Comment by krylon
5 years ago
This may be a naive question, but I cannot help but wonder: As C and Rust aim to be link-compatible, wouldn't it be easier to gradually replace parts of the Linux kernel with Rust code?
The only technical problem I can think of is that Rust may not be available for all CPU architectures Linux supports, but this is just speculation on my part having done no research on the matter.
The Rust for Linux kernel project aims to enable writing Linux kernel device drivers in Rust.
See: https://lwn.net/Articles/862018/
The issue of Rust's LLVM based compiler toolchain not targeting all CPU archs is intended to be solved by the gcc-rs project.
See: https://lwn.net/Articles/871283/
I think the approach the Rust for Linux project is taking is wise: Not about outright re-writes but more about focusing on those subsystems where Rust's intrinsic safety and security properties helps the most.
Last I checked, one of the big problems was the Rust panics when you run out of memory, which is unacceptable in the kernel. Is there any progress on that?
Out of the box Rust doesn't provide any way to allocate heap memory, so, you can't run out if it.
Your ordinary userspace apps use std (the standard library) which relies on the alloc crate, and that provides heap allocation which indeed panics if the allocation fails. Because it correctly guesses that your "clever" strategy to handle allocation failure actually isn't and will just triple fault anyway so it should cut to the chase.
The kernel obviously doesn't have std, and Rust for Linux implements its own alloc crate.
Linus' requirement that you can fail memory allocation just means all the calls in alloc that can actually allocate memory now return Result to indicate whether the allocation was successful. This isn't how you'd do things in userspace, but, this isn't userspace so fine.
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Yes, this is called "fallible allocations." You add methods with the "try_" prefix that work like the existing methods, except they return a Result which fails if it's out of memory instead of panicking.
We have a light / temporary fork of the Rust stdlib allocator with fallible allocation support: https://github.com/Rust-for-Linux/linux/tree/rust/rust/alloc
See e.g. https://github.com/Rust-for-Linux/linux/commit/487d7578bd036...
Yes a lot. https://rust-for-linux.github.io/docs/alloc/alloc/index.html
>> one of the big problems was the Rust panics when you run out of memory, which is unacceptable in the kernel.
If the kernel is running out of memory, IMHO that's a bug. The kernel is ultimately responsible for memory management right? It needs to prioritize itself over everything else or the system is in trouble.
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That's what may happen with the Linux kernel over time, now that it allows parts written in Rust.
The limitation of that approach is that all internal interfaces (which includes most data structures) have to be C-compatible. You can get much of the safety benefits of Rust, but lose a lot of Rust's ergonomics.
I don't think that the Linux kernel has decided (yet) to allow rust code into the kernel proper, has it?
It hasn't. But the attitude towards using Rust for device driver development has now gotten very positive, especially post the recent kernel summit.
An LWN article about Rust in Linux that's very relevant to this discussion: https://lwn.net/Articles/829858/
Google has decided to go forward anyway.
Just like Android Linux compiles just fine with clang for the last five years or so, it now makes use of Rust.
https://source.android.com/setup/build/rust/building-rust-mo...
If upstream ever cares to support clang or Rust, that is another matter.
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Yes, gradual replacement is easier and more practical for large projects. However, typical C APIs and idioms are different from idiomatic Rust. Rust uses more type-system features, generics, iterators, RAII, and prefers tree-like data structures and far fewer pointers (no linked lists!).
C rewritten in Rust is still very C-like, and requires refactorings that you can't do until it's all Rust.
BTW: There are two GCC-based Rust implementations in the works, so compatibility with exotic platforms is going to be solved.
A practical problem is that in order to gain the benefit of linking Rust and C code, one has to give up Rust's wonderful guarantees at the interface. So until islands of Rust meet up, these interact through a C ABI, and have to expose C-like behavior.
But you still get those guarantees and benefits in the Rust implementation itself, even if not at the interface (to non-Rust code). By a similar argument, a standalone Rust program "gives up" its guarantees whenever the process makes a call to libc or to the operating system (syscalls), but this isn't really a practical problem.
Sure, I agree. The point is that if the total corpus of Rust code is too isolated into too small islands, then the boundary effects of each island may eat up a lot of the benefit.
I still think the project is really cool and worthwhile.
The world really needs _two_ projects, a practical one that is interoperable with the here and now, to get incremental improvements in security from bits reimplemented in Rust.
And then, in the longer term, someone ought to write a book about operating system implementation in Rust that ignores Linux interoperability and focuses on readability, maintainability by showing use of Rust's idiomatic style.
It’s not naive, it’s just been speculated and talked about incessantly since Rust came out. The one naive part is underestimating how long that would take - Linux is one of the largest codebases on Earth. It would require massive coordination over years to rewrite, and in the end all you would get is the exact same product.
It’s just not an interesting or unique perspective, and pretty silly to suggest seriously.
The Linux kernel is massive, so writing any part of it in Rust is an enormous undertaking. Even a few functions at a time, we're talking decades.
Well, writing a kernel from scratch that supports as many CPU architectures and devices as Linux would take about as long, assuming one could attract a critical mass of developers.
Using an incremental approach at least gives us some benefits in the near future.
I'm not even saying a fresh start would be a bad idea. But incrementally replacing parts of Linux seems like a more promising approach, IMHO.