Comment by NobodyNada
3 days ago
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.
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/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.
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.
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
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> And the rules of Rust have tripped up even senior Rust developers.
Yeah, even senior Rust devs make mistakes. Thanks to Miri, we can catch such mistakes. No reasonable person would expect even senior Rust devs to be magic superheroes that can write tricky unsafe code without making any mistake.
How confident are you that glibc has zero Undefined Behavior? I rather doubt it. The Rust standard library has its entire test suite (well, almost everything, except for some parts in std::fs and std::net) run through Miri. That's not a proof there's no UB in corner cases not covered by the tests, but it means we are much, much more likely to find such bugs and fix them than comparable C code.
C has an opt-out that works sometimes, if a compatible type exists. Rust has an opt-out that works always: use raw pointers (or interior mutable shared references) for all accesses, and you can stop worrying about aliasing altogether.
I think that way of describing it is really weird.
In C, you do not use any special keywords to opt into or opt out of TBAA, instead it is the rule by default that one must follow. I do not consider that 'opting out'. One can disable that in some compilers by disabling 'strict aliasing', as the Linux kernel does, but that is usually on a whole-program basis and not standard.
In Rust, using raw pointers is using a different mechanism, and mutable references are always 'no aliasing'.
An example of opting in would be C's "restrict" keyword, where one opts into a similar constraint to that of Rust's 'no aliasing' for mutable references.
>use raw pointers (or interior mutable shared references) for all accesses, and you can stop worrying about aliasing altogether.
And dereferencing a raw pointer requires 'unsafe', right? And if one messee the rules up for it, theN UB.
Can you confirm that the interaction between raw pointers and mutable references still requires care? Is this comment accurate?
>It is safe to hold a raw pointer, const T or mut T, at the same time as a mutable reference, &mut T, to the same data. However, it is Undefined Behaviour if you deference that raw pointer while the mutable reference is still live.
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the opt-out in C is to use char*, which is specifically allowed to alias other types.
There are also GCC extensions to build other types that may alias.
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Thank you for your kind words. :)
One example of MIRI not being guaranteed to handle all cases.
https://github.com/rust-lang/rust/pull/139553#issuecomment-2...
The above issue was part of diagnosing UB in Rust stdlib.
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. ;)
>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.
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