Compiling a single linux kernel module with matching vermagic

I recently got locked out of a server because of configuration mistake and had to rescue a FreeBSD server by netbooting Ubuntu (only image provided by the host that booted). My root was running on UFS and as expected Ubuntu does not come with a write enabled UFS kernel module.

I set up a virtual build box on my local machine as it was faster than setting everything up on the netboot’d image (plus since it was totally in memory, if anything went wrong it’d be best to setup tooling offline). But just in case we need to remember, the dependencies are

libncurses-dev
libssl
build-essentials
bc

After booting into the Ubuntu 16.04 image, we can then fetch the kernel source by doing

apt-get source linux-image-`uname -r`

This would install /usr/src/linux-headers-version (replace version) with the kernel source. Then we need to scp the Makefile found in the directory to the same directory on our build box.

We can then enable UFS_FS_WRITE=y in the kernel config. Then call

make oldconfig
make prepare
make modules_prepare
make M=fs/ufs/ modules

Then the resulting ufs.ko can be scp’d back to the server to be loaded. The vermagic will match if the same Makefile was used across both kernel compilations.