Comment by overfeed

14 hours ago

> 95% of rust is safe rust.

Based on the raw number of assorted crates, which has no bearing on kernel code. The more relevant question is, can a performant, cross-architecture, kernel ring-buffer be written in safe Rust?

3 comments

overfeed

steveklabnik 12 hours ago

Hubris, an embedded RTOS-like used in production by Oxide, has ~4% unsafe code in the kernel last I checked. There’s a ring buffer implementation that has one unsafe, for unchecked indexing: https://github.com/oxidecomputer/hubris/blob/master/lib/ring... (this of course does not mean that it is the one ring buffer to rule them all, but it’s to demonstrate that yes, it is at least possible to have one with minimum unsafe.)

It’s always a way lower number than folks assume. Even in spaces that have higher than average usage.

saghm 1 hour ago

I've always had the impression that people who haven't actually tried to write low-level code in Rust to try to find out where the actual boundary of where they would need unsafe is tend not to realize how far you can push something and build safe abstractions on top of it. Almost every time I've had to wrap an unsafe API, I've been able to find a way to eliminate at least one of the invariants that are documented as needed for safety from propagating upwards, and there have been plenty of times that the specific circumstances of my use-case allowed me to eliminate it entirely.
The entirety of safe Rust is built upon unsafe Rust that's abstracted like this. The fact that you sometimes need unsafe isn't a mark against Rust, but literally the entire premise of the language and the exact problem it's designed to solve.

josephg 13 hours ago

I doubt it, but you can probably get pretty close.

This is something a lot of people misunderstand about unsafe rust. The safe / unsafe distinction isn't at the crate level. You don't say "this entire module opts out of safety checks". Unsafe is a granular thing. The unsafe keyword doesn't turn off the borrow checker. It just lets you dereference pointers (and do a few other tricks).

Systems code written in rust often has a few unsafe functions which interact with the actual hardware. But all the high level logic - which is usually most of the code by volume - can be written using safe, higher level abstractions.

"Can all of io_uring be written in safe rust?" - probably not, no. But could you write the vast majority of io_uring in safe rust? Almost certainly. This bug is a great example. In this case, the problematic function was this one:

    static void io_zcrx_return_niov_freelist(struct net_iov *niov)
    {
        struct io_zcrx_area *area = io_zcrx_iov_to_area(niov);

        spin_lock_bh(&area->freelist_lock);
        area->freelist[area->free_count++] = net_iov_idx(niov);
        spin_unlock_bh(&area->freelist_lock);
    }

At a glance, this function absolutely could have been written in safe rust. And even if it was unsafe, array lookups in rust are still bounds checked.