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Comment by burntsushi | Hacker News Reader

Calling an unsafe function can only be done inside an unsafe block.

I assume OP s point is that unsoundness or UB in an unsafe block is not contained in that block, but can taint safe code anywhere in the program. Which is true.

only if that block, or something in its "trusted set" (my term, for things in the same module that can access private members) is incorrect, though.

safety/unsafety in Rust is factorable, which is a key part of the reason Rust is useful at all.

Rest of your program blindly believes that the interface of your "trusted set" is safe because you owned that responsibility when you marked it unsafe.

If you have a memory safety bug in that interface, then you can taint the rest of the program's memory safety as well, correct?

Yes, that's the whole point though.

I think that is too strong. Consider this code:

    unsafe { libc::fork() };
    let v = vec![1];

This may hang or even crash (malloc in the child) but there's no sense in which the unsafe block is "incorrect." Some unsafe code has unavoidable implications for the entire program.

Indeed, some uses of unsafe aren't meaningfully factorable. File backed memory maps or shared memory with other processes are other important real world examples that are difficult or impossible to meaningfully factor in the context of a single process. But I still think that saying "safety in Rust is factorable" is accurate beyond a mere first approximation. Figuring out how to encapsulate unsafety is, in my experience, the essential creative aspect to using unsafe at all. Because if safety wasn't something you could encapsulate, then Rust really wouldn't be what it is. (I have spent many long hours thinking about how and to what extent the safety of file backed memory maps could be encapsulated. Which is to say, factoring safety is really the ultimate goal, even if certain things remain beyond that goal.)

Yes, it's true, but that wasn't what Animats said. And even if that was Animats' point, it doesn't invalidate the GP. The key value proposition of Rust isn't that "you'll never have memory safety bugs," but rather, that if your unsafe code is correct and sound, then safe Rust will be free of memory safety bugs. The important bit is the implication that grants one the power to make that sort of reasoning. It's important precisely because it limits the places in your code that need to be audited.

This is indeed a very powerful property. However, I have a question: is it a regular practice to ensure that code marked unsafe is actually safe for whichever parameters it receives? Or could the safety of some unsafe code depend on the way its safe wrapper is called?

If you have an unsafe function---that is, a function that is unsafe to call---then it is common practice to document the precise preconditions that the caller must uphold in order to ensure safety. This may indeed include passing a correct parameter. For example, the preconditions of the slice method `get_unchecked` require the caller to verify that the index provided is in bounds, otherwise the behavior is UB.

If you have a safe function that uses unsafe internally, then all possible invocations of that function should be safe. If this isn't true, then we call those sorts of APIs unsound and they are strongly discouraged. David Tolnay wrote a great blog post about it: https://docs.rs/dtolnay/0.0.7/dtolnay/macro._03__soundness_b...

Than you for the details, that makes a lot of sense!

I’m unsure what you’re responding to. As I understood it, the comment took issue with saying that you can make guarantees about safe Rust but not unsafe Rust because unsafe Rust is not verified in that way and opens the door to violating guarantees in your “safe” code if your bugs leak out.

Can you rephrase or restate your point? It's really hard to understand..

“Your comment doesn’t really make sense, https://news.ycombinator.com/item?id=24027224 “

I don't know the veracity of the claims but it's pretty crystal clear.

safe becomes unsafe really quickly

That may be what you understood, but that's not what Animats said. See my reply above for more of an elaboration: https://news.ycombinator.com/item?id=24028359

The function containing the unsafe block can be called from safe code. So unsafe functionality can be exported. Said unsafe functionality might not be safe for all possible inputs. That's a classic hole, as with APIs that can be exploited.

Then that's a "function containing an unsafe block," not an "unsafe function." In any case, this is still a misunderstanding of what staticassertion said. I elaborated more here: https://news.ycombinator.com/item?id=24028359

My bigger point here is that you're misunderstanding the advocated Rust value proposition. The value proposition isn't literally "Rust will forever and always eliminate all memory safety bugs in safe Rust." That's silly and no serious person with any credibility would double down on that claim.

They say can call though, so they are correct: if you can call then you are in an unsafe context.

Yes...? But they're responding to a point about safe Rust. See my other comment: https://news.ycombinator.com/item?id=24028359

But you can expose unsafe code with a safe API, right?

The point of the GP was that any safe code using this safe API could in fact be memory unsafe is there is a bug in the unsafe implementation.

Yes but it's easy to take this too far and conclude that e.g. Javascript is not memory safe because browsers are written in C++ and they have to interface with the kernel which is written in C. At some point you simply need to trust that the current implementation is correct and bug free. This is also a problem with formal verification. What verifies the verification?

Yes, of course you can. But that's not what Animats said. Animats specifically said "unsafe function," which means you need to use the unsafe annotation to call it. See my other reply for more elaboration: https://news.ycombinator.com/item?id=24028359

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