netdev - virtio_net: can change MTU after installing program

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Message-ID: <7df5bb7f-ea69-7673-642b-f174e45a1e64@digitalocean.com>
Date:   Tue, 25 Feb 2020 20:32:14 -0700
From:   David Ahern <dahern@...italocean.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: virtio_net: can change MTU after installing program

Another issue is that virtio_net checks the MTU when a program is
installed, but does not restrict an MTU change after:

# ip li sh dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 xdp qdisc fq_codel
state UP mode DEFAULT group default qlen 1000
    link/ether 5a:39:e6:01:a5:36 brd ff:ff:ff:ff:ff:ff
    prog/xdp id 13 tag c5595e4590d58063 jited

# ip li set dev eth0 mtu 8192

# ip li sh dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8192 xdp qdisc fq_codel
state UP mode DEFAULT group default qlen 1000

The simple solution is:

@@ -2489,6 +2495,8 @@ static int virtnet_xdp_set(struct net_device *dev,
struct bpf_prog *prog,
                }
        }

+       dev->max_mtu = prog ? max_sz : MAX_MTU;
+
        return 0;

 err:

The complicated solution is to implement ndo_change_mtu.

The simple solution causes a user visible change with 'ip -d li sh' by
showing a changing max mtu, but the ndo has a poor user experience in
that it just fails EINVAL (their is no extack) which is confusing since,
for example, 8192 is a totally legit MTU. Changing the max does return a
nice extack message.