virtio_net: validate ethtool port setting and explain the user validation

BugLink: http://bugs.launchpad.net/bugs/1581132

We should validate the port setting that we got from the user and check
if it's what we've set it to (PORT_OTHER), also add explanation that
ignoring advertising is good as long as we don't have autonegotiation.
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 0cf3ace9)
Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/net/virtio_net.c

@@ -1388,11 +1388,13 @@ static bool virtnet_validate_ethtool_cmd(const struct ethtool_cmd *cmd)
 	struct ethtool_cmd diff1 = *cmd;
 	struct ethtool_cmd diff2 = {};
 
-	/* advertising and cmd are usually set, ignore port because we set it */
+	/* cmd is always set so we need to clear it, validate the port type
+	 * and also without autonegotiation we can ignore advertising
+	 */
 	ethtool_cmd_speed_set(&diff1, 0);
+	diff2.port = PORT_OTHER;
 	diff1.advertising = 0;
 	diff1.duplex = 0;
-	diff1.port = 0;
 	diff1.cmd = 0;
 
 	return !memcmp(&diff1, &diff2, sizeof(diff1));