netfilter: conntrack: remove two args from resolve_clash

ctinfo is whats taken from the skb, i.e.
ct = nf_ct_get(skb, &ctinfo).

We do not pass 'ct' and instead re-fetch it from the skb.
Just do the same for both netns and ctinfo.

Also add a comment on what clash resolution is supposed to do.
While at it, one indent level can be removed.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_core.c

@@ -894,31 +894,64 @@ static void nf_ct_acct_merge(struct nf_conn *ct, enum ip_conntrack_info ctinfo,
 	}
 }
 
-/* Resolve race on insertion if this protocol allows this. */
+/**
+ * nf_ct_resolve_clash - attempt to handle clash without packet drop
+ *
+ * @skb: skb that causes the clash
+ * @h: tuplehash of the clashing entry already in table
+ *
+ * A conntrack entry can be inserted to the connection tracking table
+ * if there is no existing entry with an identical tuple.
+ *
+ * If there is one, @skb (and the assocated, unconfirmed conntrack) has
+ * to be dropped.  In case @skb is retransmitted, next conntrack lookup
+ * will find the already-existing entry.
+ *
+ * The major problem with such packet drop is the extra delay added by
+ * the packet loss -- it will take some time for a retransmit to occur
+ * (or the sender to time out when waiting for a reply).
+ *
+ * This function attempts to handle the situation without packet drop.
+ *
+ * If @skb has no NAT transformation or if the colliding entries are
+ * exactly the same, only the to-be-confirmed conntrack entry is discarded
+ * and @skb is associated with the conntrack entry already in the table.
+ *
+ * Returns NF_DROP if the clash could not be resolved.
+ */
 static __cold noinline int
-nf_ct_resolve_clash(struct net *net, struct sk_buff *skb,
-		    enum ip_conntrack_info ctinfo,
-		    struct nf_conntrack_tuple_hash *h)
+nf_ct_resolve_clash(struct sk_buff *skb, struct nf_conntrack_tuple_hash *h)
 {
 	/* This is the conntrack entry already in hashes that won race. */
 	struct nf_conn *ct = nf_ct_tuplehash_to_ctrack(h);
 	const struct nf_conntrack_l4proto *l4proto;
-	enum ip_conntrack_info oldinfo;
-	struct nf_conn *loser_ct = nf_ct_get(skb, &oldinfo);
+	enum ip_conntrack_info ctinfo;
+	struct nf_conn *loser_ct;
+	struct net *net;
+
+	loser_ct = nf_ct_get(skb, &ctinfo);
 
 	l4proto = nf_ct_l4proto_find(nf_ct_protonum(ct));
-	if (l4proto->allow_clash &&
-	    !nf_ct_is_dying(ct) &&
-	    atomic_inc_not_zero(&ct->ct_general.use)) {
+	if (!l4proto->allow_clash)
+		goto drop;
+
+	if (nf_ct_is_dying(ct))
+		goto drop;
+
+	if (!atomic_inc_not_zero(&ct->ct_general.use))
+		goto drop;
+
 	if (((ct->status & IPS_NAT_DONE_MASK) == 0) ||
 	    nf_ct_match(ct, loser_ct)) {
 		nf_ct_acct_merge(ct, ctinfo, loser_ct);
 		nf_conntrack_put(&loser_ct->ct_general);
-			nf_ct_set(skb, ct, oldinfo);
+		nf_ct_set(skb, ct, ctinfo);
 		return NF_ACCEPT;
 	}
+
 	nf_ct_put(ct);
-	}
+drop:
+	net = nf_ct_net(loser_ct);
 	NF_CT_STAT_INC(net, drop);
 	return NF_DROP;
 }
@@ -1036,7 +1069,7 @@ __nf_conntrack_confirm(struct sk_buff *skb)
 
 out:
 	nf_ct_add_to_dying_list(ct);
-	ret = nf_ct_resolve_clash(net, skb, ctinfo, h);
+	ret = nf_ct_resolve_clash(skb, h);
 dying:
 	nf_conntrack_double_unlock(hash, reply_hash);
 	NF_CT_STAT_INC(net, insert_failed);