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Date:	Sun, 02 Dec 2012 00:40:47 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, chas@....nrl.navy.mil,
	krzysiek@...lesie.net
Subject: Re: [PATCH 00/17] ATM fixes for pppoatm/br2684

On Sat, 2012-12-01 at 17:33 +0000, David Woodhouse wrote:
> 
> Very glad I added the BUILD_BUG_ON on the cb struct size now. Perhaps
> there should be a generic helper for that? Something like
>  skb_cb_cast(struct foo_cb, skb) could do it automatically...?

Something like this, perhaps? Using skb_cast_cb() would then make it
fairly much impossible to accidentally overflow the size of the skb cb.

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index f2af494..b4cb2cf 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -567,6 +567,11 @@ static inline struct rtable *skb_rtable(const struct sk_buff *skb)
 	return (struct rtable *)skb_dst(skb);
 }
 
+#define skb_cast_cb(skb, type)  ({				\
+	BUILD_BUG_ON(sizeof(type) > sizeof((skb)->cb));		\
+	(type *)(skb)->cb;					\
+	})
+
 extern void kfree_skb(struct sk_buff *skb);
 extern void skb_tx_error(struct sk_buff *skb);
 extern void consume_skb(struct sk_buff *skb);
diff --git a/drivers/atm/solos-pci.c b/drivers/atm/solos-pci.c
index 6619a8a..8dccf82 100644
--- a/drivers/atm/solos-pci.c
+++ b/drivers/atm/solos-pci.c
@@ -97,7 +97,7 @@ struct solos_skb_cb {
 };
 
 
-#define SKB_CB(skb)		((struct solos_skb_cb *)skb->cb)
+#define SKB_CB(skb)	skb_cast_cb(skb, struct solos_skb_cb)
 
 #define PKT_DATA	0
 #define PKT_COMMAND	1
@@ -1324,8 +1324,6 @@ static struct pci_driver fpga_driver = {
 
 static int __init solos_pci_init(void)
 {
-	BUILD_BUG_ON(sizeof(struct solos_skb_cb) > sizeof(((struct sk_buff *)0)->cb));
-
 	printk(KERN_INFO "Solos PCI Driver Version %s\n", VERSION);
 	return pci_register_driver(&fpga_driver);
 }


-- 
dwmw2


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