Hi Steve, Here the email which hopefully provides enough context. Regards, Arend -------- Forwarded Message -------- Subject: Question on drivers using skb_unclone Date: Fri, 14 Apr 2017 10:03:01 +0100 From: James Hughes <james.hughes@xxxxxxxxxxxxxxx> To: netdev@xxxxxxxxxxxxxxx CC: UNGLinuxDriver@xxxxxxxxxxxxx, brcm80211-dev-list.pdl@xxxxxxxxxxxx Netdevs, We have recently got to the bottom of an issue which we have been encountering on a Raspberry Pi being used as an access point, and we need a bit of advice on the correct way of fixing the issue. The set up is a Raspberry Pi 3 running hostapd on its inbuilt wireless adaptor (Brcm43438 ), bridged to the built in ethernet adaptor (smsc9514). Using the standard drivers for these devices as of 4.9 (looking at 4.11, no changes noted that would affect the issue). We were encountering an error in the Brcm Wireless driver that after investigation was a skb_buff being corrupted by an action in the smsc Ethernet driver. Further digging shows that the bridge was cloning an incoming skb from the Ethernet, then sending it out to both the Ethernet and wlan ports. This mean that both the Ethernet driver and the wireless driver were looking at the same physical data, and one or both were altering that data in different way (varied headers) , which mean that headers were corrupted. This was only happening on broadcast packets. We can fix the issue by, in the drivers, by using skb_unclone in appropriate places in the driver. So now to the questions. Is adding the unclone to the drivers the correct way of dealing with this issue? Examining other drivers shows that unclone is not particularly common, presumably in many cases, where the driver does not alter the skb, it doesn't matter. However, in any case where a driver may add header information to the skb (as is the case with the Brcm wireless driver), when the incoming skb has been cloned, the results must surely be undetermined unless an unclone is performed. Or, is the bridging code making a mistake by cloning the skb and passing it to multiple recipients. Thanks James Hughes Raspberry Pi
