Revisiting the MAP_SHARED_VALIDATE hack

One of the the most commonly repeated mistakes in system-call design is a failure to check for unknown flags wherever flags are accepted. If there is ever a point where callers can get away with setting unknown flags, then adding new flags becomes a hazardous act. In the case of mmap(), though, developers found a clever way around this problem. A recent discussion has briefly called that approach into question, though, and raised the issue of what constitutes a kernel regression. No changes are forthcoming as a result, but the discussion does provide an opportunity to look at both the specific hack and how the kernel community decides whether a change is a regression or not.

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