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by tatetian16
359 days ago
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> if an unprivileged task is unsafe, it's still unprivileged. Meanwhile the unsafe code that requires extra verification... I am sorry that the doc is a kind of misleading. I wrote that... The statement need to be interpreted in the context of framekernel. An entire Rust-based framekernel runs in the kernel space but is logically partitioned into the two halves: the privileged OS framework and the de-privileged OS services. Here, "privileged" means safe + unsafe Rust kernel code, whereas "de-privileged" means all safe Rust kernel code. And this is all about the kernel code. Framekernels do not put restrictions on the languages of user-space programs. |
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The kernel is logically divided into two parts: the privileged OS framework (akin to a microkernel) and the de-privileged OS services. Only the privileged framework is allowed to use unsafe, while the de-privileged services must be written in safe Rust completely. As the TCB, the privileged framework encapsulates all low-level, hardware-oriented unsafe operations behind safe APIs. Using these safe APIs, the de-privileged OS services can implement all kinds of OS functionalities, including device drivers.
Ostd provides a small yet expressive set of safe OS development abstractions, covering safe user-kernel interactions, safe kernel logic, and safe kernel-peripheral interactions. Of particular note is the untyped memory abstraction, which addresses the challenge of safely handling externally-modifiable memory (e.g., MMIO or DMA-capable memory) – a longstanding obstacle in safe driver development.
So the privileged part is privileged because it does unsafe stuff. It's also quite minimal, so that the "business logic" of a driver or similar can be implemented using safe code in the de-privileged code, which is de-privileged because it doesn't need privileged unsafe access. At least that's my understanding.
[1]: https://arxiv.org/abs/2506.03876