Re: [PATCH net-next V2] gro: introduce gro_mac_header_len

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From: Roland Dreier <roland@xxxxxxxxxx>
Date: Mon, 6 Feb 2012 12:01:13 -0800

> On Mon, Feb 6, 2012 at 11:56 AM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>> But this needs a little bit of work since the qdisc_skb_cb ends with a
>> variable length array, but we can put an upper bound on this just like
>> we do for skb->cb[] itself to fix this issue.
>>
>> I'll toss something together.
> 
> OK thanks.  We need something like 20 bytes of space to stash an
> ipoib LL addr.  Which I guess leaves 28 bytes for sch use.

I just pushed the following into net-next, you have 24 bytes
available.

--------------------
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@xxxxxxxxxxxxx>
---
 include/net/sch_generic.h |    9 ++++++++-
 net/sched/sch_choke.c     |    3 +--
 net/sched/sch_netem.c     |    3 +--
 net/sched/sch_sfb.c       |    3 +--
 net/sched/sch_sfq.c       |    5 ++---
 5 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index f6bb08b..55ce96b 100644
--- a/include/net/sch_generic.h
+++ b/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;
diff --git a/net/sched/sch_choke.c b/net/sched/sch_choke.c
index e465064..7e267d7 100644
--- a/net/sched/sch_choke.c
+++ b/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;
 }
 
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 2776012..e83d61c 100644
--- a/net/sched/sch_netem.c
+++ b/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;
 }
 
diff --git a/net/sched/sch_sfb.c b/net/sched/sch_sfb.c
index 96e42ca..d7eea99 100644
--- a/net/sched/sch_sfb.c
+++ b/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;
 }
 
diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 67494ae..60d4718 100644
--- a/net/sched/sch_sfq.c
+++ b/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,
-- 
1.7.7.6

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