Subject: Re: [PATCH v3 2/2] macvtap: Allow tap features change when IFF_VNET_HDR is disabled.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 15 Aug 2013 21:48:44 +0300

On Thu, Aug 15, 2013 at 02:39:39PM -0400, Vlad Yasevich wrote:
> On 08/15/2013 01:33 PM, Michael S. Tsirkin wrote:
> >On Thu, Aug 15, 2013 at 01:02:53PM -0400, Vlad Yasevich wrote:
> >>When the user turns off IFF_VNET_HDR flag, attempts to change
> >>offload features via TUNSETOFFLOAD do not work.  This could cause
> >>GSO packets to be delivered to the user when the user is
> >>not prepared to handle them.
> >>
> >>To solve, allow processing of TUNSETOFFLOAD when IFF_VNET_HDR is
> >>disabled.  Also, take the setting of IFF_VNET_HDR into consideration
> >>when determining the feature set to apply to incommig traffic.
> >>If IFF_VNET_HDR is set, we use user-specfied feature set.  if the
> >>IFF_VNET_HDR is clear, we mask off all tun-specific offload features,
> >>since user can not be notified of the possbile offloads.
> >>
> >>Signed-off-by: Vlad Yasevich <[email-masked]>
> >>---
> >>  drivers/net/macvtap.c | 30 ++++++++++++++++++++++++------
> >>  1 file changed, 24 insertions(+), 6 deletions(-)
> >>
> >>diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> >>index 8121358..df45a59 100644
> >>--- a/drivers/net/macvtap.c
> >>+++ b/drivers/net/macvtap.c
> >>@@ -269,6 +269,28 @@ static void macvtap_del_queues(struct net_device *dev)
> >>  		sock_put(&qlist[j]->sk);
> >>  }
> >>
> >>+/* Determine features to be applied to the packet as it
> >
> >/*
> >  * Please use comments
> >  * like this
> >  */
> >
> >>+ * gets sent to the tap.  The features depend on the offloads
> >>+ * set by user and by whether or not IFF_VNET_HDR is enabled.
> >>+ */
> >>+static netdev_features_t macvtap_skb_features(struct sk_buff *skb,
> >>+					      struct macvtap_queue *q)
> >>+{
> >>+	struct macvlan_dev *vlan = netdev_priv(skb->dev);
> >>+	netdev_features_t features = netif_skb_features(skb);
> >>+
> >>+	/* If VNET_HDR is enabled, user can receive offload info so
> >>+	 * use user-specified tap_features.
> >>+	 * Otherwise, mask off all tap offload features.
> >>+	 */
> >>+	if (q->flags & IFF_VNET_HDR)
> >>+		features &= vlan->tap_features;
> >>+	else
> >>+		features &= ~TUN_OFFLOADS;
> >>+
> >>+	return features;
> >>+}
> >>+
> >
> >Okay, but this means we need to do netdev_update_features
> >on SETIFF since it changes q->flags.
> 
> Why?
>  The whole point it not to change features on SETIFF, so that
> they are preserved for when the flags change again.  It just that
> flag values either allow whatever offload user specified (correctly
> masked via netdev_update_features) or turn off all offloads at the tap.
> 
> -vlad

That's the point. As it is, if flag value changes, we don't call
netdev_update_features so offloads aren't masked/unmasked.

Example:

1. SETOFFLOADS to enable
2. VNET = 0

offloads are still enabled

> >>  /*
> >>   * Forward happens for data that gets sent from one macvlan
> >>   * endpoint to another one in bridge mode. We just take
> >>@@ -276,9 +298,9 @@ static void macvtap_del_queues(struct net_device *dev)
> >>   */
> >>  static int macvtap_forward(struct net_device *dev, struct sk_buff *skb)
> >>  {
> >>-	struct macvlan_dev *vlan = netdev_priv(dev);
> >>  	struct macvtap_queue *q = macvtap_get_queue(dev, skb);
> >>  	netdev_features_t features;
> >>+
> >>  	if (!q)
> >>  		goto drop;
> >>
> >>@@ -289,7 +311,7 @@ static int macvtap_forward(struct net_device *dev, struct sk_buff *skb)
> >>  	/* Apply the forward feature mask so that we perform segmentation
> >>  	 * according to users wishes.
> >>  	 */
> >>-	features = netif_skb_features(skb) & vlan->tap_features;
> >>+	features = macvtap_skb_features(skb, q);
> >>  	if (netif_needs_gso(skb, features)) {
> >>  		struct sk_buff *segs = __skb_gso_segment(skb, features, false);
> >>
> >>@@ -1161,10 +1183,6 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
> >>  			    TUN_F_TSO_ECN | TUN_F_UFO))
> >>  			return -EINVAL;
> >>
> >>-		/* TODO: only accept frames with the features that
> >>-			 got enabled for forwarded frames */
> >>-		if (!(q->flags & IFF_VNET_HDR))
> >>-			return  -EINVAL;
> >>  		rtnl_lock();
> >>  		ret = set_offload(q, arg);
> >>  		rtnl_unlock();
> >>--
> >>1.8.1.4