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

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Date:	Thu, 03 May 2012 06:37:46 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Basil Gor <basil.gor@...il.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v3 2/2] macvtap: restore vlan header on user read

"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

>> > Signed-off-by: Basil Gor <basil.gor@...il.com>
>> > ---
>> >  drivers/net/macvtap.c |   11 ++++++++++-
>> >  1 files changed, 10 insertions(+), 1 deletions(-)
>> >
>> > diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> > index 0427c65..28d2678 100644
>> > --- a/drivers/net/macvtap.c
>> > +++ b/drivers/net/macvtap.c
>> > @@ -1,6 +1,7 @@
>> >  #include <linux/etherdevice.h>
>> >  #include <linux/if_macvlan.h>
>> >  #include <linux/interrupt.h>
>> > +#include <linux/if_vlan.h>
>> >  #include <linux/nsproxy.h>
>> >  #include <linux/compat.h>
>> >  #include <linux/if_tun.h>
>> > @@ -753,13 +754,21 @@ static ssize_t macvtap_aio_write(struct kiocb *iocb, const struct iovec *iv,
>> >  
>> >  /* Put packet to the user space buffer */
>> >  static ssize_t macvtap_put_user(struct macvtap_queue *q,
>> > -				const struct sk_buff *skb,
>> > +				struct sk_buff *skb,
>> >  				const struct iovec *iv, int len)
>> >  {
>> >  	struct macvlan_dev *vlan;
>> >  	int ret;
>> >  	int vnet_hdr_len = 0;
>> >  
>> > +	if (vlan_tx_tag_present(skb)) {
>> > +		skb = __vlan_put_tag(skb, vlan_tx_tag_get(skb));
>> > +		if (unlikely(!skb))
>> > +			return -ENOMEM;
>> > +
>> > +		skb->vlan_tci = 0;
>> > +	}
>> > +
>> >  	if (q->flags & IFF_VNET_HDR) {
>> >  		struct virtio_net_hdr vnet_hdr;
>> >  		vnet_hdr_len = q->vnet_hdr_sz;
>> --
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