genetlink: add genlmsg_iput() API

Add some APIs and helpers required for convenient construction
of replies and notifications based on struct genl_info.
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-8-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/genetlink.h

@@ -113,7 +113,7 @@ struct genl_info {
 	struct netlink_ext_ack *extack;
 };
 
-static inline struct net *genl_info_net(struct genl_info *info)
+static inline struct net *genl_info_net(const struct genl_info *info)
 {
 	return read_pnet(&info->_net);
 }
@@ -270,6 +270,32 @@ genl_info_dump(struct netlink_callback *cb)
 	return &genl_dumpit_info(cb)->info;
 }
 
+/**
+ * genl_info_init_ntf() - initialize genl_info for notifications
+ * @info:   genl_info struct to set up
+ * @family: pointer to the genetlink family
+ * @cmd:    command to be used in the notification
+ *
+ * Initialize a locally declared struct genl_info to pass to various APIs.
+ * Intended to be used when creating notifications.
+ */
+static inline void
+genl_info_init_ntf(struct genl_info *info, const struct genl_family *family,
+		   u8 cmd)
+{
+	struct genlmsghdr *hdr = (void *) &info->user_ptr[0];
+
+	memset(info, 0, sizeof(*info));
+	info->family = family;
+	info->genlhdr = hdr;
+	hdr->cmd = cmd;
+}
+
+static inline bool genl_info_is_ntf(const struct genl_info *info)
+{
+	return !info->nlhdr;
+}
+
 int genl_register_family(struct genl_family *family);
 int genl_unregister_family(const struct genl_family *family);
 void genl_notify(const struct genl_family *family, struct sk_buff *skb,
@@ -278,6 +304,32 @@ void genl_notify(const struct genl_family *family, struct sk_buff *skb,
 void *genlmsg_put(struct sk_buff *skb, u32 portid, u32 seq,
 		  const struct genl_family *family, int flags, u8 cmd);
 
+static inline void *
+__genlmsg_iput(struct sk_buff *skb, const struct genl_info *info, int flags)
+{
+	return genlmsg_put(skb, info->snd_portid, info->snd_seq, info->family,
+			   flags, info->genlhdr->cmd);
+}
+
+/**
+ * genlmsg_iput - start genetlink message based on genl_info
+ * @skb: skb in which message header will be placed
+ * @info: genl_info as provided to do/dump handlers
+ *
+ * Convenience wrapper which starts a genetlink message based on
+ * information in user request. @info should be either the struct passed
+ * by genetlink core to do/dump handlers (when constructing replies to
+ * such requests) or a struct initialized by genl_info_init_ntf()
+ * when constructing notifications.
+ *
+ * Returns pointer to new genetlink header.
+ */
+static inline void *
+genlmsg_iput(struct sk_buff *skb, const struct genl_info *info)
+{
+	return __genlmsg_iput(skb, info, 0);
+}
+
 /**
  * genlmsg_nlhdr - Obtain netlink header from user specified header
  * @user_hdr: user header as returned from genlmsg_put()