROTO_NUM */
+		+ NLA_TYPE_SIZE(u_int32_t)	/* CTA_ID */
+		+ NLA_TYPE_SIZE(u_int32_t)	/* CTA_STATUS */
+		+ 2 * nla_total_size(0)		/* CTA_COUNTERS_ORIG|REPL */
+		+ 2 * NLA_TYPE_SIZE(uint64_t)	/* CTA_COUNTERS_PACKETS */
+		+ 2 * NLA_TYPE_SIZE(uint64_t)	/* CTA_COUNTERS_BYTES */
+		+ NLA_TYPE_SIZE(u_int32_t)	/* CTA_TIMEOUT */
+		+ nla_total_size(0)		/* CTA_PROTOINFO */
+		+ nla_total_size(0)		/* CTA_HELP */
+		+ nla_total_size(NF_CT_HELPER_NAME_LEN)	/* CTA_HELP_NAME */
+		+ NLA_TYPE_SIZE(u_int32_t)	/* CTA_SECMARK */
+		+ 2 * nla_total_size(0)		/* CTA_NAT_SEQ_ADJ_ORIG|REPL */
+		+ 2 * NLA_TYPE_SIZE(u_int32_t)	/* CTA_NAT_SEQ_CORRECTION_POS */
+		+ 2 * NLA_TYPE_SIZE(u_int32_t)	/* CTA_NAT_SEQ_CORRECTION_BEFORE */
+		+ 2 * NLA_TYPE_SIZE(u_int32_t)	/* CTA_NAT_SEQ_CORRECTION_AFTER */
+		+ NLA_TYPE_SIZE(u_int32_t);	/* CTA_MARK */
+
+#undef NLA_TYPE_SIZE
+
+	rcu_read_lock();
+	l3proto = __nf_ct_l3proto_find(tuple->src.l3num);
+	len += l3proto->nla_size;
+
+	l4proto = __nf_ct_l4proto_find(tuple->src.l3num, tuple->dst.protonum);
+	len += l4proto->nla_size;
+	rcu_read_unlock();
+
+	return alloc_skb(len, gfp);
+}
+
 static int ctnetlink_conntrack_event(struct notifier_block *this,
 				     unsigned long events, void *ptr)
 {
@@ -438,7 +501,7 @@ static int ctnetlink_conntrack_event(struct notifier_block *this,
 	if (!item->report && !nfnetlink_has_listeners(group))
 		return NOTIFY_DONE;
 
-	skb = alloc_skb(NLMSG_GOODSIZE, GFP_ATOMIC);
+	skb = ctnetlink_alloc_skb(tuple(ct, IP_CT_DIR_ORIGINAL), GFP_ATOMIC);
 	if (!skb)
 		return NOTIFY_DONE;