netfilter: nfnetlink: allow to check for generation ID

This patch allows userspace to specify the generation ID that has been
used to build an incremental batch update.

If userspace specifies the generation ID in the batch message as
attribute, then nfnetlink compares it to the current generation ID so
you make sure that you work against the right baseline. Otherwise, bail
out with ERESTART so userspace knows that its changeset is stale and
needs to respin. Userspace can do this transparently at the cost of
taking slightly more time to refresh caches and rework the changeset.

This check is optional, if there is no NFNL_BATCH_GENID attribute in the
batch begin message, then no check is performed.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nfnetlink.h

@@ -65,4 +65,16 @@ struct nfgenmsg {
 #define NFNL_MSG_BATCH_BEGIN		NLMSG_MIN_TYPE
 #define NFNL_MSG_BATCH_END		NLMSG_MIN_TYPE+1
 
+/**
+ * enum nfnl_batch_attributes - nfnetlink batch netlink attributes
+ *
+ * @NFNL_BATCH_GENID: generation ID for this changeset (NLA_U32)
+ */
+enum nfnl_batch_attributes {
+        NFNL_BATCH_UNSPEC,
+        NFNL_BATCH_GENID,
+        __NFNL_BATCH_MAX
+};
+#define NFNL_BATCH_MAX			(__NFNL_BATCH_MAX - 1)
+
 #endif /* _UAPI_NFNETLINK_H */