For unfortunate reasons beyond the scope of this entry, I've recently needed to once again take a look at the Ubuntu kernel source code, as they're patched and modified and so on from the upstream versions. There are some things where I'll just look at the official kernel source, but for certain sorts of issues, nothing short of the real kernel we're using (or something close to it) will do. Every time I do this I have to re-discover or re-work out how to get a copy of this source code, so this time around I'm going to write it down. The necessary disclaimer is that Ubuntu may change all of this in the future.

There are two approaches you can take here, depending on what you want. I'll start with the more obvious one, using apt-get. In theory, what you should be able to do is to just 'apt-get source' on the kernel that you're running, in other words generally:

apt-get source linux-image-$(uname -r)

If you try this, you will get a 'linux-signed' source package, which does not actually contain any kernel source. It turns out that what you really want is the source package for the linux-image-unsigned package. This is the 'linux' source package, so you can do either of the following commands:

apt-get source linux-image-unsigned-$(uname -r)
apt-get source linux

In either case, you end up with the most recent kernel source package, which is not necessarily the source code to the kernel that you're actually running. Unfortunately there's no guarantee that Ubuntu still has the linux source package for your specific kernel available as a source package; they appear to purge old ones from mirrors, just as they purge the old binary packages.

The other approach is the one recommended by Ubuntu and which a successful 'apt-get source' will nag you about, which is to clone it from Ubuntu's kernel git tree. At the moment (here at the start of 2019), you can find information on this in Ubuntu's KernelGitGuide wiki page or their page on kernel source (note that this page is incorrect about apt-get). The Ubuntu 18.04 LTS git tree is here, although per Ubuntu's wiki page, you should clone it with the git: protocol URLs. As you can see, there are a variety of tags and branches in the repo, and I think they're all reasonably obvious (and there's some explanation of them in the Ubuntu wiki pages).

In the 'apt-get source' version, the Ubuntu package changelog is in debian/changelog; this is what you want to consult if you're looking for relevant bug fixes and so on. In the git version, the changelog is debian.master/changelog and the 'debian' directory has other things. In the git version, Ubuntu generally commits individual Ubuntu changes as individual git commits, which means that you can use 'git log' to scan for Ubuntu changes to particular files or directories of interest. Because the git version has tags for specific releases, it's also the easiest way to see the Ubuntu kernel tree as of a specific linux-image version (or to see the differences between two of them, perhaps in a sub-tree).

For instance, suppose you want to see all changes since Ubuntu's 4.15.0-30 in some areas of the tree. You could do:

git log Ubuntu-4.15.0-30.32.. -- fs/sysfs fs/namei.c fs/inode.c fs/dcache.c

A specific commit can then be shown with 'git show <id>' as usual, which will show you the diff as well as its commit message.

Ubuntu's kernels have the ZFS on Linux kernel module code in top-level 'spl' and 'zfs' directories. The user level tools are in a separate set of packages with their own source package, 'zfs-linux'. Apt-getting this source package will produce a claim that it is really maintained in Debian GIT, but I'm not sure that's actually true for the Ubuntu version. While this source package still includes the kernel module source code, I believe only the user level stuff is used. I'm not sure how Ubuntu manages changes to all of this, but they appear to keep everything in sync somehow.

(In Ubuntu 20.04, there will likely only be a single 'zfs' directory, since ZFS on Linux has merged the 'spl' package into the 'zfs' one in its development version.)

In theory Ubuntu has instructions on building packages from the git version here, and they were even updated within the past year. In practice I have no idea, since I haven't ever built a new Ubuntu kernel package, but I would probably be inclined to start from 'apt-get source linux', because that's at least in a format that I sort of understand. Of course, if I had to build a modified version of a specific, no longer current linux-image version, I might have to use the git version because that's the only way I can get the exact source code.

(Perhaps this means we should be routinely saving the kernel source packages for important kernels that we use. Sadly it's now too late to do this for the kernel that our Linux fileservers use; we froze their kernel version some time ago and the 'linux' source package for it is long gone now.)