Subject: Re: [PATCH net-next v2 6/9] net: use core MTU range checking in virt drivers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 21 Oct 2016 06:36:25 +0300

On Thu, Oct 20, 2016 at 10:37:20PM -0400, Jarod Wilson wrote:
> On Thu, Oct 20, 2016 at 11:23:54PM +0300, Michael S. Tsirkin wrote:
> > On Thu, Oct 20, 2016 at 01:55:21PM -0400, Jarod Wilson wrote:
> ...
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index fad84f3..720809f 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -1419,17 +1419,6 @@ static const struct ethtool_ops virtnet_ethtool_ops = {
> > >  	.set_settings = virtnet_set_settings,
> > >  };
> > >  
> > > -#define MIN_MTU 68
> > > -#define MAX_MTU 65535
> > > -
> > > -static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
> > > -{
> > > -	if (new_mtu < MIN_MTU || new_mtu > MAX_MTU)
> > > -		return -EINVAL;
> > > -	dev->mtu = new_mtu;
> > > -	return 0;
> > > -}
> > > -
> > >  static const struct net_device_ops virtnet_netdev = {
> > >  	.ndo_open            = virtnet_open,
> > >  	.ndo_stop   	     = virtnet_close,
> > > @@ -1437,7 +1426,6 @@ static const struct net_device_ops virtnet_netdev = {
> > >  	.ndo_validate_addr   = eth_validate_addr,
> > >  	.ndo_set_mac_address = virtnet_set_mac_address,
> > >  	.ndo_set_rx_mode     = virtnet_set_rx_mode,
> > > -	.ndo_change_mtu	     = virtnet_change_mtu,
> > >  	.ndo_get_stats64     = virtnet_stats,
> > >  	.ndo_vlan_rx_add_vid = virtnet_vlan_rx_add_vid,
> > >  	.ndo_vlan_rx_kill_vid = virtnet_vlan_rx_kill_vid,
> > > @@ -1748,6 +1736,9 @@ static bool virtnet_validate_features(struct virtio_device *vdev)
> > >  	return true;
> > >  }
> > >  
> > > +#define MIN_MTU ETH_MIN_MTU
> > > +#define MAX_MTU ETH_MAX_MTU
> > > +
> > 
> > Can we drop these btw?
> 
> Bah. Yeah. Should have just used them directly. I didn't add ETH_MAX_MTU
> until after doing the virtio_net changes, so I missed that.
> 
> > >  static int virtnet_probe(struct virtio_device *vdev)
> > >  {
> > >  	int i, err;
> > > @@ -1821,6 +1812,10 @@ static int virtnet_probe(struct virtio_device *vdev)
> > >  
> > >  	dev->vlan_features = dev->features;
> > >  
> > > +	/* MTU range: 68 - 65535 */
> > > +	dev->min_mtu = MIN_MTU;
> > > +	dev->max_mtu = MAX_MTU;
> > > +
> > >  	/* Configuration may specify what MAC to use.  Otherwise random. */
> > >  	if (virtio_has_feature(vdev, VIRTIO_NET_F_MAC))
> > >  		virtio_cread_bytes(vdev,
> > > @@ -1875,8 +1870,10 @@ static int virtnet_probe(struct virtio_device *vdev)
> > >  		mtu = virtio_cread16(vdev,
> > >  				     offsetof(struct virtio_net_config,
> > >  					      mtu));
> > > -		if (virtnet_change_mtu(dev, mtu))
> > > +		if (mtu < dev->min_mtu || mtu > dev->max_mtu)
> > 
> > In fact the > max_mtu branch does not make sense since a 16 bit
> > value can't exceed MAX_MTU.
> 
> Hm. mtu is declared as an int, not sure if there's any sort of type
> promotion to be worried about (not an area I know much/anything about).

Not by design, that's for sure.

> Certainly something that could be looked into as a minor optimization,
> though it's only in a probe path and shouldn't hurt anything, so ... meh?

Right. Aaron said he's working on a patch that essentially does
dev->max_mtu = mtu after validation, so this part will look
a bit silly there.

> -- 
> Jarod Wilson
> [email-masked]