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Comment by dochtman | Hacker News Reader

unsafe code blocks are required to uphold the invariants of the safe Rust code surrounding it. So yes, if you fail to uphold those invariants (and that's proven to be pretty tricky), your program is not safe.

My experience is that you hardly ever need unsafe Rust code. If your ownership model doesn't the borrow checker, there are of course also other avenues like ref counts or even an actual garbage collecter (which you can chose to apply only to values whose ownership situation is not aligned with the stack). Only when those options are unable to give you the performance you want you'll need to reach for unsafe code.

Other scenarios where you'll need unsafe might include things like SIMD (although safe wrappers exist) or FFI, but hopefully we'll evolve the Rust ecosystem over time to concentrate the unsafe code in a few places by building abstractions, where they are thoroughly reviewed and well tested.

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> My experience is that you hardly ever need unsafe Rust code.

I doubt that experience applies equally to kernel development.

They may apply better than one might think: the example SPI driver posted at https://lwn.net/Articles/863459/ only has two cases of unsafe.

Of course, the kernel:: -modules it uses undoubtedly have more, but I imagine the idea is to provide abstractions that allow actual driver code not to use unsafe.

And then there's stuff like file system code, that—I imagine—would require no more uses of unsafe than a database server written in Rust.

Because it's normal (indeed expected) to build safe abstractions on top of necessarily unsafe code, a lot of the kernel by volume shouldn't need to be unsafe at all.

For example suppose that we're back in the late 1990s and so for some reason dozens of different yet roughly equivalent 100Mbps fast Ethernet PCI cards are available, each offering the same functionality but with slightly different register layouts, interrupts and quirks. We are of course writing drivers for them in Rust.

Obviously at some fundamental level "configure PCI bus" isn't safe - if you get it wrong that'll be bad. But as the author of mediocre Ethernet driver #42 you aren't implementing "configure PCI bus" you're just calling a safe abstraction that worked for the other 41 drivers.

Handling userspace calls to twiddle the Ethernet interface is potentially unsafe too, but likewise that will have been factored out into common code and you're just talking to a safe abstraction.

Rust's type system and borrow checking makes it easier to write a safe abstraction for relatively low level ideas like "Only one device can own this MMIO register" or even "this can be 0x03 or 0x06 or even 0x18 but it can't be all zeroes, that's not a thing".

This sort of stuff is boring and in principle already not particularly unsafe in C today, this sort of abstraction is already done - but of course in C you've no practical way to get the promises Rust offers for safe code.

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