Smart pointers for the kernel

I’m not very well versed in kernel development. But I am a Rust dev and have observed the discussion about Rust in Linux with interest… Having said that, this part of the article has me baffled:

>> implementing these features for a smart-pointer type with a malicious or broken Deref (the trait that lets a programmer dereference a value) implementation could break the guarantees Rust relies on to determine when objects can be moved in memory. (…) [In] keeping with Rust's commitment to ensuring safe code cannot cause memory-safety problems, the RFC also requires programmers to use unsafe (specifically, implementing an unsafe marker trait) as a promise that they've read the relevant documentation and are not going to break Pin.

To the uninformed this seems like crossing the very boundary that you wanted Rust to uphold? Yes it’s only an impl Trait but still… I can hear the C devs now. ‘We pinky promise to clean up after our mallocs too!’

Rust’s whole premise of guaranteed memory safety through compiletime checks has always been undermined when confronted with the reality that certain foundational operations must still be implemented using unsafe. Inevitably folks concede that lower level libraries will have these unsafe blocks and still expect higher level code to trust them, and at that point we’ve essentially recreated the core paradigm of C: trust in the programmer’s diligence. Yeah Rust makes this trust visible, but it doesn’t actually eliminate it in “hard” code.

The punchline here, so to speak, is that for all Rust’s claims to revolutionize safety, it simply(!) formalizes the same unwritten social contract C developers have been meandering along with for decades. The uniqueness boils down to “we still trust the devs, but at least now we’ve made them swear on it in writing”.

The difference is every line of C can do something wrong while very few lines of Rust can. It's much easier to scrutinize a small well contained class with tools like formal methods than a sprawling codebase.

If you limited wrong to "memory safe" and also ignore that unsafe parts violating invariants can make safe parts of Rust to be wrong.

> If you limited wrong to "memory safe"

Yes, because this is a discussion about the value of "unsafe", so we're only talking about the wrongs that are enabled by "unsafe".

> and also ignore that unsafe parts violating invariants can make safe parts of Rust to be wrong.

If I run a line of code that corrupts memory, and the program crashes 400 lines later, I don't say the spot where it crashes is wrong, I say the memory corrupting line is wrong. So I disagree with you here.

It does not invalidate an argument that you do not want to talk about it.

Regarding the second point: yes, you can then blame the "unsafe" part but the issue is that the problem might not be so localized as the notion of "only auditing unsafe blocks is sufficient" implies. You may need to understand the subtle interaction of unsafe blocks with the rest of the program.

> the problem might not be so localized as the notion of "only auditing unsafe blocks is sufficient" implies

It depends on what you consider "problem" can mean. An unsafe function needs someone to write unsafe in order to call it, and it's on that calling code to make sure the conditions needed to call the unsafe function are met.

If that function itself is safe, but still let's you trigger the unsafe function unsafely? That function, which had to write 'unsafe', has a bug: either it's not upholding the preconditions of the unsafe function it's calling, or it _can't_ uphold the preconditions without their own callers also being in on it, in which case they themselves need to be an unsafe function (and consider whether their design is a good one).

In this way, you'll always find unsafe 'near' the bug.

In other words, somebody made an error somewhere.

You're thinking of C; Rust forced that somebody to write unsafe near it to create the bug.

The bug that can lead to a violation of assumptions required for safety of the unsafe block can be elsewhere. One can hope that it is near the bloc, but there is nothing in Rust enforcing this.