Legal aspects

GPLv2 licensing termsedit

Initially, Torvalds released Linux under a license which forbade any commercial use. This was changed in version 0.12 by a switch to the GNU General Public License version 2 (GPLv2). This license allows distribution and sale of possibly modified and unmodified versions of Linux but requires that all those copies be released under the same license and be accompanied by the complete corresponding source code. Torvalds has described licensing Linux under the GPLv2 as the "best thing I ever did".

The Linux kernel is licensed explicitly only under version 2 of the GPL, without offering the licensee the option to choose "any later version", which is a common GPL extension. The official git branch of Torvalds contains documentation that explains the kernel development process to people who want to work with the community and contribute code; it clearly states that "Any contributions which are not covered by a GPLv compatible license will not be accepted into the kernel.".

There was considerable debate about how easily the license could be changed to use later GPL versions (including version 3), and whether this change is even desirable. Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is released only under version 2. However, the terms of the GPL state that if no version is specified, then any version may be used, and Alan Cox pointed out that very few other Linux contributors had specified a particular version of the GPL.

In September 2006, a survey of 29 key kernel programmers indicated that 28 preferred GPLv2 to the then-current GPLv3 draft. Torvalds commented, "I think a number of outsiders... believed that I personally was just the odd man out because I've been so publicly not a huge fan of the GPLv3." This group of high-profile kernel developers, including Torvalds, Greg Kroah-Hartman and Andrew Morton, commented on mass media about their objections to the GPLv3. They referred to clauses regarding DRM/tivoization, patents, "additional restrictions" and warned a Balkanisation of the "Open Source Universe" by the GPLv3. Linus Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism even years later.

Loadable kernel modulesedit

It is debated whether some loadable kernel modules (LKMs) are to be considered derivative works under copyright law, and thereby whether or not they fall under the terms of the GPL.

In accordance with the license rules, LKMs using only a public subset of the kernel interfaces are non-derived works, thus Linux gives system administrators the mechanisms to load out-of-tree binary objects into the kernel address space.

There are some out-of-tree loadable modules that make legitimate use of the dma_buf kernel feature. GPL compliant code can certainly use it. However, a different possible use case would be Nvidia Optimus that pairs a fast GPU with an Intel integrated GPU, where the Nvidia GPU writes into the Intel framebuffer when it is active. But, Nvidia cannot use this infrastructure because it necessitates bypassing a rule that can only be used by LKMs that are also GPL. Alan Cox replied on LKML, rejecting a request from one of their engineers to remove this technical enforcement from the API. Torvalds clearly stated on the LKML that "I claim that binary-only kernel modules ARE derivative "by default"'".

On the other hand, Torvalds has also said that "one gray area in particular is something like a driver that was originally written for another operating system (i.e. clearly not a derived work of Linux in origin). THAT is a gray area, and _that_ is the area where I personally believe that some modules may be considered to not be derived works simply because they weren't designed for Linux and don't depend on any special Linux behaviour". Proprietary graphics drivers, in particular, are heavily discussed.

Firmware binary blobsedit

The official kernel, that is the Linus git branch at the kernel.org repository, does not contain any kind of proprietary code; however Linux can search the filesystems to locate proprietary firmware, drivers, and other executable modules (collectively known as "binary blobs"), then it can load and link them into the kernel space. Whenever proprietary modules are loaded into Linux, the kernel marks itself as being "tainted" and therefore bug reports from tainted kernels will often be ignored by developers.

When it is needed (e.g., for accessing boot devices or for speed) firmware can be built-in to the kernel, this means building the firmware into vmlinux; however this is not always a viable option for technical or legal issues (e.g., it is not permitted to firmware that is non-GPL compatible).

Trademarkedit

Linux is a registered trademark of Linus Torvalds in the United States, the European Union, and some other countries. A legal battle over the trademark began in 1996, when William Della Croce, a lawyer who was never involved in the development of Linux, started requesting licensing fees for the use of the word Linux. After it was proven that the word was in common use long before Della Croce's claimed first use, the trademark was awarded to Torvalds.