Comment by tester89

The FSF has always been pretty clear on this: you use a linker (static or dynamic) = it applies; you don't = it doesn't. They even wrote LGPL with this distinction in mind, and introduced exceptions to yacc (bison) to accommodate non-free software.

In case of binary releases, you can request the sources of the relevant subcomponent (e.g. the kernel). The component boundaries are pretty clear wrt Linux: Torvalds has made it quite clear early on, that the kernel's GPL2 does not apply to anything in the user space.

Here also, the important distinction between GPL 2 & 3: with GPL3, it would be a breach of the license to ship code on a device that does not allow the end user to update that code. Which has effectively pushed everyone away from GPL3-licensed software.

IMHO the move to GPL3 has likely caused more harm than good to the FOSS ecosystem; in some alternative universe, GPL3 never happened, most of Android's userspace is GPL2, and we get the source for everything. In both universes we still don't get to deploy changes to devices we own, so IMHO the GPL3 won us nothing.

The short answer is copyright law and jurisprudence. The whole purpose of copyleft is to flip draconian copyright regimes over and make them protect users instead, so the GPL generally has the most maximalist stance allowed by copyright law. If copyright law would say that combining or extending software in a particular way is not fair use then the GPL generally would render the combination GPL.

In practice, GPLv2 would not be viral in the way you describe unless you can show that all of Android is a derived work of Linux (not true). GPLv3 would require users be avle to replace components under said license which has an impact on how such an appliance need to work (though the GPLv2 does also have somewhat related text about "the scripts which control installation") but wouldn't expand the scope of code under the license, just the terms.

> the OS is effectively one single unit: all linked together

If your appliance runs linux it has separate components just like desktop linux.

You want to do as little as possible in kernel space, and depending on the appliance there isn't even any need for it.

So, like desktop linux, you can have closed source binaries on top of the kernel.

There's a concept of "separate works", see for example https://www.gnu.org/licenses/gpl-faq.html#GPLCompatInstaller .

Tangentially, I assumed that the GPL must have some built-in exception for running non-GPL userspace programs on top of a GPLed kernel (similar to the System Library exception). However, it seems like it doesn't, since the Linux kernel has its own exception to allow this: https://spdx.org/licenses/Linux-syscall-note.html.

> when does GPL stop being infectious?

Either a) when the license has an explicit exemption (such as at glibc or the kernel's userspace interfaces) or b) when something ceases to be a "derivative work" in copyright terms (which is ultimately a legal question for lawyers).