There are a lot of bad answers posted to this question ranging from insisting on running adb as root (which should not be touted as the only or even recommended solution) to solving completely unrelated issues.

I’ve used this answer posted by Alex P on Stakoverflow and it works first time!

Here is the single shortest and most universal recipe for taking care of permissions for all adb and fastboot devices at once:

Run the following commands in their order

Step 1

echo ‘ACTION==”add”, SUBSYSTEM==”usb”, ENV{DEVTYPE}==”usb_device”,
ENV{ID_USB_INTERFACES}==”*:ff420?:*”, MODE=”0666″‘ | sudo tee /etc/udev/rules.d/99-android.rules

Step 2

sudo udevadm control –reload-rules
sudo udevadm trigger –verbose –action=add –subsystem-match=usb

Step 3

adb kill-server sudo ./adb start-server adb devices

Step 4

adb devices

The device is now listed well instead of ???????