net: Make qdisc_skb_cb upper size bound explicit.

Just like skb->cb[], so that qdisc_skb_cb can be encapsulated inside
of other data structures.

This is intended to be used by IPoIB so that it can remember
addressing information stored at hard_header_ops->create() time that
it can fetch when the packet gets to the transmit routine.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -220,9 +220,16 @@ struct tcf_proto {
 
 struct qdisc_skb_cb {
 	unsigned int		pkt_len;
-	long			data[];
+	unsigned char		data[24];
 };
 
+static inline void qdisc_cb_private_validate(const struct sk_buff *skb, int sz)
+{
+	struct qdisc_skb_cb *qcb;
+	BUILD_BUG_ON(sizeof(skb->cb) < sizeof(unsigned int) + sz);
+	BUILD_BUG_ON(sizeof(qcb->data) < sz);
+}
+
 static inline int qdisc_qlen(const struct Qdisc *q)
 {
 	return q->q.qlen;

net/sched/sch_choke.c

@@ -148,8 +148,7 @@ struct choke_skb_cb {
 
 static inline struct choke_skb_cb *choke_skb_cb(const struct sk_buff *skb)
 {
-	BUILD_BUG_ON(sizeof(skb->cb) <
-		sizeof(struct qdisc_skb_cb) + sizeof(struct choke_skb_cb));
+	qdisc_cb_private_validate(skb, sizeof(struct choke_skb_cb));
 	return (struct choke_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 

net/sched/sch_netem.c

@@ -130,8 +130,7 @@ struct netem_skb_cb {
 
 static inline struct netem_skb_cb *netem_skb_cb(struct sk_buff *skb)
 {
-	BUILD_BUG_ON(sizeof(skb->cb) <
-		sizeof(struct qdisc_skb_cb) + sizeof(struct netem_skb_cb));
+	qdisc_cb_private_validate(skb, sizeof(struct netem_skb_cb));
 	return (struct netem_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 

net/sched/sch_sfb.c

@@ -94,8 +94,7 @@ struct sfb_skb_cb {
 
 static inline struct sfb_skb_cb *sfb_skb_cb(const struct sk_buff *skb)
 {
-	BUILD_BUG_ON(sizeof(skb->cb) <
-		sizeof(struct qdisc_skb_cb) + sizeof(struct sfb_skb_cb));
+	qdisc_cb_private_validate(skb, sizeof(struct sfb_skb_cb));
 	return (struct sfb_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 

net/sched/sch_sfq.c

@@ -166,9 +166,8 @@ struct sfq_skb_cb {
 
 static inline struct sfq_skb_cb *sfq_skb_cb(const struct sk_buff *skb)
 {
-       BUILD_BUG_ON(sizeof(skb->cb) <
-               sizeof(struct qdisc_skb_cb) + sizeof(struct sfq_skb_cb));
-       return (struct sfq_skb_cb *)qdisc_skb_cb(skb)->data;
+	qdisc_cb_private_validate(skb, sizeof(struct sfq_skb_cb));
+	return (struct sfq_skb_cb *)qdisc_skb_cb(skb)->data;
 }
 
 static unsigned int sfq_hash(const struct sfq_sched_data *q,