Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets when XDP is enabled

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Mon, 23 Jul 2018 00:13:03 +0900, Toshiaki Makita wrote:
> From: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
> 
> All oversized packets including GSO packets are dropped if XDP is
> enabled on receiver side, so don't send such packets from peer.
> 
> Drop TSO and SCTP fragmentation features so that veth devices themselves
> segment packets with XDP enabled. Also cap MTU accordingly.
> 
> Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>

Is there any precedence for fixing up features and MTU like this?  Most
drivers just refuse to install the program if settings are incompatible.

> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 78fa08cb6e24..f5b72e937d9d 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -542,6 +542,23 @@ static int veth_get_iflink(const struct net_device *dev)
>  	return iflink;
>  }
>  
> +static netdev_features_t veth_fix_features(struct net_device *dev,
> +					   netdev_features_t features)
> +{
> +	struct veth_priv *priv = netdev_priv(dev);
> +	struct net_device *peer;
> +
> +	peer = rtnl_dereference(priv->peer);
> +	if (peer) {
> +		struct veth_priv *peer_priv = netdev_priv(peer);
> +
> +		if (peer_priv->_xdp_prog)
> +			features &= ~NETIF_F_GSO_SOFTWARE;
> +	}
> +
> +	return features;
> +}
> +
>  static void veth_set_rx_headroom(struct net_device *dev, int new_hr)
>  {
>  	struct veth_priv *peer_priv, *priv = netdev_priv(dev);
> @@ -591,14 +608,33 @@ static int veth_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>  				goto err;
>  			}
>  		}
> +
> +		if (!old_prog) {
> +			peer->hw_features &= ~NETIF_F_GSO_SOFTWARE;
> +			peer->max_mtu = PAGE_SIZE - VETH_XDP_HEADROOM -
> +				peer->hard_header_len -
> +				SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +			if (peer->mtu > peer->max_mtu)
> +				dev_set_mtu(peer, peer->max_mtu);
> +		}
>  	}
>  
>  	if (old_prog) {
> -		if (!prog && dev->flags & IFF_UP)
> -			veth_disable_xdp(dev);
> +		if (!prog) {
> +			if (dev->flags & IFF_UP)
> +				veth_disable_xdp(dev);
> +
> +			if (peer) {
> +				peer->hw_features |= NETIF_F_GSO_SOFTWARE;
> +				peer->max_mtu = ETH_MAX_MTU;
> +			}
> +		}
>  		bpf_prog_put(old_prog);
>  	}
>  
> +	if ((!!old_prog ^ !!prog) && peer)
> +		netdev_update_features(peer);
> +
>  	return 0;
>  err:
>  	priv->_xdp_prog = old_prog;
> @@ -643,6 +679,7 @@ static const struct net_device_ops veth_netdev_ops = {
>  	.ndo_poll_controller	= veth_poll_controller,
>  #endif
>  	.ndo_get_iflink		= veth_get_iflink,
> +	.ndo_fix_features	= veth_fix_features,
>  	.ndo_features_check	= passthru_features_check,
>  	.ndo_set_rx_headroom	= veth_set_rx_headroom,
>  	.ndo_bpf		= veth_xdp,