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> This is a great way to see why invalid UTF-8 strings and unicode chars cause undefined behaviour in Rust. What does "undefined behavior" mean without a spec?

> This is a great way to see why invalid UTF-8 strings and unicode chars cause undefined behaviour in Rust. What does "undefined behavior" mean without a spec? Wouldn't the behavior rustc produces today be de-facto defined behavior? It seems like the contention is violating some transmute constraint, but does this not result in reproducible runtime behavior? In what context are you framing "soundness"? EDIT: I'm honestly befuddled why anyone would downvote this. I certainly don't think this is detracting from the conversation at all—how can you understand the semantics of the above comment without understanding what the intended meaning of "undefined behavior" or "soundness" is?

mmastrac 1y ago
> What does "undefined behavior" mean without a spec? While not as formalized as C/C++, Rust's "spec" exists in the reference, nomicon, RFCs and documentation. I believe that there is a desire for a spec, but enough resources exist that the community can continue without one with no major negative side-effects (unless you want to re-implement the compiler from scratch, I suppose). The compiler may exploit "lack of UB" for optimizations, e.g., using a known-invalid value as a niche, optimizing away safety checks, etc. > Wouldn't the behavior rustc produces today be de-facto defined behavior? Absolutely not. Bugs are fixed and the behaviour changes. Not often, but it happens. This post probably answers a lot of your reply as well: https://jacko.io/safety_and_soundness.html https://jacko.io/safety_and_soundness.html
- NoTeslaThrow 1y ago
  EDIT: > While not as formalized as C/C++, Rust's "spec" exists in the reference, nomicon, RFCs and documentation. I believe that there is a desire for a spec, but enough resources exist that the community can continue without one with no major negative side-effects (unless you want to re-implement the compiler from scratch, I suppose). Thank you, I was unaware that this is a thing. > This post probably answers a lot of your reply as well: https://jacko.io/safety_and_soundness.html https://jacko.io/safety_and_soundness.html This appears to also rely on "undefined behavior" as a meaningful term.
  - mmastrac 1y ago
    > This appears to also rely on "undefined behavior" as a meaningful term. I assure you it is a meaningful term: https://llvm.org/docs/UndefinedBehavior.html https://llvm.org/docs/UndefinedBehavior.html
    - NoTeslaThrow 1y ago
      Ok, but in the context of the language at hand? Presumably the IR has distinct semantics from the language that generates the IR. Does UB just strictly resolve to LLVM UB? That's very reasonable!
      - fc417fc802 1y ago
        No. UB is a term of art here. Consider a hypothetical non-LLVM full reimplementation of the compiler. If it optimizes and there are invalid assumptions then there is likely UB. LLVM isn't involved in that case though.
        NoTeslaThrow 1y ago
        > If it optimizes and there are invalid assumptions then there is likely UB. It's the distinguishing from bugs that concerns me.
        lmm 1y ago
        Anything a compiler does with code which is UB is not a bug in the compiler. That's pretty much the definition of UB.
        fc417fc802 1y ago
        I don't follow. Isn't UB a subset of bugs or alternatively a follow on consequence that causes observable behavior to further deviate?
        NoTeslaThrow 1y ago
newpavlov 1y ago
"Undefined behavior" means that the compiler can apply optimizations which assume that it does not happen, resulting in an incorrect code. A simpler example is `Option<NonZeroU8>`, the compiler assumes that `NonZeroU8` can never contain 0, thus it can use 0 as value for `None`. Now, if you take a reference to the inner `NonZeroU8` stored in `Some` and write 0 to it, you changed `Some` to `None`, while other optimizations may rely on the assumption that references to the content of `Some` can not flip the enum variant to `None`. You don't need a full language spec to declare something UB. And, arguably, from the compiler correctness perspective, there is no fundamental difference between walls of prose in the C/C++ "spec" and the "informal spec" currently used by Rust. (Well, there is the CompCert exception, but it's quite far from the mainstream compilers in many regards)
- NoTeslaThrow 1y ago
  > resulting in an incorrect code. Incorrect with respect to an assumption furnished where? Your sibling comment mentions RFCs—is this behavior tied to some kind of documented expectation? > A simpler example is `Option<NonZeroU8>`, the compiler assumes that `NonZeroU8` can never contain 0, thus it can use 0 as value for `None`. Now, if you take a reference to the inner `NonZeroU8` stored in `Some` and write 0 to it, you changed `Some` to `None`, while other optimizations may rely on the assumption that references to the content of `Some` can not flip the enum variant to `None`. That seems to be the intended behavior, unless I'm reading incorrectly. Why else would you write a 0 to it? Also, does this not require using the `unsafe` keyword? So is tricking the compiler into producing the behavior you described not the expected and intended behavior?
  - newpavlov 1y ago
    >Incorrect with respect to an assumption furnished where? In the definition of the `NonZeroU8` type. Or in a more practical terms, in LLVM, when we generate LLVM IR we communicate this property to LLVM and it in turn uses it to apply optimizations to our code. >Also, does this not require using the `unsafe` keyword? Yes, it requires `unsafe` and the point is that writing 0 to `NonZeroU8` is UB since it breaks the locality principle critical for correctness of optimizations. Applying just one incorrect (because of the broken assumption) optimization together with numerous other (correct) optimizations can easily lead to very surprising results, which are practically impossible to predict and debug. This is why it's considered such anathema to have UB in code, since having UB in one place may completely break code somewhere far away.
deleted 1y ago
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duckerude 1y ago
It means that anything strange that happens next isn't a language bug. Whether something is a bug or not is sometimes hard to pin down because there's no formal spec. Most of the time it's pretty clear though. Most software doesn't have a formal spec and manages to categorize bugs anyway.
- NoTeslaThrow 1y ago
  > It means that anything strange that happens next isn't a language bug. This is even more vague. The language is getting blamed regardless. This makes no sense.
  - dwattttt 1y ago
    No: the language defined that e.g. a NonZeroU8 can't contain 0, and the only way it could is via illegal means. You don't need a formal proof to describe that. To try to characterise what any compiler, hypothetical or not, does if you nonetheless produce one (again, via means that aren't valid) isn't meaningful.
ben0x539 1y ago
> I'm honestly befuddled why anyone would downvote this. I think there's two parts to this. First, there's a bit of a history of people making disingenious jabs at Rust for not having an "ISO C++" style spec. Typically people would try to suggest that Rust can't be ready for production or shouldn't receive support in other ecosystems without being certified by some manner of international committee. Second, Rust by now has an extensive tradition of people discussing memory safety invariants, what soundness means, formal models of what is a valid memory access, desirable optimizations, etc, etc, so your question what undefined behavior means could be taken to be, like, polemically reductive or dismissive. In context I don't think it's what you're doing, but I would also not be surprised if a lot of people reading Rust-related HN discussions are just super tired of anything that even slightly looks like an effort to re-litigate undefined behavior from first principles, because it tends to derail more specific discussions.
- NoTeslaThrow 1y ago
  Tbh, I just really hate the term "undefined behavior". It really feels like laziness in terms of what the possible damage might entail.
  - arlort 1y ago
    It is a term of art in compilers/language design though, isn't it? If you break an invariant the compiler is relying on for optimization then you can't say for sure what the effect after all optimisation passes or in future versions of the compiler will be. It's just "undefined"
  - imtringued 1y ago
    Yeah I personally think the problem isn't undefined behavior itself, but the C development culture where undefined behavior is sprinkled all over the language to the point where it has become unavoidable plus the inevitable assignment of blame onto C developers, because everyone knows there is enough time in the day for fuzzing your entire code base.
  - Dylan16807 1y ago
    In a situation like this, causing UB is basically saying you deliberately corrupted your memory. How are you supposed to be specific about what the possible damage might entail for corrupted memory? If you have a function with an "if" or a "while" or a "switch" in it, and you break the variable being evaluated, you might cause the program to skip over the choices and run whatever happens to be next in memory. What's the non-lazy listing of possible outcomes at that point?

mmastrac 1y ago

> What does "undefined behavior" mean without a spec? While not as formalized as C/C++, Rust's "spec" exists in the reference, nomicon, RFCs and documentation. I believe that there is a desire for a spec, but enough resources exist that the community can continue without one with no major negative side-effects (unless you want to re-implement the compiler from scratch, I suppose). The compiler may exploit "lack of UB" for optimizations, e.g., using a known-invalid value as a niche, optimizing away safety checks, etc. > Wouldn't the behavior rustc produces today be de-facto defined behavior? Absolutely not. Bugs are fixed and the behaviour changes. Not often, but it happens. This post probably answers a lot of your reply as well: https://jacko.io/safety_and_soundness.html https://jacko.io/safety_and_soundness.html

NoTeslaThrow 1y ago
EDIT: > While not as formalized as C/C++, Rust's "spec" exists in the reference, nomicon, RFCs and documentation. I believe that there is a desire for a spec, but enough resources exist that the community can continue without one with no major negative side-effects (unless you want to re-implement the compiler from scratch, I suppose). Thank you, I was unaware that this is a thing. > This post probably answers a lot of your reply as well: https://jacko.io/safety_and_soundness.html https://jacko.io/safety_and_soundness.html This appears to also rely on "undefined behavior" as a meaningful term.
- mmastrac 1y ago
  > This appears to also rely on "undefined behavior" as a meaningful term. I assure you it is a meaningful term: https://llvm.org/docs/UndefinedBehavior.html https://llvm.org/docs/UndefinedBehavior.html
  - NoTeslaThrow 1y ago
    Ok, but in the context of the language at hand? Presumably the IR has distinct semantics from the language that generates the IR. Does UB just strictly resolve to LLVM UB? That's very reasonable!
    - fc417fc802 1y ago
      No. UB is a term of art here. Consider a hypothetical non-LLVM full reimplementation of the compiler. If it optimizes and there are invalid assumptions then there is likely UB. LLVM isn't involved in that case though.
      - NoTeslaThrow 1y ago
        > If it optimizes and there are invalid assumptions then there is likely UB. It's the distinguishing from bugs that concerns me.
        lmm 1y ago
        Anything a compiler does with code which is UB is not a bug in the compiler. That's pretty much the definition of UB.
        fc417fc802 1y ago
        I don't follow. Isn't UB a subset of bugs or alternatively a follow on consequence that causes observable behavior to further deviate?
        NoTeslaThrow 1y ago

newpavlov 1y ago

"Undefined behavior" means that the compiler can apply optimizations which assume that it does not happen, resulting in an incorrect code. A simpler example is `Option<NonZeroU8>`, the compiler assumes that `NonZeroU8` can never contain 0, thus it can use 0 as value for `None`. Now, if you take a reference to the inner `NonZeroU8` stored in `Some` and write 0 to it, you changed `Some` to `None`, while other optimizations may rely on the assumption that references to the content of `Some` can not flip the enum variant to `None`. You don't need a full language spec to declare something UB. And, arguably, from the compiler correctness perspective, there is no fundamental difference between walls of prose in the C/C++ "spec" and the "informal spec" currently used by Rust. (Well, there is the CompCert exception, but it's quite far from the mainstream compilers in many regards)

NoTeslaThrow 1y ago
> resulting in an incorrect code. Incorrect with respect to an assumption furnished where? Your sibling comment mentions RFCs—is this behavior tied to some kind of documented expectation? > A simpler example is `Option<NonZeroU8>`, the compiler assumes that `NonZeroU8` can never contain 0, thus it can use 0 as value for `None`. Now, if you take a reference to the inner `NonZeroU8` stored in `Some` and write 0 to it, you changed `Some` to `None`, while other optimizations may rely on the assumption that references to the content of `Some` can not flip the enum variant to `None`. That seems to be the intended behavior, unless I'm reading incorrectly. Why else would you write a 0 to it? Also, does this not require using the `unsafe` keyword? So is tricking the compiler into producing the behavior you described not the expected and intended behavior?
- newpavlov 1y ago
  >Incorrect with respect to an assumption furnished where? In the definition of the `NonZeroU8` type. Or in a more practical terms, in LLVM, when we generate LLVM IR we communicate this property to LLVM and it in turn uses it to apply optimizations to our code. >Also, does this not require using the `unsafe` keyword? Yes, it requires `unsafe` and the point is that writing 0 to `NonZeroU8` is UB since it breaks the locality principle critical for correctness of optimizations. Applying just one incorrect (because of the broken assumption) optimization together with numerous other (correct) optimizations can easily lead to very surprising results, which are practically impossible to predict and debug. This is why it's considered such anathema to have UB in code, since having UB in one place may completely break code somewhere far away.

deleted 1y ago

[deleted]

duckerude 1y ago

It means that anything strange that happens next isn't a language bug. Whether something is a bug or not is sometimes hard to pin down because there's no formal spec. Most of the time it's pretty clear though. Most software doesn't have a formal spec and manages to categorize bugs anyway.

NoTeslaThrow 1y ago
> It means that anything strange that happens next isn't a language bug. This is even more vague. The language is getting blamed regardless. This makes no sense.
- dwattttt 1y ago
  No: the language defined that e.g. a NonZeroU8 can't contain 0, and the only way it could is via illegal means. You don't need a formal proof to describe that. To try to characterise what any compiler, hypothetical or not, does if you nonetheless produce one (again, via means that aren't valid) isn't meaningful.

ben0x539 1y ago

> I'm honestly befuddled why anyone would downvote this. I think there's two parts to this. First, there's a bit of a history of people making disingenious jabs at Rust for not having an "ISO C++" style spec. Typically people would try to suggest that Rust can't be ready for production or shouldn't receive support in other ecosystems without being certified by some manner of international committee. Second, Rust by now has an extensive tradition of people discussing memory safety invariants, what soundness means, formal models of what is a valid memory access, desirable optimizations, etc, etc, so your question what undefined behavior means could be taken to be, like, polemically reductive or dismissive. In context I don't think it's what you're doing, but I would also not be surprised if a lot of people reading Rust-related HN discussions are just super tired of anything that even slightly looks like an effort to re-litigate undefined behavior from first principles, because it tends to derail more specific discussions.

NoTeslaThrow 1y ago
Tbh, I just really hate the term "undefined behavior". It really feels like laziness in terms of what the possible damage might entail.
- arlort 1y ago
  It is a term of art in compilers/language design though, isn't it? If you break an invariant the compiler is relying on for optimization then you can't say for sure what the effect after all optimisation passes or in future versions of the compiler will be. It's just "undefined"
- imtringued 1y ago
  Yeah I personally think the problem isn't undefined behavior itself, but the C development culture where undefined behavior is sprinkled all over the language to the point where it has become unavoidable plus the inevitable assignment of blame onto C developers, because everyone knows there is enough time in the day for fuzzing your entire code base.
- Dylan16807 1y ago
  In a situation like this, causing UB is basically saying you deliberately corrupted your memory. How are you supposed to be specific about what the possible damage might entail for corrupted memory? If you have a function with an "if" or a "while" or a "switch" in it, and you break the variable being evaluated, you might cause the program to skip over the choices and run whatever happens to be next in memory. What's the non-lazy listing of possible outcomes at that point?