Re: [PATCH net] net: sched: shrink struct qdisc_skb_cb to 28 bytes

On Thu, 2014-09-18 at 09:32 -0700, Eric Dumazet wrote:

On Thu, 2014-09-18 at 09:26 -0700, Stephen Hemminger wrote:

quoted

On Thu, 18 Sep 2014 08:02:05 -0700
Eric Dumazet [off-list ref] wrote:

quoted

From: Eric Dumazet <edumazet@google.com>

We cannot make struct qdisc_skb_cb bigger without impacting IPoIB,
or increasing skb->cb[] size.

Commit e0f31d849867 ("flow_keys: Record IP layer protocol in
skb_flow_dissect()") broke IPoIB.

Only current offender is sch_choke, and this one do not need an
absolutely precise flow key.

If we store 17 bytes of flow key, its more than enough. (Its the actual
size of flow_keys if it was a packed structure, but we might add new
fields at the end of it later)

Signed-off-by: Eric Dumazet <edumazet@google.com>

Can we add BUILD_BUG to stop next time something smacks this.

I though we had.

Maybe IPoIB lacks one.

Or, do you have an idea ?

Seems straightforward ...

Or can you carry this fix for me ?

Thanks

[PATCH] ipoib: validate struct ipoib_cb size

To catch future errors sooner.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---

diff --git a/drivers/infiniband/ulp/ipoib/ipoib.h b/drivers/infiniband/ulp/ipoib/ipoib.h
index 3edce617c31b..d7562beb5423 100644
--- a/drivers/infiniband/ulp/ipoib/ipoib.h
+++ b/drivers/infiniband/ulp/ipoib/ipoib.h

@@ -131,6 +131,12 @@ struct ipoib_cb {
 	u8			hwaddr[INFINIBAND_ALEN];
 };
 
+static inline struct ipoib_cb *ipoib_skb_cb(const struct sk_buff *skb)
+{
+	BUILD_BUG_ON(sizeof(skb->cb) < sizeof(struct ipoib_cb));
+	return (struct ipoib_cb *)skb->cb;
+}
+
 /* Used for all multicast joins (broadcast, IPv4 mcast and IPv6 mcast) */
 struct ipoib_mcast {
 	struct ib_sa_mcmember_rec mcmember;

diff --git a/drivers/infiniband/ulp/ipoib/ipoib_main.c b/drivers/infiniband/ulp/ipoib/ipoib_main.c
index 1310acf6bf92..13e6e0431592 100644
--- a/drivers/infiniband/ulp/ipoib/ipoib_main.c
+++ b/drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -716,7 +716,7 @@ static int ipoib_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct ipoib_dev_priv *priv = netdev_priv(dev);
 	struct ipoib_neigh *neigh;
-	struct ipoib_cb *cb = (struct ipoib_cb *) skb->cb;
+	struct ipoib_cb *cb = ipoib_skb_cb(skb);
 	struct ipoib_header *header;
 	unsigned long flags;
 

@@ -813,7 +813,7 @@ static int ipoib_hard_header(struct sk_buff *skb,
 			     const void *daddr, const void *saddr, unsigned len)
 {
 	struct ipoib_header *header;
-	struct ipoib_cb *cb = (struct ipoib_cb *) skb->cb;
+	struct ipoib_cb *cb = ipoib_skb_cb(skb);
 
 	header = (struct ipoib_header *) skb_push(skb, sizeof *header);