Copyleft-next and the kernel

But as a rule of thumb, the threshold of originality is generally quite low. If you just fixed a typo, you probably can't claim copyright on the fix alone. If you (re)wrote or substantially refactored* a whole function, you probably can claim copyright on that.

* You might argue that refactoring should not be eligible for copyright protection because it makes no functional change to the software. This is incorrect. Copyright does not care at all about functionality, and "it's functional" is in fact an argument *against* copyright protection (e.g. you might not be able to copyright a "hello world" program, because there's usually only one reasonable way to write such a program). Copyright protects the creative expression of an idea or concept (but not the idea or concept itself); refactoring code to make it clearer or easier for a human to understand is definitely the right sort of work to merit copyright protection. The only thing that matters is scale: whether you refactored enough code and made enough changes to that code.