genl: Add genlmsg_new_unicast() for unicast message allocation

Allocates a new sk_buff large enough to cover the specified payload
plus required Netlink headers. Will check receiving socket for
memory mapped i/o capability and use it if enabled. Will fall back
to non-mapped skb if message size exceeds the frame size of the ring.

Signed-of-by: Thomas Graf <[e-mail]>
Reviewed-by: Daniel Borkmann <[e-mail]>
Signed-off-by: Jesse Gross <[e-mail]>

include/net/genetlink.h

···
  * @attrs: netlink attributes
  * @_net: network namespace
  * @user_ptr: user pointers
+ * @dst_sk: destination socket
  */
 struct genl_info {
 	u32			snd_seq;
···
 	struct net *		_net;
 #endif
 	void *			user_ptr[2];
+	struct sock *		dst_sk;
 };
 
 static inline struct net *genl_info_net(struct genl_info *info)
···
 		 struct sk_buff *skb, struct net *net, u32 portid,
 		 u32 group, struct nlmsghdr *nlh, gfp_t flags);
 
+struct sk_buff *genlmsg_new_unicast(size_t payload, struct genl_info *info,
+				    gfp_t flags);
 void *genlmsg_put(struct sk_buff *skb, u32 portid, u32 seq,
 		  struct genl_family *family, int flags, u8 cmd);
 

net/netlink/genetlink.c

···
 EXPORT_SYMBOL(genl_unregister_family);
 
 /**
+ * genlmsg_new_unicast - Allocate generic netlink message for unicast
+ * @payload: size of the message payload
+ * @info: information on destination
+ * @flags: the type of memory to allocate
+ *
+ * Allocates a new sk_buff large enough to cover the specified payload
+ * plus required Netlink headers. Will check receiving socket for
+ * memory mapped i/o capability and use it if enabled. Will fall back
+ * to non-mapped skb if message size exceeds the frame size of the ring.
+ */
+struct sk_buff *genlmsg_new_unicast(size_t payload, struct genl_info *info,
+				    gfp_t flags)
+{
+	size_t len = nlmsg_total_size(genlmsg_total_size(payload));
+
+	return netlink_alloc_skb(info->dst_sk, len, info->snd_portid, flags);
+}
+EXPORT_SYMBOL_GPL(genlmsg_new_unicast);
+
+/**
  * genlmsg_put - Add generic netlink header to netlink message
  * @skb: socket buffer holding the message
  * @portid: netlink portid the message is addressed to
···
 	info.genlhdr = nlmsg_data(nlh);
 	info.userhdr = nlmsg_data(nlh) + GENL_HDRLEN;
 	info.attrs = attrbuf;
+	info.dst_sk = skb->sk;
 	genl_info_net_set(&info, net);
 	memset(&info.user_ptr, 0, sizeof(info.user_ptr));