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| ▲ | josephg 3 hours ago | parent | next [-] | | No they really don't. 95% of rust is safe rust[1]. Also unsafe rust doesn't remove bounds checks. arr[idx] is bounds checked in every context. You can opt out of array bounds checking by writing unsafe { arr.get_unchecked(idx) } . But thats incredibly rare in practice. [1] https://cs.stanford.edu/~aozdemir/blog/unsafe-rust-syntax/ | | |
| ▲ | overfeed an hour ago | parent [-] | | > 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? | | |
| ▲ | josephg an hour ago | parent [-] | | 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. |
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| ▲ | saghm 2 hours ago | parent | prev | next [-] | | "unsafe Rust" is not a binary; you don't opt into it for every single line of code. Given that the entire premise behind the idea that using C instead of Rust is fine is that people should be able to pay close attention and not make mistakes like this, having the number of places you need to look be a tiny fraction of the overall code that's explicitly marked as unsafe is a massive difference from C where literally every line of the code could be hiding stuff like this. | |
| ▲ | Jtsummers 3 hours ago | parent | prev [-] | | > except nearly everyone uses unsafe rust Really? Why? I've not used Rust outside of some fairly small efforts, but I've never found a reason to reach for unsafe. So why is "nearly everyone" else using it? | | |
| ▲ | dvt 3 hours ago | parent [-] | | Let's say you want to call win32 (or Mac) OS functions, all of a sudden you're doing all kinds of wonky pointer stuff because that's how these operating systems have been architected. Doing unsafe stuff is pretty inevitable if you want to do anything non-hello-world-ish. | | |
| ▲ | Jtsummers 3 hours ago | parent | next [-] | | > Doing unsafe stuff is pretty inevitable if you want to do anything non-hello-world-ish. So the vast majority of Rust projects involve writing at least one unsafe block? Is that really your claim? | | |
| ▲ | greiskul 3 hours ago | parent [-] | | And even if you do end up writing an unsafe block, that should be a massive flag that the code in said block should deserve extra comments on why it is safe, and extra unit tests on verifying that it does not blow up. How do you know the unsafe operation is safe? What are the preconditions the code block has? Write it down, review it, test it. | | |
| ▲ | saghm 2 hours ago | parent [-] | | Exactly; I feel like a lot of people seem to misunderstand what Rust is trying to solve. It's fundamentally not trying to make unsafe code impossible; it's making the number of places you need to audit it a tiny fraction of your codebase compared to needing to audit the entirety of a C or C++ codebase. When I'm doing code reviews, you'd better believe I'm going to spend some extra time on any unsafe block I see to figure out if it's necessary and if so, if it's actually safe safe (with the default assumption for both of those being that they're not until I can convince myself otherwise). | | |
| ▲ | skydhash an hour ago | parent [-] | | The thing is you can actually write quite good C code (see OpenBSD project). The power of C is that it's pragmatic. It lets you write code with you taking the full responsibility of being a responsible person. To err is human, but we developed a set of practices to handle this (by making sure the gun is unloaded and the safety is on before storing it to avoid putting holes in feet). I like type checking and other compile time checks, but sometimes they feel very ceremonial. And all of them are inference based, so they still relies on the axiom being right and that the chain of rules is not broken somewhere. And in the end they are annotations, not the runtime algorithm. |
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| ▲ | dralley 2 hours ago | parent | prev | next [-] | | A tiny fraction of programs need to use win32 or Mac OS functions beyond the standard library or other safe wrappers for said functions. | |
| ▲ | josephg 3 hours ago | parent | prev | next [-] | | Making use of win32 functions doesn't turn off bounds checking in your rust code. | |
| ▲ | amluto 3 hours ago | parent | prev [-] | | So what? Just because you used the keyword `unsafe` to call an unsafe API does not mean that you are going to use unsafe pointer access to write to a vector. |
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