[DCCP]: Fix skb->cb conflicts with IP

dev_queue_xmit() and the other IP output functions expect to get a skb
with clear or properly initialized skb->cb. Unlike TCP and UDP, the
dccp_skb_cb doesn't contain a struct inet_skb_parm at the beginning,
so the DCCP-specific data is interpreted by the IP output functions.
This can cause false negatives for the conditional POST_ROUTING hook
invocation, making the packet bypass the hook.

Add a inet_skb_parm/inet6_skb_parm union to the beginning of
dccp_skb_cb to avoid clashes. Also add a BUILD_BUG_ON to make
sure it fits in the cb.

[ Combined with patch from Gerrit Renker to remove two now unnecessary
  memsets of IPCB(skb)->opt ]

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 0 deletions

diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index e3f5d37b84be..c91d3c1fd30d 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -1057,6 +1057,9 @@ static int __init dccp_init(void)
         int ehash_order, bhash_order, i;
         int rc = -ENOBUFS;
 
+        BUILD_BUG_ON(sizeof(struct dccp_skb_cb) >
+                     FIELD_SIZEOF(struct sk_buff, cb));
+
         dccp_hashinfo.bind_bucket_cachep =
                 kmem_cache_create("dccp_bind_bucket",
                                   sizeof(struct inet_bind_bucket), 0,