The EU law, also linked in the sibling comment, has some direct conditions. Interoperability of interfaces are allowed on the condition that it does not prejudi

The EU law, also linked in the sibling comment, has some direct conditions. Interoperability of interfaces are allowed on the condition that it does not prejudice the legitimate interest of the copyright holder, and it does not conflict with a normal exploitation of the program, and is independent. This is likely the reason why Linus toward had said that non-free drivers may be legal, may in other cases not be, and it depend on the exact details. A driver that itself was intertwined with the internals of the kernel, or would conflict with the exploitation of the linux kernel, or cause conflict with the legitimate interest of the copyright holders, might be a derivative work. The linux kernel has published clear boundaries for this (https://www.kernel.org/doc/html/latest/process/license-rules.html https://www.kernel.org/doc/html/latest/process/license-rules...), like the the syscall interface. Drivers that do not respect those boundaries may be more likely to fall outside of the EU law, but as with most legal discussions it would be a gliding scale. Readline is one of those more odd theoretical case where the specific main work is the library itself. This makes questions like "legitimate interest of the copyright holder" and "normal exploitation of the program" a bit more complex. It would however never become a real legal case since anyone accused of infringement can just replace the small library with an compatible alternative and stop distributing the GNU Readline library. Since compliance is generally the goal by FSF I doubt they or any company would be willing to spend money on lawyers to fight over it. In contrast, Unity sells their game engine library, so companies that tried to bypass copyright (by not paying) and "link" their games with existing installed versions on users computers would likely still end up in court. My money would also be on unity winning that battle.