main/postmarketos-mkinitfs: unudhcpd can be started with (unnamed net_device)

Describe your issue

What's the expected behaviour?

unudhcpd doesn't start if there is no usb network iface

What's the current behaviour?

unudhcpd get's started with (unnamed net_device)

How to reproduce your issue?

1. Make sure your device doesn't have USB Device Controller (/sys/class/udc/ is empty)
2. Boot.
3. Observe initramfs log spamming unudhcpd restarts:

Setup usb network
  /sys/class/android_usb does not exist, skipping android_usb
  Setting up an USB gadget through configfs
  No USB Device Controller available
Starting unudhcpd with server ip 172.16.42.1, client ip: 172.16.42.2
ifconfig: SIOCSIFADDR: No such device
  Using interface (unnamed net_device)
  Starting the DHCP daemon
Trying to start server with parameters: Server IP addr: 172.16.42.1:67, client IP addr: 172.16.42.2, interface: (unnamed net_device)
Unable to bind to interface: No such device
(...)
Trying to bind to interface: (unnamed net_device)
Server quit, retrying in 1 second...
Trying to start server with parameters: Server IP addr: 172.16.42.1:67, client IP addr: 172.16.42.2, interface: (unnamed net_device)
Trying to bind to interface: (unnamed net_device)
Server quit, retrying in 1 second...

(loops forever spamming tty1)

What device are you using?

acer-aspire1

On what postmarketOS version did you encounter the issue?

• edge (master branch)

• v23.12

• v23.06 (supported until 2024-01-18)

• I confirm that the issue still is present after running sudo apk upgrade -a

Additional info

We get to this because setup_usb_network_configfs() first creates the gadget but assigns it a usb controller at the very last step, so if there is no UDC, the gadget is left half-done. Then start_unudhcpd() checks ifname prop of the gadget but doesn't check if it's garbage as it assumes that only empty string is "no interface".

One could fix this on a per-device basis by disabling dhcpd but I think it's nicer to fix the detection for all devices.