mctp: serial: enforce fixed MTU

The current serial driver requires a maximum MTU of 68, and it doesn't
make sense to set a MTU below the MCTP-required baseline (of 68) either.

This change sets the min_mtu & max_mtu of the mctp netdev, essentially
disallowing changes. By using these instead of a ndo_change_mtu op, we
get the netlink extacks reported too.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/mctp/mctp-serial.c b/drivers/net/mctp/mctp-serial.c
index 85b407f4df112f9e64898f22d50bb788154511ac..b0e14a63b10d4038933993b4d8c9c651848732e6 100644 (file)
--- a/drivers/net/mctp/mctp-serial.c
+++ b/drivers/net/mctp/mctp-serial.c
@@ -410,7 +410,14 @@ static const struct net_device_ops mctp_serial_netdev_ops = {
 static void mctp_serial_setup(struct net_device *ndev)
 {
        ndev->type = ARPHRD_MCTP;
+
+       /* we limit at the fixed MTU, which is also the MCTP-standard
+        * baseline MTU, so is also our minimum
+        */
        ndev->mtu = MCTP_SERIAL_MTU;
+       ndev->max_mtu = MCTP_SERIAL_MTU;
+       ndev->min_mtu = MCTP_SERIAL_MTU;
+
        ndev->hard_header_len = 0;
        ndev->addr_len = 0;
        ndev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;