Magic exists by necessity alone

Magic exists by necessity alone

Posted Feb 13, 2008 0:19 UTC (Wed) by jd (guest, #26381)
In reply to: This code is there not for performance by khim
Parent article: vmsplice(): the making of a local root exploit

Well, yes, but ideally you'd have some kind of abstraction. Since the numbers and arithmetic are "magic", they must also be impermanent and subject to continual experimentation. Possibly by a coder, possibly by the person compiling, possibly by the OS itself. As such, those need to be the components that are most easily identified and changed in a consistant way - in terms of calculations and type ranges.

Making this "pure" beyond a certain level is, agreed, problematic. You don't have infinite CPU cycles and although the compiler can do some optimization, it can't turn an abstract, general solution into something tuned to a considerably narrower range of special cases that are usable in practice that are further constrained by the implementation details of a given architecture, which is all any real-world physical computer can ever be.

A way round this would be to have some sort of hypothetical generic architecture, which implemented the formal "pure" solution but was never - and could never - actually used in practice. As all usable architectures would necessarily be perfect subsets of the "pure" solution, it no longer matters if it is easy to read, you know it's just a re-arranged and contracted form.

However, here you run into a problem. This assumes a totally generic "pure" solution even exists, wholly independent of any architecture. Since you can't use such a solution, test-run such a solution or in many cases reverse-map onto such a solution, it's not obvious you could ever show a pure solution was indeed generic or was the solution the specific implementations were specific implementations of.

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