Using Gadget Filesystem under Ubuntu

I wanted to emulate a USB device in Linux for testing purpose. This can be done easily using the Gadget API. However I quickly discovered that this feature was not present in Ubuntu, even in the form of a kernel module in linux-extra. The only solution remaining was to rebuild the missing kernel module manually.

The main issue with a Linux kernel is that both the API and the ABI are not stable, meaning:

• You can’t use any source version of the module. The kernel API is evolving and you are likely to miss new or deprecated API. The current version of the dummy_hcd module does not compile due to newly introduced APIs. The matching kernel source is available in the linux-extra package.
• You can’t keep a module you built for one version to another version. You must always rebuilt your module to match the current kernel ABI. Ideally the module must be known from modprobe to be correctly loaded along its dependencies. This is solved by using DKMS.

Now the proces was fairly simple.

1. Install the package linux-source to have the kernel sources matching the currently installed kernel.

This part is fairly easy.

sudo aptitude install linux-source

2. Setup a build process that would extract the source of the dummy_hcd kmod from these sources and,

For a Makefile, you can retrieve the current version of the kernel from uname plus some grep.

SVERSION := $(shell uname -r | grep -o "^[^-]*")

Then extract and move the dummy_hcd.c file from the current kernel source to the current folder.

dummy_hcd.c: /usr/src/linux-source-$(SVERSION)/linux-source-$(SVERSION).tar.bz2
tar -xjvf $^ linux-source-$(SVERSION)/drivers/usb/gadget/udc/dummy_hcd.c &&\
    cp linux-source-$(SVERSION)/drivers/usb/gadget/udc/dummy_hcd.c $@

3. Build the kmod using the kernel Makefile.

Following the basic example found in the documentation.

obj-m := dummy_hcd.o
KVERSION := $(shell uname -r)

all: dummy_hcd.c
    $(MAKE) -C /lib/modules/$(KVERSION)/build M=$(PWD) modules

clean:
    $(MAKE) -C /lib/modules/$(KVERSION)/build M=$(PWD) clean

4. Wrap everything into a DKMS module.

First we should install DKMS.

sudo aptitude install dkms

Then add a dkms.conf to ls /usr/src/dummy_hcd-0.1/ along with the Makefile we previously created.

PACKAGE_NAME="dummy_hcd"
PACKAGE_VERSION="0.1"
CLEAN="make clean"
MAKE[0]="make all KVERSION=$kernelver"
BUILT_MODULE_NAME[0]="dummy_hcd"
DEST_MODULE_LOCATION[0]="/extra"
AUTOINSTALL="yes"

We can then add our DKMS module to be built and added to our kernel modules.

dkms add -m dummy_hcd -v 0.1
dkms build -m dummy_hcd -v 0.1
dkms install -m dummy_hcd -v 0.1

Finally, we can test our module is working by loading it and mounting the Gadget Filesystem.

modprobe dummy_hcd
mount -t gadgetfs gadgetfs /dev/gadgetfs

The complete project is published on Github.