Subject: Re: [PATCH] macvlan/macvtap: Fix vlan tagging on user read

From: [email-masked] (Eric W. Biederman)
Date: Wed, 18 Apr 2012 11:54:52 -0700

Basil Gor <[email-masked]> 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.
>
> Scenario tested:
> kvm guests (that use vlans) migration from bridged network to macvtap
> revealed that packets delivered to guests are always untagged. Dumping
> and comparing sk_buff in case of tap and macvtap driver showed that
> macvtap does not restore vlan_tci.
>
> With current patch applied I was able to get working network, kvm guests
> get correctly tagged packets and can reach each other when macvtap in
> bridge mode (both with no vlans and through vlan interfaces).

My first impression is that this is the wrong place to add a vlan
header back.

You need to keep the vlan information in vlan_tci until just
before the packet is delivered to userspace. Which would suggest
the best place for these games is macvtap_put_user.

Elsewhere vlan headers should not be explicitly stored in the packet.

At least that was the rule last I looked.

Eric

> Signed-off-by: Basil Gor <[email-masked]>
> ---
>  drivers/net/macvtap.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index 0427c65..a6802b9 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>
> @@ -254,6 +255,14 @@ static int macvtap_forward(struct net_device *dev, struct sk_buff *skb)
>  	if (skb_queue_len(&q->sk.sk_receive_queue) >= dev->tx_queue_len)
>  		goto drop;
>  
> +	if (vlan_tx_tag_present(skb)) {
> +		skb = __vlan_put_tag(skb, vlan_tx_tag_get(skb));
> +		if (unlikely(!skb))
> +			return NET_RX_DROP;
> +
> +		skb->vlan_tci = 0;
> +	}
> +
>  	skb_queue_tail(&q->sk.sk_receive_queue, skb);
>  	wake_up_interruptible_poll(sk_sleep(&q->sk), POLLIN | POLLRDNORM | POLLRDBAND);
>  	return NET_RX_SUCCESS;