USB gadget functionalities in Android

I started working on Android stuffs this summer. While I mainly work on the USB layer within the Linux kernel, I do sometimes need to look into the Android framework, to see if I could achieve my goal from the Android user space. One of the questions I have got is to find the USB configuration of the phone (e.g., MTP, adb, and etc) when connected with the host machine. And here comes this post.

1. lsusb

The most straightforward way to have the USB information of the phone is to plug it into a Linux box, and run ‘lsusb -t‘ or ‘lsusb -v‘. Things get annoying when I tried to pull similar information from the phone directly.

2. adb

If you have adb with root permission, go to ‘/sys/class/android_usb/android0‘. You will find all the kernel USB gadget functionalities builtin, and the configurations, e.g., which functionalities are enabled in the phone right now. This won’t work if adb runs as ‘shell’ user.

3. USB Options

When the phone is connected with the host machine, the USB option menu would pop up providing more options: charging, MTP, PTP, and etc. This should be the easiest way in most cases, however, would fail if you are not playing with your own phone (e.g., pin protected, and yes, I am talking about “remote” phones in the device farm). It is also possible that some USB gadget functionalities are NOT exposed to the menu at all, e.g., a hiddent USB CDC/ACM functionality.

4. USB Settings

Some vendors provide secret dial codes to trigger a USB Setting menu with some combinations of different gadget functionalities for testing/debugging purpose. E.g., for Samsung Galaxy phones, use ‘*#0808#‘. Let me know if you have similar dial codes from other vendors.

5. UsbManager

Now it is time to look into the Android framework. Can we write an app to show the USB configuration/information? UsbManager.java[1] is the implementation in Android to control the USB hardware. To differentiate the USB host mode and the USB gadget mode, the Android APIs have USB Host APIs and USB Accessory API[2], both of which are managed by the UsbManager. For instance, getDeviceList() in the UsbManager returns a list of UsbDevice connected with the phone. As expected, UsbDevice class is the instantiation of a USB device defined by the USB spec. We could go and find the configuration, interfaces, and endpoints, just like what we could do with libusb. Accordingly, getAccessoryList() returns a list of UsbAccessory the phone behaves. Here is the problem – UsbAccessory is NOT abstracted as UsbDevice! The only information from UsbAccessory you could get is device information, such as manufacturer, product, serial number, and etc. In short, no interface-level information in the UsbAccessory!

Nevertheless, UsbManager may still be the answer if we look the latest implementation[3]. ‘isFunctionEnabled()’ and ‘containsFunction()’ seem to be promising to detect hiddent functionalities, which may not be shown in the USB option menu. Unfortunately, this still depends on the vendor-specific customization – whether or not the USB function is exposed to the Android framework (platform), and managed by the UsbManager.

Refs:
[1] https://developer.android.com/reference/android/hardware/usb/UsbManager.html
[2] https://developer.android.com/guide/topics/connectivity/usb/index.html
[3] https://github.com/android/platform_frameworks_base/blob/master/core/java/android/hardware/usb/UsbManager.java

About daveti

Interested in kernel hacking, compilers, machine learning and guitars.
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