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Comment by gpderetta | Hacker News Reader

> Considering that it's Undefined Behavior, quite possibly.

Is it thought? Certainly it is according the C and C++ standards, but POSIX adds:

> References to unmapped addresses shall result in a SIGSEGV signal

While time-traveling-UB is a theoretical possibility, practically POSIX compliant compilers won't reorder around potentially trapping operations (they will do the reverse, they might remove a null check if made redundant by a prior potentially trapping dereference) .

A real concern is if a null pointer is dereferenced with a large attacker-controlled offset that can avoid the trap, but that's more of an issue of failing to bound check.

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Under your interpretation, neither gcc nor clang are POSIX compliant. Because in practice all these optimizing compilers will reorder memory accesses without bothering to prove that the pointers involved are valid -- the compiler just assumes that the pointers are valid, which is justified because otherwise the program would have undefined behavior.

Actually you are right, what I said about reordering is nonsense. The compiler will definitely reorder non-aliasing accesses. There are much weaker properties that are preserved.

I am not so sure. Assuming the program does not do anything undefined is sort of the worst possible take on leaving the behavior undefined in the first place. I mean the behavior was left undefined so that "something" could be done. the language standard just does not know what that "something" is. Hell the compiler could do nothing and that would make more sense.

But to make optimizations pretending it is an invariant, it can't happen, when the specification clearly says it could happen. That's wild, and I would argue out of specification.

Undefined Behavior has a very specific meaning in the C and (respectively) C++ standards. And it is this.

I think you may be misunderstand how optimization works. It's not "AHA! Undefined Behavior, I'll poke at that!".

It's more that compilers are allowed to assume that some things just "don't happen"... and can optimize based on that. And it does make sense... if you were told to optimize a thing given certain rules, how would you do it?

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