usb-gadget

A Dart library for turning any Linux device with a USB controller into a USB peripheral — keyboard, mouse, storage, network adapter, audio device, or your own custom protocol.

Requires a device with a USB Device Controller (UDC). Embedded boards (Raspberry Pi 4/5, Zero 2 W) and Android phones with NDK support (e.g. OnePlus 7 Pro) work great; standard PCs typically don't have one.

  dart pub add usb_gadget

Requirements

sudo apt-get install libaio-dev                  # library required by this package
sudo modprobe libcomposite                       # kernel module for USB gadget support
sudo mount -t configfs none /sys/kernel/config   # if not already mounted

Example

import 'dart:io';
import 'dart:typed_data';

import 'package:usb_gadget/usb_gadget.dart';

class Keyboard extends HIDFunction {
  Keyboard() : super(
    name: 'keyboard',
    descriptor: .fromList([
      0x05, 0x01, 0x09, 0x06, 0xA1, 0x01, 0x05, 0x07,
      0x19, 0xE0, 0x29, 0xE7, 0x15, 0x00, 0x25, 0x01,
      0x75, 0x01, 0x95, 0x08, 0x81, 0x02, 0x95, 0x01,
      0x75, 0x08, 0x81, 0x01, 0x95, 0x06, 0x75, 0x08,
      0x15, 0x00, 0x25, 0x65, 0x05, 0x07, 0x19, 0x00,
      0x29, 0x65, 0x81, 0x00, 0xC0,
    ]),
    protocol: 1,
    subClass: 1,
    reportLength: 8,
  );

  void sendKey(int keyCode, {int modifiers = 0}) =>
      file..writeFromSync(Uint8List(8)
        ..[0] = modifiers
        ..[2] = keyCode)..writeFromSync(Uint8List(8));
}

Future<void> main() async {
  final keyboard = Keyboard();
  final gadget = Gadget(
    name: 'hid_keyboard',
    id: Id(vendor: 0x1234, product: 0x5679),
    class_: .interfaceSpecific(),
    strings: {
      .enUS: const .new(
        manufacturer: 'ACME Corp',
        product: 'USB Keyboard',
        serialnumber: 'KB001',
      ),
    },
    configs: [.new(description: 'A Keyboard', functions: [keyboard])],
  );

  try {
    final reg = await gadget.register();
    reg.bind(defaultUDC);
    await reg.udc?.awaitState(.configured);
    await Future.delayed(const .new(milliseconds: 100));
    [0x0B, 0x08, 0x0F, 0x0F, 0x12, 0x2C, 0x1A, 0x12, 0x15, 0x0F, 0x07, 0x28]
    // Write "hello world\n"
    .forEach(keyboard.sendKey);
    await ProcessSignal.sigint.watch().first;
  } finally {
    await gadget.remove();
  }
}

dart compile exe bin/app.dart -o my_gadget && sudo ./my_gadget

dart compile exe \
-DUSB_GADGET_LOG=true \
-DUSB_GADGET_LOG_COLORS=true \
-DUSB_GADGET_LOG_LEVEL=0 \
bin/app.dart -o my_gadget

Kernel configuration

Most embedded distributions ship with the required options already enabled. To verify:

zcat /proc/config.gz | grep -E 'CONFIG_USB_(GADGET|CONFIGFS|FUNCTIONFS)'

Android

No pre-built Dart SDK exists for Android — you must cross-compile both the SDK and libaio and deploy to the device. The resulting binary runs from the Android command line only, with access to dart:core APIs only.

gclient sync
./tools/build.py --mode=release --arch=arm64 --os=android create_sdk
adb push out/android/ReleaseAndroidARM64/dart-sdk /data/local/tmp/dart-sdk

git clone https://pagure.io/libaio.git && cd libaio

export TOOLCHAIN=<PATH>/dart-sdk/sdk/third_party/android_tools/ndk/toolchains/llvm/prebuilt/<ARCH>
export CC=$TOOLCHAIN/bin/aarch64-linux-android21-clang
export AR=$TOOLCHAIN/bin/llvm-ar
export RANLIB=$TOOLCHAIN/bin/llvm-ranlib

make clean && make CC="$CC" AR="$AR" RANLIB="$RANLIB"
adb push src/libaio.so.1.0.2 /data/local/tmp/
adb shell ln -s /data/local/tmp/libaio.so.1.0.2 /data/local/tmp/libaio.so

adb push /path/to/your/project /data/local/tmp/project && adb shell
su
export PATH=/data/local/tmp/dart-sdk/bin:$PATH PUB_CACHE=/data/local/tmp/.pub-cache/
cd /data/local/tmp/project && dart pub get
dart compile exe bin/your_app.dart -o app && chmod +x app

# Detach UDC from the Android USB stack
ORIGINAL=$(getprop sys.usb.config)
setprop sys.usb.config none && sleep 1
mount -t configfs none /sys/kernel/config

LD_LIBRARY_PATH=/data/local/tmp ./app
sudo ./app

setprop sys.usb.config "$ORIGINAL"

Credits & References

• https://github.com/vpelletier/python-functionfs
• https://github.com/surban/usb-gadget
• https://www.beyondlogic.org/usbnutshell/usb1.shtml
• https://www.kernel.org/doc/Documentation/filesystems/configfs/configfs.txt
• https://www.kernel.org/doc/html/latest/driver-api/usb/gadget.html