Subject: Re: [PATCH net] net: usb: usbnet: fix name regression

From: Dominique MARTINET <[email-masked]>
Date: Mon, 2 Dec 2024 12:50:15 +0900

Hi,

Oliver Neukum wrote on Thu, Oct 17, 2024 at 09:18:37AM +0200:
> The fix for MAC addresses broke detection of the naming convention
> because it gave network devices no random MAC before bind()
> was called. This means that the check for the local assignment bit
> was always negative as the address was zeroed from allocation,
> instead of from overwriting the MAC with a unique hardware address.

So we hit the exact inverse problem with this patch: our device ships an
LTE modem which exposes a cdc-ethernet interface that had always been
named usb0, and with this patch it started being named eth1, breaking
too many hardcoded things expecting the name to be usb0 and making our
devices unable to connect to the internet after updating the kernel.

Long term we'll probably add an udev rule or something to make the name
explicit in userspace and not risk this happening again, but perhaps
there's a better way to keep the old behavior?

(In particular this hit all stable kernels last month so I'm sure we
won't be the only ones getting annoyed with this... Perhaps reverting
both patches for stable branches might make sense if no better way
forward is found -- I've added stable@ in cc for heads up/opinions)

> +++ b/drivers/net/usb/usbnet.c
> @@ -1767,7 +1767,8 @@ usbnet_probe (struct usb_interface *udev, const struct usb_device_id *prod)
>  		// can rename the link if it knows better.
>  		if ((dev->driver_info->flags & FLAG_ETHER) != 0 &&
>  		    ((dev->driver_info->flags & FLAG_POINTTOPOINT) == 0 ||
> -		     (net->dev_addr [0] & 0x02) == 0))
> +		     /* somebody touched it*/
> +		     !is_zero_ether_addr(net->dev_addr)))

... or actually now I'm looking at it again, perhaps is the check just
backwards, or am I getting this wrong?
previous check was rename if (mac[0] & 0x2 == 0), which reads to me as
"nobody set the 2nd bit"
new check now renames if !is_zero, so renames if it was set, which is
the opposite?...

>  			strscpy(net->name, "eth%d", sizeof(net->name));
>  		/* WLAN devices should always be named "wlan%d" */

Thanks,
-- 
Dominique Martinet