virtio_net: MAC address releated breakage if there is no MAC area in config

[Christian Borntraeger <borntraeger@de.ibm.com>]

Hello,

commit 9c46f6d42f1b5627c49a5906cb5b315ad8716ff0
Author: Alex Williamson <alex.williamson@hp.com>
Date:   Wed Feb 4 16:36:34 2009 -0800
virtio_net: Allow setting the MAC address of the NIC

Introduced an unconditional config->set to the MAC address field of
the device config.


+       } else {
                random_ether_addr(dev->dev_addr);
+               vdev->config->set(vdev, offsetof(struct virtio_net_config, mac),
+                                 dev->dev_addr, dev->addr_len);
+       }

Since our kuli userspace sample does not set VIRTIO_NET_F_MAC, there is no
config space assigned for this device. When virtio_net tries to overwrite
the non-existing field, this triggers a bug.

virtio_net.h specifies:
struct virtio_net_config
{
        /* The config defining mac address (if VIRTIO_NET_F_MAC) */
        __u8 mac[6];
[...]

I read this as the mac config field is optional (similar to all the optional
fields we added in virtio_blk later).

I see two options:
1. Change our sample userspace to always allocate the config (like lguest and
qemu)
2. Change the kernel code to not write into the config unless a specific feature
bit is set. (e.g. VIRTIO_NET_F_SETMAC)


Opinions?

PS: The same is true for virtnet_set_mac_address. it crashes as well