netfilter: nf_tables: nft_obj_filter fits into cb->ctx

No need to allocate it if one may just use struct netlink_callback's
scratch area for it.
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -7690,7 +7690,7 @@ struct nft_obj_dump_ctx {
 static int nf_tables_dump_obj(struct sk_buff *skb, struct netlink_callback *cb)
 {
 	const struct nfgenmsg *nfmsg = nlmsg_data(cb->nlh);
-	struct nft_obj_dump_ctx *ctx = cb->data;
+	struct nft_obj_dump_ctx *ctx = (void *)cb->ctx;
 	struct net *net = sock_net(skb->sk);
 	int family = nfmsg->nfgen_family;
 	struct nftables_pernet *nft_net;
@@ -7752,34 +7752,28 @@ static int nf_tables_dump_obj(struct sk_buff *skb, struct netlink_callback *cb)
 
 static int nf_tables_dump_obj_start(struct netlink_callback *cb)
 {
+	struct nft_obj_dump_ctx *ctx = (void *)cb->ctx;
 	const struct nlattr * const *nla = cb->data;
-	struct nft_obj_dump_ctx *ctx = NULL;
 
-	ctx = kzalloc(sizeof(*ctx), GFP_ATOMIC);
-	if (!ctx)
-		return -ENOMEM;
+	BUILD_BUG_ON(sizeof(*ctx) > sizeof(cb->ctx));
 
 	if (nla[NFTA_OBJ_TABLE]) {
 		ctx->table = nla_strdup(nla[NFTA_OBJ_TABLE], GFP_ATOMIC);
-		if (!ctx->table) {
-			kfree(ctx);
+		if (!ctx->table)
 			return -ENOMEM;
-		}
 	}
 
 	if (nla[NFTA_OBJ_TYPE])
 		ctx->type = ntohl(nla_get_be32(nla[NFTA_OBJ_TYPE]));
 
-	cb->data = ctx;
 	return 0;
 }
 
 static int nf_tables_dump_obj_done(struct netlink_callback *cb)
 {
-	struct nft_obj_dump_ctx *ctx = cb->data;
+	struct nft_obj_dump_ctx *ctx = (void *)cb->ctx;
 
 	kfree(ctx->table);
-	kfree(ctx);
 
 	return 0;
 }