netdev - [PATCH net-next v2 2/3] mctp: serial: enforce fixed MTU

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Date:   Thu, 25 Nov 2021 14:07:38 +0800
From:   Jeremy Kerr <jk@...econstruct.com.au>
To:     netdev@...r.kernel.org
Cc:     Matt Johnston <matt@...econstruct.com.au>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Subject: [PATCH net-next v2 2/3] 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@...econstruct.com.au>
---
 drivers/net/mctp/mctp-serial.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/mctp/mctp-serial.c b/drivers/net/mctp/mctp-serial.c
index 85b407f4df11..b0e14a63b10d 100644
--- 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;
-- 
2.30.2