Why do they allow unsafe parts in linux kernel in the first place? Why rewriting C code into unsafe rust?
As for why there is unsafe in the kernel? There are things, especially in a kernel, that cannot be expressed in safe Rust.
Still, having smaller sections of unsafe is a boon because you isolate these locations of elevated power, meaning they are auditable and obvious. Rust also excels at wrapping unsafe in safe abstractions that are impossible to misuse. A common comparison point is that in C your entire program is effectively unsafe, whereas in Rust it's a subset.
> Still, having smaller sections of unsafe is a boon because you isolate these locations of elevated power, meaning they are auditable and obvious.
The Rustonomicon makes it very clear that it is generally insufficient to only verify correctness of Rust-unsafe blocks. If the absence of UB in a Rust-unsafe block depends on Rust-not-unsafe code in the surrounding module, potentially the whole module has to be verified for correctness. And that assumes that the module has correct encapsulation, otherwise even more may have to be verified. And a single buggy change to Rust-not-unsafe code can cause UB, if a Rust-unsafe block somewhere depends on that code to be correct.
This is nonsense.
You'd still need unsafe blocks because a kernel requires shared mutable memory in places.
This is like saying "If it compiles, it works", which is absolute nonsense as well.
I was referring to the current unsafe blocks used for Rust->C FFI. Obviously OS code in any language will need to perform low-level operations, those unsafe blocks are never going away.
You need direct shared mutable memory access with runtime locking even in the pure-Rust parts. That's kinda what OSes need, actually. Some things (Maybe DMA, possibly Page Table mutation, register saving/loading, as a few examples) can't be compile-time checked.
In fact, I would guess that if you gradually moved the Linux code over to Rust, at the end of it you'd still have maybe 50% of it in unsafe blocks.
So, no - your claim is no different than "if it compiles it works".