A while back I showed that it is possible to work around the anti-Tivoization provision of GPLv3. I've since realized that the GPL itself has a similar loophole.
The entire thing is predicated on the idea that if you change a program, you've created a "derived work" which therefore falls under the same copyright protection as the original. But is a change necessarily a derived work? If it doesn't include any of the original program, I don't think it necessarily would be if the change was distributed as a delta from the original. Normally a diff file says things like "in file x, look for the line that contains y and change it to z". Because this file will contain the line "y" (and even the filename "x", if it could be considered a creative work) it is a derived work of the original program. But one were to use a diff format that says "in the third file, change line 327 to z" that would contain no copyrighted information from the original, and could be distributed without restriction. There really isn't any difference between a code change and "mere aggregation".
What does this mean? Suppose you're Microsoft and you want to include GCC in Windows, modified with some proprietary algorithms and you don't want to release the source code. It turns out that this can be done. All Microsoft would have to do is include in the installer some code that will download the source (or binary) from a known location (perhaps from GNU servers, or a trusted third party's servers) patch it and (if necessary) compile it. Hey presto, there's the MS-GCC binary. The patch could be in the form of obfuscated code, object files or even (with some difficulty) binary deltas as long as care was taken to ensure none of the GNU-copyrighted works ended up in the Microsoft-distributed patch. Microsoft would not be committing copyright violation since none of the things they actually distributed contained any GNU-copyrighted code. The end-user would not be committing copyright violation either since they aren't distributing the software.
Essentially, with this hack, we've shown that the GPL is essentially equivalent to a BSD license. This isn't something that can be fixed by changing the license - it's a fundamental fact of copyright law.
The reason people don't do this is because it's more convenient to just release changes under GPL (so that a compiled binary of the aggregate can be released) than go through this hacky patching process. So it doesn't mean that copyleft isn't worth doing - in fact, I have heard it said that there are important parts of gcc which would never have been contributed if their authors' hands were not forced by the GPL.
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