net: cdc_ncm: avoid changing RX/TX buffers on MTU changes

[ Upstream commit 1dfddff5 ]

NCM buffer sizes are negotiated with the device independently of
the network device MTU.  The RX buffers are allocated by the
usbnet framework based on the rx_urb_size value set by cdc_ncm. A
single RX buffer can hold a number of MTU sized packets.

The default usbnet change_mtu ndo only modifies rx_urb_size if it
is equal to hard_mtu.  And the cdc_ncm driver will set rx_urb_size
and hard_mtu independently of each other, based on dwNtbInMaxSize
and dwNtbOutMaxSize respectively. It was therefore assumed that
usbnet_change_mtu() would never touch rx_urb_size.  This failed to
consider the case where dwNtbInMaxSize and dwNtbOutMaxSize happens
to be equal.

Fix by implementing an NCM specific change_mtu ndo, modifying the
netdev MTU without touching the buffer size settings.
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/net/usb/cdc_mbim.c

@@ -100,7 +100,7 @@ static const struct net_device_ops cdc_mbim_netdev_ops = {
 	.ndo_stop             = usbnet_stop,
 	.ndo_start_xmit       = usbnet_start_xmit,
 	.ndo_tx_timeout       = usbnet_tx_timeout,
-	.ndo_change_mtu       = usbnet_change_mtu,
+	.ndo_change_mtu       = cdc_ncm_change_mtu,
 	.ndo_set_mac_address  = eth_mac_addr,
 	.ndo_validate_addr    = eth_validate_addr,
 	.ndo_vlan_rx_add_vid  = cdc_mbim_rx_add_vid,

drivers/net/usb/cdc_ncm.c

@@ -41,6 +41,7 @@
 #include <linux/module.h>
 #include <linux/netdevice.h>
 #include <linux/ctype.h>
+#include <linux/etherdevice.h>
 #include <linux/ethtool.h>
 #include <linux/workqueue.h>
 #include <linux/mii.h>
@@ -687,6 +688,33 @@ static void cdc_ncm_free(struct cdc_ncm_ctx *ctx)
 	kfree(ctx);
 }
 
+/* we need to override the usbnet change_mtu ndo for two reasons:
+ *  - respect the negotiated maximum datagram size
+ *  - avoid unwanted changes to rx and tx buffers
+ */
+int cdc_ncm_change_mtu(struct net_device *net, int new_mtu)
+{
+	struct usbnet *dev = netdev_priv(net);
+	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
+	int maxmtu = ctx->max_datagram_size - cdc_ncm_eth_hlen(dev);
+
+	if (new_mtu <= 0 || new_mtu > maxmtu)
+		return -EINVAL;
+	net->mtu = new_mtu;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(cdc_ncm_change_mtu);
+
+static const struct net_device_ops cdc_ncm_netdev_ops = {
+	.ndo_open	     = usbnet_open,
+	.ndo_stop	     = usbnet_stop,
+	.ndo_start_xmit	     = usbnet_start_xmit,
+	.ndo_tx_timeout	     = usbnet_tx_timeout,
+	.ndo_change_mtu	     = cdc_ncm_change_mtu,
+	.ndo_set_mac_address = eth_mac_addr,
+	.ndo_validate_addr   = eth_validate_addr,
+};
+
 int cdc_ncm_bind_common(struct usbnet *dev, struct usb_interface *intf, u8 data_altsetting)
 {
 	const struct usb_cdc_union_desc *union_desc = NULL;
@@ -861,6 +889,9 @@ int cdc_ncm_bind_common(struct usbnet *dev, struct usb_interface *intf, u8 data_
 	/* add our sysfs attrs */
 	dev->net->sysfs_groups[0] = &cdc_ncm_sysfs_attr_group;
 
+	/* must handle MTU changes */
+	dev->net->netdev_ops = &cdc_ncm_netdev_ops;
+
 	return 0;
 
 error2:

include/linux/usb/cdc_ncm.h

@@ -133,6 +133,7 @@ struct cdc_ncm_ctx {
 };
 
 u8 cdc_ncm_select_altsetting(struct usb_interface *intf);
+int cdc_ncm_change_mtu(struct net_device *net, int new_mtu);
 int cdc_ncm_bind_common(struct usbnet *dev, struct usb_interface *intf, u8 data_altsetting);
 void cdc_ncm_unbind(struct usbnet *dev, struct usb_interface *intf);
 struct sk_buff *cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign);