dccp: Mechanism to resolve CCID dependencies

This adds a hook to resolve features whose value depends on the choice of
CCID. It is done at the server since it can only be done after the CCID
values have been negotiated; i.e. the client will add its CCID preference
list on the Change options sent in the Request, which will be reconciled
with the local preference list of the server.

The concept is documented on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/feature_negotiation/\
				implementation_notes.html#ccid_dependencies

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 37 insertions, 0 deletions

diff --git a/net/dccp/feat.c b/net/dccp/feat.c
index 4c4144147325..4f86a48723f6 100644
--- a/net/dccp/feat.c
+++ b/net/dccp/feat.c
@@ -166,6 +166,18 @@ static void dccp_feat_entry_destructor(struct dccp_feat_entry *entry)
  * - SP values are always freshly allocated
  * - list is sorted in increasing order of feature number (faster lookup)
  */
+static struct dccp_feat_entry *dccp_feat_list_lookup(struct list_head *fn_list,
+                                                     u8 feat_num, bool is_local)
+{
+        struct dccp_feat_entry *entry;
+
+        list_for_each_entry(entry, fn_list, node)
+                if (entry->feat_num == feat_num && entry->is_local == is_local)
+                        return entry;
+                else if (entry->feat_num > feat_num)
+                        break;
+        return NULL;
+}
 
 /**
  * dccp_feat_entry_new  -  Central list update routine (called by all others)
@@ -560,6 +572,31 @@ int dccp_feat_finalise_settings(struct dccp_sock *dp)
         return 0;
 }
 
+/**
+ * dccp_feat_server_ccid_dependencies  -  Resolve CCID-dependent features
+ * It is the server which resolves the dependencies once the CCID has been
+ * fully negotiated. If no CCID has been negotiated, it uses the default CCID.
+ */
+int dccp_feat_server_ccid_dependencies(struct dccp_request_sock *dreq)
+{
+        struct list_head *fn = &dreq->dreq_featneg;
+        struct dccp_feat_entry *entry;
+        u8 is_local, ccid;
+
+        for (is_local = 0; is_local <= 1; is_local++) {
+                entry = dccp_feat_list_lookup(fn, DCCPF_CCID, is_local);
+
+                if (entry != NULL && !entry->empty_confirm)
+                        ccid = entry->val.sp.vec[0];
+                else
+                        ccid = dccp_feat_default_value(DCCPF_CCID);
+
+                if (dccp_feat_propagate_ccid(fn, ccid, is_local))
+                        return -1;
+        }
+        return 0;
+}
+
 static int dccp_feat_update_ccid(struct sock *sk, u8 type, u8 new_ccid_nr)
 {
         struct dccp_sock *dp = dccp_sk(sk);