netfilter: nf_tables: add destroy_clone expression

Before this patch, cloned expressions are released via ->destroy. This
is a problem for the new connlimit expression since the ->destroy path
drop a reference on the conntrack modules and it unregisters hooks. The
new ->destroy_clone provides context that this expression is being
released from the packet path, so it is mirroring ->clone(), where
neither module reference is dropped nor hooks need to be unregistered -
because this done from the control plane path from the ->init() path.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -745,6 +745,8 @@ struct nft_expr_ops {
 						      const struct nft_expr *expr);
 	void				(*destroy)(const struct nft_ctx *ctx,
 						   const struct nft_expr *expr);
+	void				(*destroy_clone)(const struct nft_ctx *ctx,
+							 const struct nft_expr *expr);
 	int				(*dump)(struct sk_buff *skb,
 						const struct nft_expr *expr);
 	int				(*validate)(const struct nft_ctx *ctx,

net/netfilter/nf_tables_api.c

@@ -4046,8 +4046,16 @@ void nft_set_elem_destroy(const struct nft_set *set, void *elem,
 	nft_data_release(nft_set_ext_key(ext), NFT_DATA_VALUE);
 	if (nft_set_ext_exists(ext, NFT_SET_EXT_DATA))
 		nft_data_release(nft_set_ext_data(ext), set->dtype);
-	if (destroy_expr && nft_set_ext_exists(ext, NFT_SET_EXT_EXPR))
-		nf_tables_expr_destroy(&ctx, nft_set_ext_expr(ext));
+	if (destroy_expr && nft_set_ext_exists(ext, NFT_SET_EXT_EXPR)) {
+		struct nft_expr *expr = nft_set_ext_expr(ext);
+
+		if (expr->ops->destroy_clone) {
+			expr->ops->destroy_clone(&ctx, expr);
+			module_put(expr->ops->type->owner);
+		} else {
+			nf_tables_expr_destroy(&ctx, expr);
+		}
+	}
 	if (nft_set_ext_exists(ext, NFT_SET_EXT_OBJREF))
 		(*nft_set_ext_obj(ext))->use--;
 	kfree(elem);

net/netfilter/nft_counter.c

@@ -258,6 +258,7 @@ static const struct nft_expr_ops nft_counter_ops = {
 	.eval		= nft_counter_eval,
 	.init		= nft_counter_init,
 	.destroy	= nft_counter_destroy,
+	.destroy_clone	= nft_counter_destroy,
 	.dump		= nft_counter_dump,
 	.clone		= nft_counter_clone,
 };