[NETLINK]: New message building macros

 NLMSG_PUT_ANSWER(skb, nlcb, type, length)
   Start a new netlink message as answer to a request,
   returns the message header.

 NLMSG_END(skb, nlh)
   End a netlink message, fixes total message length,
   returns skb->len.

 NLMSG_CANCEL(skb, nlh)
   Cancel the building process and trim whole message
   from skb again, returns -1.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -171,9 +171,22 @@ __nlmsg_put(struct sk_buff *skb, u32 pid, u32 seq, int type, int len)
 }
 
 #define NLMSG_PUT(skb, pid, seq, type, len) \
-({ if (skb_tailroom(skb) < (int)NLMSG_SPACE(len)) goto nlmsg_failure; \
+({	if (skb_tailroom(skb) < (int)NLMSG_SPACE(len)) \
+		goto nlmsg_failure; \
 	__nlmsg_put(skb, pid, seq, type, len); })
 
+#define NLMSG_PUT_ANSWER(skb, cb, type, len) \
+	NLMSG_PUT(skb, NETLINK_CB((cb)->skb).pid, \
+		  (cb)->nlh->nlmsg_seq, type, len)
+
+#define NLMSG_END(skb, nlh) \
+({	(nlh)->nlmsg_len = (skb)->tail - (unsigned char *) (nlh); \
+	(skb)->len; })
+
+#define NLMSG_CANCEL(skb, nlh) \
+({	skb_trim(skb, (unsigned char *) (nlh) - (skb)->data); \
+	-1; })
+
 extern int netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 			      struct nlmsghdr *nlh,
 			      int (*dump)(struct sk_buff *skb, struct netlink_callback*),