netfilter: nft_compat: use .release_ops and remove list of extension

Add .release_ops, that is called in case of error at a later stage in
the expression initialization path, ie. .select_ops() has been already
set up operations and that needs to be undone. This allows us to unwind
.select_ops from the error path, ie. release the dynamic operations for
this extension.

Moreover, allocate one single operation instead of recycling them, this
comes at the cost of consuming a bit more memory per rule, but it
simplifies the infrastructure.

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

include/net/netfilter/nf_tables.h

@@ -690,10 +690,12 @@ static inline void nft_set_gc_batch_add(struct nft_set_gc_batch *gcb,
 	gcb->elems[gcb->head.cnt++] = elem;
 }
 
+struct nft_expr_ops;
 /**
  *	struct nft_expr_type - nf_tables expression type
  *
  *	@select_ops: function to select nft_expr_ops
+ *	@release_ops: release nft_expr_ops
  *	@ops: default ops, used when no select_ops functions is present
  *	@list: used internally
  *	@name: Identifier
@@ -706,6 +708,7 @@ static inline void nft_set_gc_batch_add(struct nft_set_gc_batch *gcb,
 struct nft_expr_type {
 	const struct nft_expr_ops	*(*select_ops)(const struct nft_ctx *,
 						       const struct nlattr * const tb[]);
+	void				(*release_ops)(const struct nft_expr_ops *ops);
 	const struct nft_expr_ops	*ops;
 	struct list_head		list;
 	const char			*name;

net/netfilter/nf_tables_api.c

@@ -2172,6 +2172,7 @@ struct nft_expr *nft_expr_init(const struct nft_ctx *ctx,
 {
 	struct nft_expr_info info;
 	struct nft_expr *expr;
+	struct module *owner;
 	int err;
 
 	err = nf_tables_expr_parse(ctx, nla, &info);
@@ -2191,7 +2192,11 @@ struct nft_expr *nft_expr_init(const struct nft_ctx *ctx,
 err3:
 	kfree(expr);
 err2:
-	module_put(info.ops->type->owner);
+	owner = info.ops->type->owner;
+	if (info.ops->type->release_ops)
+		info.ops->type->release_ops(info.ops);
+
+	module_put(owner);
 err1:
 	return ERR_PTR(err);
 }