I understand "do not break userspace" differently than you do. You seem to understand it as "do not make a change that could hypothetically break userspace."

I understand "do not break userspace" differently than you do. You seem to understand it as "do not make a change that could hypothetically break userspace." I understand it as "do not make a change that is known to break userspace." There are hundreds, if not thousands, of little behaviors that a userspace program could hypothetically depend on. In many cases, it would be impossible to change things without breaking observed behavior. A great many changes fall in to the bucket "visibly changes behavior, but it is unlikely that anything depends on that specific behavior." When the assumption that no userspace program is broken turns out to be false, the correct response is to revert that change, and avoid breaking the userspace (hopefully this happens in an RC, before a stable release). What's incorrect, is to insist that the userspace program shouldn't have relied on that behavior. In the referenced rant ("Mauro, SHUT THE FUCK UP!"), the problem wasn't so much that Mauro had changed the errno used (though Linus did take issue with the new errno); the big issue was that when Mauro learned that the change broke pulseaudio, Mauro tried to argue that it was a bug in pulseaudio for relying on what the errno was.