netdev - Re: [PATCH v3 2/2] macvtap: restore vlan header on user read

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Date:	Thu, 3 May 2012 19:22:25 +0400
From:	Basil Gor <basil.gor@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v3 2/2] macvtap: restore vlan header on user read

On Thu, May 03, 2012 at 05:31:10PM +0300, Michael S. Tsirkin wrote:
> On Thu, May 03, 2012 at 06:37:46AM -0700, Eric W. Biederman wrote:
> > "Michael S. Tsirkin" <mst@...hat.com> writes:
> > 
> > > On Wed, Apr 25, 2012 at 10:31:25PM -0700, Eric W. Biederman wrote:
> > >> Basil Gor <basil.gor@...il.com> writes:
> > >> 
> > >> > Vlan tag is restored during buffer transmit to a network device (bridge
> > >> > port) in bridging code in case of tun/tap driver. In case of macvtap it
> > >> > has to be done explicitly. Otherwise vlan_tci is ignored and user always
> > >> > gets untagged packets.
> > >> 
> > >> We could quibble about efficiencies but this looks good except for
> > >> macvtap_recvmsg which isn't setting the auxdata for the vlan header.
> > >> 
> > >> Eric
> > >
> > > Right. I'm guessing we need to support old userspace
> > > so if there's auxdata, put vlan there but if not,
> > > put the vlan in the packet like this patch does.
> > 
> > This patch isn't horrible.
> > 
> > Still why copy the skb when you can just split the copy to userspace
> > into a couple of pieces?
> > 
> > We don't need to change the skb and changing the skb looks like
> > it is likely to confuse things and cause bugs because we are
> > not working with a consistent model of how vlan information
> > is encoded.
> > 
> > Still something needs to happen and this works in more cases even if it
> > isn't perfect.
> > 
> > Eric
> 
> Absolutely. And it's easier than I thought.
> So we can do something like the below (warning: compiled only).
> Basil - want to take a look?

Sure, I'll give it a try.
Thanks

Basil Gor

> My only concern if we put this logic in an out of way
> driver like macvtap will people remember to update it?
> Maybe better to update skb_copy_datagram_const_iovec which is in core?
> 
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index 0427c65..5a1724c 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -1,5 +1,6 @@
>  #include <linux/etherdevice.h>
>  #include <linux/if_macvlan.h>
> +#include <linux/if_vlan.h>
>  #include <linux/interrupt.h>
>  #include <linux/nsproxy.h>
>  #include <linux/compat.h>
> @@ -759,6 +760,7 @@ static ssize_t macvtap_put_user(struct macvtap_queue *q,
>  	struct macvlan_dev *vlan;
>  	int ret;
>  	int vnet_hdr_len = 0;
> +	int vlan_offset = 0;
>  
>  	if (q->flags & IFF_VNET_HDR) {
>  		struct virtio_net_hdr vnet_hdr;
> @@ -776,8 +778,29 @@ static ssize_t macvtap_put_user(struct macvtap_queue *q,
>  
>  	len = min_t(int, skb->len, len);
>  
> -	ret = skb_copy_datagram_const_iovec(skb, 0, iv, vnet_hdr_len, len);
> +	if (vlan_tx_tag_present(skb)) {
> +		struct {
> +			__be16 h_vlan_proto;
> +			__be16 h_vlan_TCI;
> +		} veth;
> + 		veth.h_vlan_proto = htons(ETH_P_8021Q);
> + 		veth.h_vlan_TCI = vlan_tx_tag_get(skb);
> +
> +		vlan_offset = offsetof(struct vlan_ethhdr, h_vlan_proto);
> +		ret = skb_copy_datagram_const_iovec(skb, 0, iv, vnet_hdr_len,
> +						    vlan_offset);
> +		if (ret)
> +			goto done;
> +		ret = memcpy_toiovecend(iv, (unsigned char *)&veth, vlan_offset,
> +					sizeof veth);
> +		if (ret)
> +			goto done;
> +		vlan_offset += sizeof veth;
> +	}
> +	ret = skb_copy_datagram_const_iovec(skb, vlan_offset, iv, vnet_hdr_len,
> +					    len);
>  
> +done:
>  	rcu_read_lock_bh();
>  	vlan = rcu_dereference_bh(q->vlan);
>  	if (vlan)
--
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