Just some informative notes for those who are not familiar with the intricacies of GPL.
I'm not detracting your point.

Disclaimer: IANAL

The GPLv2 license governs the distribution of software licensed with said license. It forces anyone who distributes a GPL binary to also distribute the source code used to build that binary.

If in order to build that particular binary you had to include some other code, then you have to provide the sources for that code as well. Furthermore, that code you provide must be distributed in turn under the GPL license. Any such "added code" becomes part of the thing you distribute and thus constitutes "derivative work".

The key thing here is that it doesn't matter if the "added code" is just some bit of extra functionality of if it's the "whole thing" while the GPL code is just one header. When you build a kernel module, you have use some GPLd headers at the very least.

This last point technically taints a binary kernel module and forces you to release all the sources used to build it and release those sources as GPL.

However there are two interesting points here:

1. Distribution

Notice how the license only mentions distribution. You are allowed to do whatever you want with a GPL source that you hold on your hands: you can change it, add some code, link it with other code. There is absolutely nothing forcing you to immediately release your derivate work to the public and thus nothing forcing you to release the sources of your derivate work.

This means, that if you build a kernel module on your machine, from a mixture of sources with GPL and an other licence; you didn't violate the GPL (whether you violated the other license, it depends on that other license).

Please read very informative:

http://zfsonlinux.org/faq.html#WhatAboutTheLicensingIssue

TL;DR: ^^^ In the case of the kernel, this prevents us from distributing ZFS as part of the kernel binary. However, there is nothing in either license that prevents distributing it in the form of a binary module or in the form of source code.

2. Exemptions

http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html

However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.

This has a much less clear interpretation in the context of kernel modules.
This clause is mainly intended to allow you to use GPL software on a proprietary operating systems as well as running proprietary software on GPL operating systems.
You can compile it: which means you can include headers provided by the compiler and OS. And you can run it: which means you can invoke system calls and/or link it with possibly proprietary libraries that make up the main interface.

I have no idea whether in the context of a kernel module, the kernel internal API can fall under this category, given that it's arguable that it doesn't make up the "main interface" of the linux OS.

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How does this relate to the article from canonical ?

They are claiming something entirely different. They claim that the ZFS kernel module is "evidently" not a derivate work on the grounds that it's self contained and they cite existing exemptions for other binary drivers such as those by nvidia.

They don't explicitly mention the exemption on the aforementioned point in the GPLv2 license text.

Thus, it's not clear how this will affect the observation of the GPL license and whether it will set a precedent at least for all cases where one can argue that the binary blob is linked against an well define interface that is meant to provide an "execution environment". (e.g. plugins, kernel modules, binaries running inside an OS). Not sure if setting this precedent will eventually erode the general case of linking though.

The line is thin, but I would have preferred some more explicit reasoning instead of "we have concluded that we are acting within the rights " and "is clearly not a derivative work", and finishing with a "As we have already reached the conclusion, we are not interested in debating license compatibility".