netfilter: deprecate NF_STOP

NF_STOP is only used by br_netfilter these days, and it can be emulated
with a combination of NF_STOLEN plus explicit call to the ->okfn()
function as Florian suggests.

To retain binary compatibility with userspace nf_queue application, we
have to keep NF_STOP around, so libnetfilter_queue userspace userspace
applications still work if they use NF_STOP for some exotic reason.

Out of tree modules using NF_STOP would break, but we don't care about
those.

Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/uapi/linux/netfilter.h

···
 #define NF_STOLEN 2
 #define NF_QUEUE 3
 #define NF_REPEAT 4
-#define NF_STOP 5
+#define NF_STOP 5	/* Deprecated, for userspace nf_queue compatibility. */
 #define NF_MAX_VERDICT NF_STOP
 
 /* we overload the higher bits for encoding auxiliary data such as the queue

net/bridge/br_netfilter_hooks.c

···
 				   struct sk_buff *skb,
 				   const struct nf_hook_state *state)
 {
-	if (skb->nf_bridge && !skb->nf_bridge->in_prerouting)
-		return NF_STOP;
+	if (skb->nf_bridge && !skb->nf_bridge->in_prerouting) {
+		state->okfn(state->net, state->sk, skb);
+		return NF_STOLEN;
+	}
 
 	return NF_ACCEPT;
 }

net/netfilter/core.c

···
 	entry = rcu_dereference(state->hook_entries);
 next_hook:
 	verdict = nf_iterate(skb, state, &entry);
-	if (verdict == NF_ACCEPT || verdict == NF_STOP) {
+	if (verdict == NF_ACCEPT) {
 		ret = 1;
 	} else if ((verdict & NF_VERDICT_MASK) == NF_DROP) {
 		kfree_skb(skb);