netfilter: nf_tables: support for recursive chain deletion

This patch sorts out an asymmetry in deletions. Currently, table and set
deletion commands come with an implicit content flush on deletion.
However, chain deletion results in -EBUSY if there is content in this
chain, so no implicit flush happens. So you have to send a flush command
in first place to delete chains, this is inconsistent and it can be
annoying in terms of user experience.

This patch uses the new NLM_F_NONREC flag to request non-recursive chain
deletion, ie. if the chain to be removed contains rules, then this
returns EBUSY. This problem was discussed during the NFWS'17 in Faro,
Portugal. In iptables, you hit -EBUSY if you try to delete a chain that
contains rules, so you have to flush first before you can remove
anything. Since iptables-compat uses the nf_tables netlink interface, it
has to use the NLM_F_NONREC flag from userspace to retain the original
iptables semantics, ie.  bail out on removing chains that contain rules.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1617,8 +1617,11 @@ static int nf_tables_delchain(struct net *net, struct sock *nlsk,
 	struct nft_af_info *afi;
 	struct nft_table *table;
 	struct nft_chain *chain;
+	struct nft_rule *rule;
 	int family = nfmsg->nfgen_family;
 	struct nft_ctx ctx;
+	u32 use;
+	int err;
 
 	afi = nf_tables_afinfo_lookup(net, family, false);
 	if (IS_ERR(afi))
@@ -1631,11 +1634,30 @@ static int nf_tables_delchain(struct net *net, struct sock *nlsk,
 	chain = nf_tables_chain_lookup(table, nla[NFTA_CHAIN_NAME], genmask);
 	if (IS_ERR(chain))
 		return PTR_ERR(chain);
-	if (chain->use > 0)
+
+	if (nlh->nlmsg_flags & NLM_F_NONREC &&
+	    chain->use > 0)
 		return -EBUSY;
 
 	nft_ctx_init(&ctx, net, skb, nlh, afi, table, chain, nla);
 
+	use = chain->use;
+	list_for_each_entry(rule, &chain->rules, list) {
+		if (!nft_is_active_next(net, rule))
+			continue;
+		use--;
+
+		err = nft_delrule(&ctx, rule);
+		if (err < 0)
+			return err;
+	}
+
+	/* There are rules and elements that are still holding references to us,
+	 * we cannot do a recursive removal in this case.
+	 */
+	if (use > 0)
+		return -EBUSY;
+
 	return nft_delchain(&ctx);
 }