I think Rust kinda has a marketing problem: the myth that "Writing in Safe Rust automatically makes your code memory-safe". It doesn't (well, it does most of the time but it isn't guaranteed), but it rather defers the responsibility to other low-level system programmers writing Unsafe code behind the scenes. And oh boy they have a fuckton of responsibility... Stacked Borrows along with various sanitizers can help when writing unsafe code, but it isn't perfect. I highly recommend anyone trying out Rust for the safety guarantees to take a look at the Rustonomicon (https://doc.rust-lang.org/nomicon/), which debunks a lot of the misconceptions around safe/unsafe Rust.
In an ideal la-la land world, there exists an abstract interpreter that can consume safe Rust code and does not enforce any contracts upon the programmer (and hence will never have any undefined behavior). However, real world hardware definitely has contracts which developers have to obey (manually! because of the constraints of actual semiconductor physics! no compiler hand-holding here!). And on top of that all major OSes (Windows, MacOS, Linux) are written in C (so you need unsafe FFI to interact with the OS).
There seems to be a perception among Rust programmers that C's status as a defacto standard came to be in spite of C's contradictions, rather than as a result of them. Skimming the Rustonomicon just now left me with the impression that at least one Rust person gets it. Though it still seemed as if the author felt the need to choose their words very carefully, lest they "make the memory model people angry".
exactly. if i'm writing safe rust and encounter memory safety issues, their origin is with my dependencies, and my responsibility is limited to having chosen such dependencies.
In practice, this makes vulnerabilities in eg. argument parsers (like the recent "baron samedit" vulnerability in sudo) incredibly unlikely.
> the myth that "Writing in Safe Rust automatically makes your code memory-safe"
Of course you're right that that isn't true as stated, but I think it's interesting to try to situate this point along a continuum of other similar points:
1. C with Valgrind and sanitizers isn't always memory safe, because those tools are limited by test coverage.
2. Python isn't always memory safe, because many libraries including the standard library call into C code.
3. Pure Python that doesn't call into any C code isn't always memory safe, because the interpreter might have bugs.
4. Provably correct Ada with a provably correct compiler isn't always memory safe, because the proof checker, the compile-time hardware, or the runtime hardware might have bugs.
I think we all agree that there are important differences between 1 and 4, beyond the simple fact that the defects get less common as you go down the list. Here are some things that stand out to me:
- In cases #2 and below, the application code isn't "at fault" for any memory unsafety that comes up, and whatever code is at fault can be fixed to restore memory safety without changing the application.
- In case #1, there's no clear boundary in any sense between "safe code" (which we know isn't at fault for memory unsafety) and "unsafe code" (which might be at fault). There may be a distinction between code that's well covered by tests and code that isn't, for example, but it's often not easy to tell which is which. In case #2 and below, the boundary is pretty clear.
- In case #1, the amount of "unsafe code" in an application probably grows linearly with the size of the application, or maybe we just consider the whole application unsafe. But in cases #3 and #4, unsafe code is confined to low-level dependencies that get a lot of "battle testing" compared to how much code is in them. Case #2 is kind of a gray area, and we need to look at what dependencies the application is using.
So where should we situate Rust in that continuum? Is being able to write unsafe Rust code more or less risky than being able to call into C? It's certainly a lot more convenient to write an `unsafe` block than to cross the FFI barrier, and maybe that convenience is dangerous. On the other hand (contrary to some common misconceptions), unsafe Rust still benefits a lot from the borrow checker and other safety features, and it might end up having a lower rate of defects for that reason. Maybe it's too early to tell?
But anyway yes, I totally agree that the Rust community has a hard time getting the messaging right about how safe code and unsafe code work. But even though this discussion is really important to Rust, I'm not sure it's a "Rust problem" per se. I think it's actually quite difficult to talk clearly and correctly and precisely about memory safety in general.
In an ideal la-la land world, there exists an abstract interpreter that can consume safe Rust code and does not enforce any contracts upon the programmer (and hence will never have any undefined behavior). However, real world hardware definitely has contracts which developers have to obey (manually! because of the constraints of actual semiconductor physics! no compiler hand-holding here!). And on top of that all major OSes (Windows, MacOS, Linux) are written in C (so you need unsafe FFI to interact with the OS).