undefined | Better HN

0 pointsdzaima2y ago0 comments

UB is far from the only source of systems not doing the desired thing - writing code that ends up at UB is as wrong as writing code that was written with an incorrect understanding of the invoked behavior.

Sure, the neat trick of a+1<a not working is perhaps undesirable, but, even if signed addition was defined to wrap, in most contexts an "a+1" subtracting four billion is not gonna be the specific thing you want it to do in your system.

Alternatively, signed overflow could be defined to return exactly 31415, which would be very concrete defined behavior, but barely if at all more useful compared to it being UB.

0 comments

3 comments · 1 top-level

AlotOfReading2y ago· 2 in thread

I hope I didn't imply that UB was the only source of bugs. It obviously isn't. It's just the only source of bugs that has the side effect of undefining the semantics of all your other code.

Just for fun let's take your example and say signed overflow returns integer pi. That now means the compiler has to implement your (hypothetical) next line checking if the result is 31415 rather than omitting it under the assumption that it's unreachable because it would imply UB. All of that code suddenly has defined behavior, even if it's silly.

dzaimaOP2y ago

But what does it get you that it's a "defined but completely unusable value" versus "undefined"? Indexing an array by it, adding it to some previously-meaningful value, or doing anything else with it, is still gonna all do practically arbitrary things.

I suppose in some cases it can lead to bugs being harder to exploit, but it's still a bug and still wrong and still should be fixed. Being defined is not a get out of exploitability free card.

(ok I do have one case where "defined but completely arbitrary" is actually meaningful over "undefined" with no reasonable alternative in C - for a floating-point x, "x==(int)x" for checking if x exactly fits in an int - e.g. gcc on aarch64 or x86+AVX (requiring -fno-trapping-math for whatever reason) optimizes that to "x==floor(x)" as an fp-to-integer cast is undefined on overflowing result)

AlotOfReading2y ago

It means you could know what the code will do, that's it. Even that's useful though. It means you can write complete formal models of the language and apply them against your code. The current situation is that you can only build partial formal models, and the assumptions those models rely on evaporate in the presence of UB. It's a really shitty way to do proofs.

Not knowing what the code will do also means that most of the safety critical code in your life is verified through a checkbox that essentially says "I promise there's no undefined behavior". For example, here's what MISRA says about undefined behavior:

    Rule 1.3: There shall be no occurrence of undefined or critical unspecified behaviour

    Analysis: Undecidable, System

It'd be nice to have at least the potential to analyze the code both as one of the people writing safety-critical code and a person who uses cars, planes, trains, etc.

1 more reply

j / k navigate · click thread line to collapse