Composite USB Gadgets on the Raspberry Pi Zero

RPi_Zero_multi-usb
original by jsmith7342, cc by-sa, others cc0/PD

Hi, Hackaday! 😀

Want your Pi Zero to emulate a keyboard, ethernet adapter, serial device, mass storage, and many more at the same time? This tutorial is for you!

The Pi Zero is all the rage – and I too am working on a Zero-based project.
[LadyAda] and [gbaman] both have written tutorials on the old series of drivers, g_{hid,ether,serial,*}. Those are not flexible at all, only allow for one loaded at each time and in case of the hid-gadget even require you to modify and recompile the kernel module.

LibComposite solves those problems by putting the configuration into userland (ConfigFS).

Keep in mind though, that at the time of writing, no windows drivers are available – you need to use Linux or Mac OS X to connect to your Pi this way.

Step 0 – configuring the SD card

Download and install the latest Raspbian Jessie onto a suitably large SD card, and expand the root partition. This has been described enough already, so I’ll just link you to the guide on Adafruit!

Step 1 – Kernel stuff

We need to use the 4.4 Kernel, which is currently (Feb. 2016) not installed on the default Raspbian image. But upgrading is easy: just run
sudo BRANCH=next rpi-update
(the ‚$‘ indicates this should be typed into the shell – don’t try to copy it)

Now, we need to enable a special „device tree overlay“:
echo "dtoverlay=dwc2" | sudo tee -a /boot/config.txt
echo "dwc2" | sudo tee -a /etc/modules

Finally, we need to enable the libcomposite driver by running:
sudo echo "libcomposite" | sudo tee -a /etc/modules

Step 2 – Configuring the gadget(s)

Now you have to define, what you want your gadget to do – just want ethernet? a keyboard? all of the above?
Configuring is done via ConfigFS, a virtual file system in /sys/. It is automatically mounted on the Pi on startup, so we can dig right in.
For a gadget called ‚USBArmory‘, a nice starting point exists, on which I will base the rest of this tutorial.

Create the config script

The configuration is volatile, so it must be run on each startup.
Create a file isticktoit_usb in /usr/bin/ using your favourite text editor. Type the following:
sudo touch /usr/bin/isticktoit_usb #create the file
sudo chmod +x /usr/bin/isticktoit_usb #make it executable
sudo nano /usr/bin/isticktoit_usb #edit the file

Afterwards, you need to run this script automatically at startup. For best performance, you can create a systemd unit file, but for now, we’ll stick to rc.local. (this is part of the old sysvinit system, but is still executed on the pi by default)
Open /etc/rc.local as root and add the following line before(!!!!) the line containing the word ‚exit‘.
sudo nano /etc/rc.local
/etc/rc.local...
/usr/bin/isticktoit_usb # libcomposite configuration
exit

Creating the gadget

This is the global configuration; so it does not matter how many USB features your Pi will use.
Feel free to change serial number, manufacturer and product name in this block.
/usr/bin/isticktoit_usb#!/bin/bash
cd /sys/kernel/config/usb_gadget/
mkdir -p isticktoit
cd isticktoit
echo 0x1d6b > idVendor # Linux Foundation
echo 0x0104 > idProduct # Multifunction Composite Gadget
echo 0x0100 > bcdDevice # v1.0.0
echo 0x0200 > bcdUSB # USB2
mkdir -p strings/0x409
echo "fedcba9876543210" > strings/0x409/serialnumber
echo "Tobias Girstmair" > strings/0x409/manufacturer
echo "iSticktoit.net USB Device" > strings/0x409/product
mkdir -p configs/c.1/strings/0x409
echo "Config 1: ECM network" > configs/c.1/strings/0x409/configuration
echo 250 > configs/c.1/MaxPower
# Add functions here
# see gadget configurations below
# End functions
ls /sys/class/udc > UDC

Now scroll down to the headings that interest you and configure those functions 😉
A full example can be found at my hardpass repository – here

Serial Adapter

This is quite useful for debugging and is the easiest to setup.
to our config-file insert the following between the ‚functions‘ comments:
/usr/bin/isticktoit_usb# Add functions here
mkdir -p functions/acm.usb0
ln -s functions/acm.usb0 configs/c.1/
# End functions

Then, enable a console on the USB serial:
sudo systemctl enable getty@ttyGS0.service
To connect to your Pi on a Linux computer, make sure screen is installed (sudo apt-get install screen) and then run:
sudo screen /dev/ttyACM0 115200
if screen terminates automatically, you need to change the device file. Consult for example dmesg:
dmesg|grep "USB ACM device"

Ethernet Adapter

first, lets add to our configuration file:
sudo nano /usr/bin/isticktoit_usb
/usr/bin/isticktoit_usb# Add functions here
mkdir -p functions/ecm.usb0
# first byte of address must be even
HOST="48:6f:73:74:50:43" # "HostPC"
SELF="42:61:64:55:53:42" # "BadUSB"
echo $HOST > functions/ecm.usb0/host_addr
echo $SELF > functions/ecm.usb0/dev_addr
ln -s functions/ecm.usb0 configs/c.1/
# End functions
ls /sys/class/udc > UDC
#put this at the very end of the file:
ifconfig usb0 10.0.0.1 netmask 255.255.255.252 up
route add -net default gw 10.0.0.2

Save and exit nano, then head over to your host PC:
When you have problems with the automatic connection (e.g. „Wired Connection 2“ on my Thinkpad), disconnect and execute this:
dmesg|grep cdc_ether
[13890.668557] cdc_ether 1-1:1.2 eth0: register 'cdc_ether' at usb-0000:00:14.0-1, CDC Ethernet Device, 48:6f:73:74:50:43
[13890.674117] usbcore: registered new interface driver cdc_ether
[13890.687619] cdc_ether 1-1:1.2 enp0s20u1i2: renamed from eth0

will tell you, how your ethernet adapter is named (and maybe renamed afterwards. Plug the interface name (enp0s20u1i2) into this line:
sudo ifconfig enp0s20u1i2 10.0.0.2 netmask 255.255.255.252 up
Then, connect via ssh to your Pi:
ssh 10.0.0.1 -l pi

Keyboard / Mouse / Joystick (HID)

Firstly, our configuration script.
sudo nano /usr/bin/isticktoit_usb
/usr/bin/isticktoit_usb# Add functions here
mkdir -p functions/hid.usb0
echo 1 > functions/hid.0/protocol
echo 1 > functions/hid.0/subclass
echo 8 > functions/hid.0/report_length
echo -ne \\x05\\x01\\x09\\x06\\xa1\\x01\\x05\\x07\\x19\\xe0\\x29\\xe7\\x15\\x00\\x25\\x01\\x75\\x01\\x95\\x08\\x81\\x02\\x95\\x01\\x75\\x08\\x81\\x03\\x95\\x05\\x75\\x01\\x05\\x08\\x19\\x01\\x29\\x05\\x91\\x02\\x95\\x01\\x75\\x03\\x91\\x03\\x95\\x06\\x75\\x08\\x15\\x00\\x25\\x65\\x05\\x07\\x19\\x00\\x29\\x65\\x81\\x00\\xc0 > functions/hid.0/report_desc
ln -s functions/hid.usb0 configs/c.1/
# End functions

The simplest way to send keystrokes is by echoing HID packets to the device file:
sudo su
echo -ne "\0\0\x4\0\0\0\0\0" > /dev/hidg0 #press the A-button
echo -ne "\0\0\0\0\0\0\0\0" > /dev/hidg0 #release all keys

This isn’t practicable however, so head over to my GitHub page and download the code onto your computer.
extract it onto the Pi’s SD card and boot it up. On the Pi:
cd PATH_TO_HARDPASS_REPO
make #compile the program
echo -n "hello world!" | sudo ./scan /dev/hidg0 1 2

the last line will write the piped in string over the HID protocol. The ‚1‘ means US-Layout, a ‚2‘ in its place would be German/Austrian layout. The second number is for entering characters not available on your keyboard (2=Linux, 3=Windows(but no windows drivers))

Mass storage

Mass storage is somewhat difficult. You cannot share the Pi’s partition with the host computer, but only a disk image file. I created a very small one to store the ethernet host configuration (sudo ifconfig …) on it.
First, lets make a disk. This is quite a long process, so follow this tutorial here. Run those commads (count=1024 describes the size in kilobytes):
dd if=/dev/zero of=~/usbdisk.img bs=1024 count=1024
mkdosfs ~/usbdisk.img

Then, it we go to our configuration again:
sudo nano /usr/bin/isticktoit_usb
/usr/bin/isticktoit_usb# Add functions here
FILE=/home/pi/usbdisk.img
mkdir -p ${FILE/img/d}
# mount -o loop,ro,offset=1048576 -t ext4 $FILE ${FILE/img/d} # FOR OLD WAY OF MAKING THE IMAGE
mount -o loop,ro, -t vfat $FILE ${FILE/img/d} # FOR IMAGE CREATED WITH DD
mkdir -p functions/mass_storage.usb0
echo 1 > functions/mass_storage.usb0/stall
echo 0 > functions/mass_storage.usb0/lun.0/cdrom
echo 0 > functions/mass_storage.usb0/lun.0/ro
echo 0 > functions/mass_storage.usb0/lun.0/nofua
echo $FILE > functions/mass_storage.usb0/lun.0/file
ln -s functions/mass_storage.usb0 configs/c.1/
# End functions

A FAT32 formatted removable drive should show up, when you plug the Pi into a computer the next time. To get access to the files stored on the disk-image from the Pi, you can unmount it completely (in the host first, then in the pi), and remount it somewhere else.

Others

There are about 20 different USB gadgets the Linux Kernel can emulate. Have a look at the Kernel documentation to find out more!

Links

Tutorial by gbaman
libcomposite in the Kernel documentation
–> hardpass – my PiZero project (Keyboad emulation) <--
–> hardpass – my PiZero project (Password Manager) <--
USBArmory Wiki Page

Revisions

Article Revisions
1.000: Initial article
1.001: Added 'Hi, Hackaday'
1.010: Fixed configuration script ('# Add functions here') (thanks, Hans)
1.011: Fixed and simplified a few things (thanks, Tom)
1.100: Fixed the 'press A-button' example (missing `-ne`) (thanks, Jim)