netdev - Re: [PATCH 1/3] vsockmon: Add tap functions.

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Message-ID: <e551618e-4a10-3973-73fe-47542758b6e1@deic.uab.cat>
Date:	Fri, 12 Aug 2016 16:15:03 +0200
From:	Gerard Garcia <ggarcia@...a.uab.cat>
To:	Stefan Hajnoczi <stefanha@...hat.com>
Cc:	netdev@...r.kernel.org, jhansen@...are.com, mst@...hat.com
Subject: Re: [PATCH 1/3] vsockmon: Add tap functions.

On 08/10/2016 01:40 PM, Stefan Hajnoczi wrote:
> On Mon, Aug 08, 2016 at 06:14:40PM +0200, ggarcia@...a.uab.cat wrote:
>> +static int __vsock_deliver_tap_skb(struct sk_buff *skb,
>> +				     struct net_device *dev)
>> +{
>> +	int ret = 0;
>> +
>> +	if (skb) {
>> +		dev_hold(dev);
>> +		/* Take skb ownership so it is not consumed in dev_queue_xmit.
>> +		 * dev_queue_xmit will drop a reference so the reference count
>> +		 * will reset.
>> +		 */
>> +		skb_get(skb);
>
> Netlink clones the skb instead of adding a reference.  I guess this is
> because the skb might be modified later on?  Perhaps there are race
> conditions if the original skb is shared.
>

Seems that it is responsibility of the functions processing the skb to 
make sure that it is not modified (without performing a copy) if it is a 
shared skb, as it is the case. vsockmon doesn't modify it and from what 
I understand it is not modified along the tx path but, of course, I 
could be missing something.

As it is not performance critical it will be safer to just clone the skb 
so I'll do that.