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As someone who has been writing a lot of unsafe Rust (mostly in an embedded context), I'm thrilled about and thankful for the work that you, your students, and

As someone who has been writing a lot of unsafe Rust (mostly in an embedded context), I'm thrilled about and thankful for the work that you, your students, and the opsem team are doing. When you're working with anything below the application level, C's confusing and underspecified rules about UB are almost impossible to keep track of, especially when it comes to aliasing and volatile/MMIO. The spec is so difficult to read and full of complicated cross-references that to actually get a practical answer you have to look for a random Stack Overflow post that may or may not have a correct interpretation of the spec, and may or may not address your specific problem. Rust right now feels a lot harder to work with, because the spec isn't done. When you have a concrete question about a piece of code, like "is this conversion from an &mut to a *mut and back sound", and you try to look for documentation on it, you get either "Nobody knows, Rust aliasing model isn't defined"; a hand-wavy explanation that is not rigorous or specific; or a model like Stack Borrows or Tree Borrows that's defined a little too formally for easy digestion :) But when I really started digging, I realized just how much cleaner Rust's semantics are. References aren't actually hard, Tree Borrows basically boils down to "while an &mut reference is live, you can only access the value through pointers or references derived from that reference". Pointer operations have straightforward semantics, there's no confusing notions of typed memory, and no UB "just because" for random things like integer overflow. It's just so much less complicated to understand than C's abstract machine. I'm really looking forward to things like MiniRust, and to an aliasing model making it into the Reference / other documentation, because at that point I feel like unsafe Rust will be way easier to write confidently and correctly than C. Congrats on the publication, and thanks again for the work you all have put into this.

gronpi 1y ago
In C, you can alias pointers if they have compatible types. Not the case in Rust for mutable references. And the rules of Rust have tripped up even senior Rust developers. https://github.com/rust-lang/rust/commit/71f5cfb21f3fd2f1740bced061c66ff112fec259 https://github.com/rust-lang/rust/commit/71f5cfb21f3fd2f1740... Without MIRI, a lot of Rust developers would be lost, as they do not even attempt to understand unsafe. And MIRI cannot and does not cover everything, no matter how good and beloved it is. It should have been possible for senior Rust developers to write UB-free code without having to hope that MIRI saves them.
- GolDDranks 1y ago
  The situation is not that bad. The rules of unsafe code were pretty badly defined back then, but they are in process of becoming a lot clearer, and like the grandparent argues, with a well-defined aliasing model like Tree Borrows, they are easier to understand than C's. If you look into the code you linked, the problem was about accessing undefined bytes though an aliased, differently-typed pointer – something you would have hard time doing in C to begin with. MaybeUninit was a new thing back then. I think that nowadays, a senior Rust developer would clear the hurdles better.
  - hamcocar 1y ago
    I am very sorry, but you do not address that TBAA, like C has by default, generally is easier than just no aliasing, like what Rust has for mutable references. This is a major difference. C code can opt into a similar kind of aliasing, namely by using _restrict_, but that is opt-in, while it is always on for Rust. And there is newer UB as well in Rust stdlib https://github.com/rust-lang/rust/pull/139553 https://github.com/rust-lang/rust/pull/139553
    - simonask 1y ago
      How is that bug in the stdlib related to any of this?
    - GolDDranks 1y ago
      > TBAA, like C has by default, generally is easier than just no aliasing [note: I assume no aliasing here to refer Rust's mutable references, not C's restrict] I don't accept that statement. Rust's mutable references are statically checked, so they are impossible to get wrong (modulo compiler bugs) if you aren't using unsafe code. Using raw pointers has no no-aliasing requirements, so again, it's easier than in C. The hard part is mixing mutable references and raw pointers. In 95% of Rust code, you are not required to do that, and you shouldn't do that. In the remaining 5%, you should understand the aliasing model. In that case, indeed, you need to know more than what TBAA requires you to know. But that's for the case where you can also _DO_ more than TBAA would allow you to do.
gronpi 1y ago
One example of MIRI not being guaranteed to handle all cases. https://github.com/rust-lang/rust/pull/139553#issuecomment-2791095042 https://github.com/rust-lang/rust/pull/139553#issuecomment-2... The above issue was part of diagnosing UB in Rust stdlib.
- ralfj 1y ago
  Yeah, concurrency bugs that only occur in very specific situations are hard to track down with a pure testing tool. However, we have some ongoing work that should make Miri a lot better at this... we are just not sure yet whether we can get it to have usable performance. ;)
  - hamcocar 1y ago
    >Yeah, concurrency bugs that only occur in very specific situations are hard to track down with a pure testing tool. It would have been better to have prevented it in the first place, and it is inconsistent with "fearless concurrency" when the Rust stdlib has UB. And there are categories of cases that Miri does not handle either, FFI being a clear example as far as I know.
    - ralfj 1y ago
      > it is inconsistent with "fearless concurrency" when the Rust stdlib has UB. It is not. "fearless X" only applies to safe code. Rust was always very clear about that. It turns out that in practice, if >90% of your code are "fearless", that actually significantly reduces the amount of bugs you have to worry about, even if the remaining 10% can still have UB. (I don't have hard numbers on how much code is unsafe, AFAIK it is much less than 10%.) But yeah, Miri cannot find all UB bugs. We are also very clear about that. But you don't have to find all UB bugs to make progress in this space compared to the status quo in C/C++.
ralfj 1y ago
Thank you for your kind words. :)

gronpi 1y ago

In C, you can alias pointers if they have compatible types. Not the case in Rust for mutable references. And the rules of Rust have tripped up even senior Rust developers. https://github.com/rust-lang/rust/commit/71f5cfb21f3fd2f1740bced061c66ff112fec259 https://github.com/rust-lang/rust/commit/71f5cfb21f3fd2f1740... Without MIRI, a lot of Rust developers would be lost, as they do not even attempt to understand unsafe. And MIRI cannot and does not cover everything, no matter how good and beloved it is. It should have been possible for senior Rust developers to write UB-free code without having to hope that MIRI saves them.

GolDDranks 1y ago
The situation is not that bad. The rules of unsafe code were pretty badly defined back then, but they are in process of becoming a lot clearer, and like the grandparent argues, with a well-defined aliasing model like Tree Borrows, they are easier to understand than C's. If you look into the code you linked, the problem was about accessing undefined bytes though an aliased, differently-typed pointer – something you would have hard time doing in C to begin with. MaybeUninit was a new thing back then. I think that nowadays, a senior Rust developer would clear the hurdles better.
- hamcocar 1y ago
  I am very sorry, but you do not address that TBAA, like C has by default, generally is easier than just no aliasing, like what Rust has for mutable references. This is a major difference. C code can opt into a similar kind of aliasing, namely by using _restrict_, but that is opt-in, while it is always on for Rust. And there is newer UB as well in Rust stdlib https://github.com/rust-lang/rust/pull/139553 https://github.com/rust-lang/rust/pull/139553
  - simonask 1y ago
    How is that bug in the stdlib related to any of this?
  - GolDDranks 1y ago
    > TBAA, like C has by default, generally is easier than just no aliasing [note: I assume no aliasing here to refer Rust's mutable references, not C's restrict] I don't accept that statement. Rust's mutable references are statically checked, so they are impossible to get wrong (modulo compiler bugs) if you aren't using unsafe code. Using raw pointers has no no-aliasing requirements, so again, it's easier than in C. The hard part is mixing mutable references and raw pointers. In 95% of Rust code, you are not required to do that, and you shouldn't do that. In the remaining 5%, you should understand the aliasing model. In that case, indeed, you need to know more than what TBAA requires you to know. But that's for the case where you can also _DO_ more than TBAA would allow you to do.

gronpi 1y ago

One example of MIRI not being guaranteed to handle all cases. https://github.com/rust-lang/rust/pull/139553#issuecomment-2791095042 https://github.com/rust-lang/rust/pull/139553#issuecomment-2... The above issue was part of diagnosing UB in Rust stdlib.

ralfj 1y ago
Yeah, concurrency bugs that only occur in very specific situations are hard to track down with a pure testing tool. However, we have some ongoing work that should make Miri a lot better at this... we are just not sure yet whether we can get it to have usable performance. ;)
- hamcocar 1y ago
  >Yeah, concurrency bugs that only occur in very specific situations are hard to track down with a pure testing tool. It would have been better to have prevented it in the first place, and it is inconsistent with "fearless concurrency" when the Rust stdlib has UB. And there are categories of cases that Miri does not handle either, FFI being a clear example as far as I know.
  - ralfj 1y ago
    > it is inconsistent with "fearless concurrency" when the Rust stdlib has UB. It is not. "fearless X" only applies to safe code. Rust was always very clear about that. It turns out that in practice, if >90% of your code are "fearless", that actually significantly reduces the amount of bugs you have to worry about, even if the remaining 10% can still have UB. (I don't have hard numbers on how much code is unsafe, AFAIK it is much less than 10%.) But yeah, Miri cannot find all UB bugs. We are also very clear about that. But you don't have to find all UB bugs to make progress in this space compared to the status quo in C/C++.

ralfj 1y ago

Thank you for your kind words. :)