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> have to imagine that in the general case it will be a translation to unsafe Rust, with occasional isolated leaf nodes being translated to safe Rust. That’s n

> have to imagine that in the general case it will be a translation to unsafe Rust, with occasional isolated leaf nodes being translated to safe Rust. That’s not what they are aiming for. FTA: “The goal is to achieve the same quality and style that a skilled Rust developer would produce” > just imagine how much harder it is to write automatic translation to borrow-checker-approved code that accounts for all the possible program space of C and all it's celebrated undefined behavior Nitpick: undefined behavior gives the compiler leeway in deciding what a program does, so the more undefined behavior a C program invokes, the easier it is to translate its code to rust. (Doing that translation in such a way that the behavior remains what gcc, clang or “most C compilers” do may be harder, but I’m not sure of that)

rectang 2y ago
> undefined behavior gives the compiler leeway in deciding what a program does, so the more undefined behavior a C program invokes, the easier it is to translate its code to rust. That's the kind of language lawyer approach that caused a rebellion in the last decade amongst C programmers against irresponsible compiler optimizations. "Who cares if your program actually works as intended? My optimization is legal according to the standard, it's your program that's written to exploit loopholes". I don't see any evidence that that's the attitude being taken by TRACTOR — I sure hope it isn't. But hell, even if the result is unreliable in practice, I suppose that if somebody gets to claim "it works" then the incentives are aligned to produce garbage.
- atiedebee 2y ago
  > Who cares if your program actually works as intended? My optimization is legal according to the standard, it's your program that's relying written to exploit loopholes". If your program invokes undefined behaviour, it's invalid and non-portable. Out of bounds array accesses are UB, yet a program containing them may just happen to work. It won't be portable even between different compiler versions. The C standard is a 2 way contract: the programmer doesn't produce code that invokes undefined behaviour, and the compiler returns a standard conforming executable
  - rectang 2y ago
    The C standard with its extensive undefined behavior causes programmers and compiler writers to be at odds. In a sane world, "undefined behavior" wouldn't be assumed to mean "the programmer must have meant for me to optimize this whole section of code away". We aren't on the same team, even if I believe that all parties are acting with the best of intentions. I don't feel that the Rust language situation incentivizes such awful conflict, and it's one of many reasons I now try really hard to avoid C and use Rust instead.
    - astrange 2y ago
      A funny thing about this problem is that it gets worse the more formally correct your implementation is. Undefined behavior is undefined, so it's outside the model, and if your program is a 100% correct implementation of a model then how can it know what to do about something outside it? But I don't think defining all behavior helps. The defined behavior could be /wrong/, and now you can't find it because the program using it is valid, so it can't be detected with UBSan.
  - Asooka 2y ago
    Doing one funny thing on platform A and a different funny thing on platform B when an edge case arises is way better than completely deleting the code on all platforms with no warning.
  - matheusmoreira 2y ago
    If undefined behavior is invalid, then reject the program instead of "optimizing" it. This "oh look undefined behavior I'm gonna turn the entire function into a no-op" nonsense is completely unacceptable. It's adversarial and borders on malicious. Null pointer check deletion can turn bugs into exploitable vulnerabilities.
    - zajio1am 2y ago
      > If undefined behavior is invalid, then reject the program instead of "optimizing" it. Undefined behavior is usually a result of runtime situation, it is usually not obvious from just the code whether it could or could not happen, so the compiler cannot reject the program. The 'UB-based' optimization is just assumption that the code is correct and therefore UB-situation could not happen in runtime.
- Someone 2y ago
  > I don't see any evidence that that's the attitude being taken by TRACTOR — I sure hope it isn't. I don’t see any way it can do otherwise. As a simple example, what would one translate this C statement to: int i; … i = abs(i); ? I would expect TRACTOR to generate (assuming 64-bit integers): let i: i64; … i = abs(i); However, that can panic in debug mode and return a negative number in release mode (https://doc.rust-lang.org/stable/std/primitive.i64.html#method.abs https://doc.rust-lang.org/stable/std/primitive.i64.html#meth...), and there’s no way for TRACTOR to know whether that makes the program “work as intended”. That code may have worked fine/fine enough) for decades because its standard library returns zero for abs(INT_MIN).
  - rectang 2y ago
    It's possible to preserve the semantics of the original program using unsafe Rust. [1] unsafe { let mut i: std::os::raw::c_int = std::mem::MaybeUninit::uninit().assume_init(); // ... i = libc::abs(i); } That's grotesque, but it is idiomatic Rust insofar as it lays bare many of the assumptions in the C code and gives the programmer the opportunity to fix them. It is what I would personally want TRACTOR to generate if it could not prove that `i` can never take on the value `libc::INT_MIN`. Given that generated code, I could then piecemeal migrate the unsafe bits to cleaner, idiomatic safe rust: possibly your code but more likely `i::wrapping_abs()` or similar. What will TRACTOR choose? At least for this example, they don't have to choose inappropriate pruning of undefined behavior. They claim the following: > The goal is to achieve the same quality and style that a skilled Rust developer would produce, thereby eliminating the entire class of memory safety security vulnerabilities present in C programs. If they're going to uphold the same "quality", the translation you presented doesn't cut it. But you may be right and they will go down the path of claiming that a garbage translation is technically valid under undefined behavior and therefore”quality” — if so, I will shun them. [1] https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=479a6c3b8fb0c51049cbc3ed28aadf74 https://play.rust-lang.org/?version=stable&mode=debug&editio...
    - artikae 2y ago
      The first line is already UB. `assume_init` requires the contents to be initialized, hence the name.
derdi 2y ago
> undefined behavior gives the compiler leeway in deciding what a program does, so the more undefined behavior a C program invokes, the easier it is to translate its code to rust. You assume that the compiler can determine what behavior is undefined. It can't. C compilers don't just look at some individual line of the program and say "oh, that's undefined, unleash the nasal demons". C compilers look at code, reason that if such-and-such variable has a certain value (say, a null or invalid pointer), then such-and-such operation is undefined (say, dereferencing that variable), and therefore on the next line that variable can be assumed not to have that bad value. Despite all the FUD, this is a very limited power. C compilers don't usually know the actual values in question, all they do is exclude some invalid ones.
- bigstrat2003 2y ago
  I (not the person you are replying to) do understand that's how compilers interact with UB. However, a wealth of experience has shown us that the assumption "UB doesn't occur" is completely false. It is, in my opinion, quite irresponsible for compiler writers to continue to use a known-false assumption when building the optimizer. I don't really care how much speed it costs, we need to stop building software on a shaky foundation like that.
  - astrange 2y ago
    Soon (or actually, already) we'll have MTE and CHERI, and then that C undefined behavior will be giving you security improvements as well as speed improvements. Can't design a system that 100% crashes on invalid behavior if you've declared that behavior is valid, because then someone is relying on it.

rectang 2y ago

> undefined behavior gives the compiler leeway in deciding what a program does, so the more undefined behavior a C program invokes, the easier it is to translate its code to rust. That's the kind of language lawyer approach that caused a rebellion in the last decade amongst C programmers against irresponsible compiler optimizations. "Who cares if your program actually works as intended? My optimization is legal according to the standard, it's your program that's written to exploit loopholes". I don't see any evidence that that's the attitude being taken by TRACTOR — I sure hope it isn't. But hell, even if the result is unreliable in practice, I suppose that if somebody gets to claim "it works" then the incentives are aligned to produce garbage.

atiedebee 2y ago
> Who cares if your program actually works as intended? My optimization is legal according to the standard, it's your program that's relying written to exploit loopholes". If your program invokes undefined behaviour, it's invalid and non-portable. Out of bounds array accesses are UB, yet a program containing them may just happen to work. It won't be portable even between different compiler versions. The C standard is a 2 way contract: the programmer doesn't produce code that invokes undefined behaviour, and the compiler returns a standard conforming executable
- rectang 2y ago
  The C standard with its extensive undefined behavior causes programmers and compiler writers to be at odds. In a sane world, "undefined behavior" wouldn't be assumed to mean "the programmer must have meant for me to optimize this whole section of code away". We aren't on the same team, even if I believe that all parties are acting with the best of intentions. I don't feel that the Rust language situation incentivizes such awful conflict, and it's one of many reasons I now try really hard to avoid C and use Rust instead.
  - astrange 2y ago
    A funny thing about this problem is that it gets worse the more formally correct your implementation is. Undefined behavior is undefined, so it's outside the model, and if your program is a 100% correct implementation of a model then how can it know what to do about something outside it? But I don't think defining all behavior helps. The defined behavior could be /wrong/, and now you can't find it because the program using it is valid, so it can't be detected with UBSan.
- Asooka 2y ago
  Doing one funny thing on platform A and a different funny thing on platform B when an edge case arises is way better than completely deleting the code on all platforms with no warning.
- matheusmoreira 2y ago
  If undefined behavior is invalid, then reject the program instead of "optimizing" it. This "oh look undefined behavior I'm gonna turn the entire function into a no-op" nonsense is completely unacceptable. It's adversarial and borders on malicious. Null pointer check deletion can turn bugs into exploitable vulnerabilities.
  - zajio1am 2y ago
    > If undefined behavior is invalid, then reject the program instead of "optimizing" it. Undefined behavior is usually a result of runtime situation, it is usually not obvious from just the code whether it could or could not happen, so the compiler cannot reject the program. The 'UB-based' optimization is just assumption that the code is correct and therefore UB-situation could not happen in runtime.
Someone 2y ago
> I don't see any evidence that that's the attitude being taken by TRACTOR — I sure hope it isn't. I don’t see any way it can do otherwise. As a simple example, what would one translate this C statement to: int i; … i = abs(i); ? I would expect TRACTOR to generate (assuming 64-bit integers): let i: i64; … i = abs(i); However, that can panic in debug mode and return a negative number in release mode (https://doc.rust-lang.org/stable/std/primitive.i64.html#method.abs https://doc.rust-lang.org/stable/std/primitive.i64.html#meth...), and there’s no way for TRACTOR to know whether that makes the program “work as intended”. That code may have worked fine/fine enough) for decades because its standard library returns zero for abs(INT_MIN).
- rectang 2y ago
  It's possible to preserve the semantics of the original program using unsafe Rust. [1] unsafe { let mut i: std::os::raw::c_int = std::mem::MaybeUninit::uninit().assume_init(); // ... i = libc::abs(i); } That's grotesque, but it is idiomatic Rust insofar as it lays bare many of the assumptions in the C code and gives the programmer the opportunity to fix them. It is what I would personally want TRACTOR to generate if it could not prove that `i` can never take on the value `libc::INT_MIN`. Given that generated code, I could then piecemeal migrate the unsafe bits to cleaner, idiomatic safe rust: possibly your code but more likely `i::wrapping_abs()` or similar. What will TRACTOR choose? At least for this example, they don't have to choose inappropriate pruning of undefined behavior. They claim the following: > The goal is to achieve the same quality and style that a skilled Rust developer would produce, thereby eliminating the entire class of memory safety security vulnerabilities present in C programs. If they're going to uphold the same "quality", the translation you presented doesn't cut it. But you may be right and they will go down the path of claiming that a garbage translation is technically valid under undefined behavior and therefore”quality” — if so, I will shun them. [1] https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=479a6c3b8fb0c51049cbc3ed28aadf74 https://play.rust-lang.org/?version=stable&mode=debug&editio...
  - artikae 2y ago
    The first line is already UB. `assume_init` requires the contents to be initialized, hence the name.

derdi 2y ago

> undefined behavior gives the compiler leeway in deciding what a program does, so the more undefined behavior a C program invokes, the easier it is to translate its code to rust. You assume that the compiler can determine what behavior is undefined. It can't. C compilers don't just look at some individual line of the program and say "oh, that's undefined, unleash the nasal demons". C compilers look at code, reason that if such-and-such variable has a certain value (say, a null or invalid pointer), then such-and-such operation is undefined (say, dereferencing that variable), and therefore on the next line that variable can be assumed not to have that bad value. Despite all the FUD, this is a very limited power. C compilers don't usually know the actual values in question, all they do is exclude some invalid ones.

bigstrat2003 2y ago
I (not the person you are replying to) do understand that's how compilers interact with UB. However, a wealth of experience has shown us that the assumption "UB doesn't occur" is completely false. It is, in my opinion, quite irresponsible for compiler writers to continue to use a known-false assumption when building the optimizer. I don't really care how much speed it costs, we need to stop building software on a shaky foundation like that.
- astrange 2y ago
  Soon (or actually, already) we'll have MTE and CHERI, and then that C undefined behavior will be giving you security improvements as well as speed improvements. Can't design a system that 100% crashes on invalid behavior if you've declared that behavior is valid, because then someone is relying on it.