I once wrote a paper called "Type Integer Considered Harmful". But I had that argument with other people in the formal verification community. Well, we didn't, back in the early 1980s. The formal verification community ignored integer overflow for far too long Rust offers some functions for it and it can often be less problematic than wrapping, for instance when dealing with the length of things.Įxplicitly ranged integers as seen in Ada are also very nice, there’s no reason that traps should only occur at the 32-bit or 64-bit boundaries. I also found the article lacking because it doesn't talk about saturation arithmetics: I haven't seen this attack mentioned before and it's probably extremely unlikely to happen in the wild, but it's possible. This prints some ASLR-protected pointer which you can use to calculate the offset, which allows you to break ASLR. This can be a pointer that will be leaking info about ASLR or a register that can contain user controlled information.Īs an example compile this with optimisations (should work the same with signed integer overflow): If you make a function call with an `undef` value you will pass what's in the register used according to the calling convention before. The backend subsequently will treat the `undef` value like it's in all registers. That's only true for signed integers, overflow is defined as wrapping for unsigned integers.Īnd even with signed integers the compilers handle it way worse than wrapping: When LLVM finds a overflow that will always happen, it won't report it (Only Clang will do that, but that's before any optimisations and thus can see far less) and optimize it to a `undef` value. Instead, if overflow occurs, Rust performs two’s complement wrapping ( add this: in case of signed integers and modular arithmetic in case of unsigned integers.> worse than C and C++’s UB-on-overflow which at least permits an implementation to trap. Isn't two's complement wrapping only applicable in case of signed numbers? How would the wrapping be called for unsigned numbers? Modular arithmetic? Does the reference need to be updated as in saying: Relying on integer overflow’s wrapping behavior is considered an error. The program won’t panic, but the variable will have a value that probably isn’t what you were expecting it to have. In the case of a u8, the value 256 becomes 0, the value 257 becomes 1, and so on. In short, values greater than the maximum value the type can hold “wrap around” to the minimum of the values the type can hold. Instead, if overflow occurs, Rust performs two’s complement wrapping. When you’re compiling in release mode with the -release flag, Rust does not include checks for integer overflow that cause panics. So do I understand right that in Rust signed integer overflows are defined as overflowing in regard two's complement wrapping or causing a panic in debug mode, while in C signed integer overflows are not well-defined? The Rust reference book regarding integer overflow gives only an example with u8 but talks about "two's complement wrapping". But it looks like it's C is the language with not-well-defined behavior here while Rust just exhibits different (but well-defined) behavior regarding whether in debug-mode or release-mode. I'm asking because a friend of mine was thinking the Rust rules on interger overflows (panicking in debug-mode, but wrapping around in release-mode) were somewhat awkward. A quick Wikipedia check confirms that, but does anyone have a more authoritative link on (signed) integer overflows being undefined in C? Exposing randomized base addresses through pointer leaksĪnyway, I'm very surprised that signed integer overflows in C are not defined as always wrapping around.The Rust compiler does not consider the following behaviors unsafe, though a programmer may (should) find them undesirable, unexpected, or erroneous. I guess calling something "undefined behavior" in C is usually different from when you call something UB in Rust? Can the "undefined behavior" in C affect anything else than the result of the integer computation? I don't think so, but the term "undefined behavior" is a bit vague here.Īlso note Rust's reference regarding behavior considered unsafe:
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