[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:
> > On Thu, 18 Sep 2014 08:02:05 -0700
> > Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > 
> > > 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>
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On Thu, 2014-09-18 at 11:00 -0700, Eric Dumazet wrote:
> On Thu, 2014-09-18 at 09:32 -0700, Eric Dumazet wrote:
> > On Thu, 2014-09-18 at 09:26 -0700, Stephen Hemminger wrote:
[]
> > 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.
[]
> diff --git a/drivers/infiniband/ulp/ipoib/ipoib.h b/drivers/infiniband/ulp/ipoib/ipoib.h
[]
> +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;
> +}

It seems better not to use const for the struct sk_buff * here.

Neither of the uses take a const struct sk_buff *

> diff --git 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);



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On Thu, 2014-09-18 at 11:07 -0700, Joe Perches wrote:

> > +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;
> > +}
> 
> It seems better not to use const for the struct sk_buff * here.
> 
> Neither of the uses take a const struct sk_buff *

Thats pretty standard, check for other similar constructs like that.


static inline struct qdisc_skb_cb *qdisc_skb_cb(const struct sk_buff *skb)
{
        return (struct qdisc_skb_cb *)skb->cb;
}

This allows uses of the helper when the skb is only read (has the const qual)



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On Thu, 2014-09-18 at 12:14 -0700, Eric Dumazet wrote:
> On Thu, 2014-09-18 at 11:07 -0700, Joe Perches wrote:
> 
> > > +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;
> > > +}
> > 
> > It seems better not to use const for the struct sk_buff * here.
> > 
> > Neither of the uses take a const struct sk_buff *
> 
> Thats pretty standard, check for other similar constructs like that.
> 
> 
> static inline struct qdisc_skb_cb *qdisc_skb_cb(const struct sk_buff *skb)
> {
>         return (struct qdisc_skb_cb *)skb->cb;
> }
> 
> This allows uses of the helper when the skb is only read (has the const qual)

I don't mind the const argument, but casting
the return to non-const seems like poor form.

btw: it seems like it's 8:5 non-const to const

$ grep -rP --include=*.[ch] -n "cb\s*\*\s*\w+\s*\(\s*(?:const\s+)?struct\s+sk_buff\s*\*\s*\w+\s*\)" *
drivers/net/ethernet/freescale/gianfar.c:2091:static inline struct txfcb *gfar_add_fcb(struct sk_buff *skb)
drivers/net/wireless/ath/ath10k/core.h:83:static inline struct ath10k_skb_cb *ATH10K_SKB_CB(struct sk_buff *skb)
include/net/mrp.h:38:static inline struct mrp_skb_cb *mrp_cb(struct sk_buff *skb)
include/net/ieee802154_netdev.h:235:static inline struct ieee802154_mac_cb *mac_cb(struct sk_buff *skb)
include/net/ieee802154_netdev.h:240:static inline struct ieee802154_mac_cb *mac_cb_init(struct sk_buff *skb)
include/net/sch_generic.h:251:static inline struct qdisc_skb_cb *qdisc_skb_cb(const struct sk_buff *skb)
include/net/codel.h:95:static struct codel_skb_cb *get_codel_cb(const struct sk_buff *skb)
include/net/garp.h:36:static inline struct garp_skb_cb *garp_cb(struct sk_buff *skb)
net/core/dev.c:2177:static struct dev_kfree_skb_cb *get_kfree_skb_cb(const struct sk_buff *skb)
net/sched/sch_choke.c:142:static inline struct choke_skb_cb *choke_skb_cb(const struct sk_buff *skb)
net/sched/sch_sfb.c:95:static inline struct sfb_skb_cb *sfb_skb_cb(const struct sk_buff *skb)
net/sched/sch_netem.c:171:static inline struct netem_skb_cb *netem_skb_cb(struct sk_buff *skb)
net/sched/sch_sfq.c:167:static inline struct sfq_skb_cb *sfq_skb_cb(const struct sk_buff *skb)


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On Thu, 2014-09-18 at 12:31 -0700, Joe Perches wrote:

> I don't mind the const argument, but casting
> the return to non-const seems like poor form.

This is the current way to do this, even if we do not like it.

C does not allow to have the same function name for different
parameters, there is nothing we can do about it at this moment.

->

static inline const struct ipoib_cb *ipoib_skb_cb(const struct sk_buff *skb)

static inline struct ipoib_cb *ipoib_skb_cb(struct sk_buff *skb)



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