Re: [PATCH net] usbnet: cap max_mtu for drivers without bind callback

[Laurent Vivier]

On 7/23/26 17:43, Jakub Kicinski wrote:
> On Fri, 17 Jul 2026 17:16:26 +0200 Laurent Vivier wrote:
> > Commit c7159e960f14 ("usbnet: limit max_mtu based on device's hard_mtu")
> > caps max_mtu inside the if (info->bind) block in usbnet_probe().
>
> Okay... but before that commit the logic was also missing,
> and that commit only refines the condition. I don't think
> that's the right commit for Fixes.

So it's:

Fixes: f77f0aee4da4 ("net: use core MTU range checking in USB NIC drivers")
Cc: jarod@xxxxxxxxxx

> > Drivers without a bind callback never enter this block, so max_mtu
> > stays at ETH_MAX_MTU.
> >
> > QEMU's usb-net device (0x0525/0xa4a2) is claimed by the cdc_subset
> > driver which has no bind callback. The guest accepts any MTU from DHCP
> > (e.g. 65520 from passt), leading to TCP segments that exceed the
> > device's 2048-byte receive buffer and are silently dropped.
> >
> > Move the max_mtu capping after the if/else block so it applies to all
> > usbnet drivers.
>
> A short explanation here why the changes was made under the bind would
> be quite useful (presumably it was done there because that's where the
> existing MTU check was, and the existing MTU check only mattered if the
> device specific callback could modify the state etc.)

Yes, I put max_mtu here because the existing mtu check was here.

> It feels like we should update the max_mtu instead of capping it later
> for devices without bind (note that only a device with bind sets
> NOMAXMTU, too)

What do you think of:

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 25518635b7b7..9ec038a1b6f5 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1794,7 +1794,7 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id 
*prod)
 	 */
 	dev->hard_mtu = net->mtu + net->hard_header_len;
 	net->min_mtu = 0;
-	net->max_mtu = ETH_MAX_MTU;
+	net->max_mtu = dev->hard_mtu - net->hard_header_len;

 	net->netdev_ops = &usbnet_netdev_ops;
 	net->watchdog_timeo = TX_TIMEOUT_JIFFIES;
@@ -1825,8 +1825,9 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id 
*prod)
 		if ((dev->driver_info->flags & FLAG_NOARP) != 0)
 			net->flags |= IFF_NOARP;

-		if ((dev->driver_info->flags & FLAG_NOMAXMTU) == 0 &&
-		    net->max_mtu > (dev->hard_mtu - net->hard_header_len))
+		if (dev->driver_info->flags & FLAG_NOMAXMTU) == 0)
+			net->max_mtu = ETH_MAX_MTU;
+		else
 			net->max_mtu = dev->hard_mtu - net->hard_header_len;

 		if (net->mtu > (dev->hard_mtu - net->hard_header_len))

Thanks,
Laurent