Re: [RFC] net: usb: usbnet: fix name regression

[Simon Horman <horms@kernel.org>]

On Tue, Oct 15, 2024 at 04:03:32PM +0200, Oliver Neukum wrote:
> 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.
> 
> The correct check for whether bind() has altered the MAC is
> done with is_zero_ether_addr
> 
> Signed-off-by: Oliver Neukum <oneukum@suse.com>
> Reported-by: Greg Thelen <gthelen@google.com>
> Diagnosed-by: John Sperbeck <jsperbeck@google.com>
> Fixes: bab8eb0dd4cb9 ("usbnet: modern method to get random MAC")
> ---
>  drivers/net/usb/usbnet.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> index ee1b5fd7b491..44179f4e807f 100644
> --- a/drivers/net/usb/usbnet.c
> +++ 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)))

Hi Oliver,

I think works for the case where a random address will be assigned
as per the cited commit. But I'm unsure that is correct wrt
to the case where ->bind assigns an address with 0x2 set in the 0th octet.

Can that occur in practice? Perhaps not because the driver would
rely on usbnet_probe() to set a random address. But if so then
it would previously have hit the "eth%d" logic, but does not anymore.

>  			strscpy(net->name, "eth%d", sizeof(net->name));
>  		/* WLAN devices should always be named "wlan%d" */
>  		if ((dev->driver_info->flags & FLAG_WLAN) != 0)
> -- 
> 2.47.0
> 
>