By the people who made the changes but never had to release them, by NDAs that people sign before they work for web companies, etc. There is nothing in the GPLv2 that says, "All your changes must be released." What it says is that if you give a modified version to another person, your changes must be GPL'd also. Since web companies do not give modified versions of the kernel to others, they are not in any way obligated to make their changes available to others, and more often than not they do not make those changes available.

> By the people who made the changes but never had to release them, by NDAs that people sign before they work for web companies, etc.

This is complete nonsense. There is no proprietary, closed-source code in the Linux kernel.

Remember, recipients of GPL code (companies in this case) are not affected by the terms of the GPL until they redistribute the code themselves, until then they can add proprietary code all they want, run it on servers and never release a thing. They are completely free to USE the code however they wish.

So in this example, no cloud company has to release changes to their running linux kernel because it was never distributed to end users, or at all.

Same for all other GPL code running on servers, users of the service, loading web pages and using APIs, are not recipients of the code under the GPL, so they have no rights under the license.

If some company decides their changes to the kernel, or nginx, or apache, or php can be released because it won't destroy their business, they frequently do so. Otherwise you'll never hear about it and changes silently remain secret.

No, there is no proprietary code in the official kernel as distributed on kernel.org. There is a very long list of companies that have their own version of the kernel, which they keep to themselves, which contains their own proprietary changes, and which they never distribute to anyone else. There is no requirement whatsoever that you give your changes to Linux to anyone, there is only a requirement that your changes be GPL'd if you do choose to distribute them.

Feel free to change the subject, but don't pretend you're addressing the original topic, which is whether the Linux kernel is or is not open-source.