netfilter: nf_tables: reintroduce the NFT_SET_CONCAT flag

Stefano originally proposed to introduce this flag, users hit EOPNOTSUPP
in new binaries with old kernels when defining a set with ranges in
a concatenation.

Fixes: f3a2181e ("netfilter: nf_tables: Support for sets with multiple ranged fields")
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -276,6 +276,7 @@ enum nft_rule_compat_attributes {
  * @NFT_SET_TIMEOUT: set uses timeouts
  * @NFT_SET_EVAL: set can be updated from the evaluation path
  * @NFT_SET_OBJECT: set contains stateful objects
+ * @NFT_SET_CONCAT: set contains a concatenation
  */
 enum nft_set_flags {
 	NFT_SET_ANONYMOUS		= 0x1,
@@ -285,6 +286,7 @@ enum nft_set_flags {
 	NFT_SET_TIMEOUT			= 0x10,
 	NFT_SET_EVAL			= 0x20,
 	NFT_SET_OBJECT			= 0x40,
+	NFT_SET_CONCAT			= 0x80,
 };
 
 /**

net/netfilter/nf_tables_api.c

@@ -3962,7 +3962,7 @@ static int nf_tables_newset(struct net *net, struct sock *nlsk,
 		if (flags & ~(NFT_SET_ANONYMOUS | NFT_SET_CONSTANT |
 			      NFT_SET_INTERVAL | NFT_SET_TIMEOUT |
 			      NFT_SET_MAP | NFT_SET_EVAL |
-			      NFT_SET_OBJECT))
+			      NFT_SET_OBJECT | NFT_SET_CONCAT))
 			return -EOPNOTSUPP;
 		/* Only one of these operations is supported */
 		if ((flags & (NFT_SET_MAP | NFT_SET_OBJECT)) ==