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General Information

This document contains useful information to know when working with the Rust support in the kernel.

Code documentation

Rust kernel code is documented using rustdoc, its built-in documentation generator.

The generated HTML docs include integrated search, linked items (e.g. types, functions, constants), source code, etc. They may be read at (TODO: link when in mainline and generated alongside the rest of the documentation):

The docs can also be easily generated and read locally. This is quite fast (same order as compiling the code itself) and no special tools or environment are needed. This has the added advantage that they will be tailored to the particular kernel configuration used. To generate them, use the rustdoc target with the same invocation used for compilation, e.g.:

make LLVM=1 rustdoc

To read the docs locally in your web browser, run e.g.:

xdg-open Documentation/output/rust/rustdoc/kernel/index.html

To learn about how to write the documentation, please see Coding Guidelines.

Extra lints

While rustc is a very helpful compiler, some extra lints and analyses are available via clippy, a Rust linter. To enable it, pass CLIPPY=1 to the same invocation used for compilation, e.g.:

make LLVM=1 CLIPPY=1

Please note that Clippy may change code generation, thus it should not be enabled while building a production kernel.

Abstractions vs. bindings

Abstractions are Rust code wrapping kernel functionality from the C side.

In order to use functions and types from the C side, bindings are created. Bindings are the declarations for Rust of those functions and types from the C side.

For instance, one may write a Mutex abstraction in Rust which wraps a struct mutex from the C side and calls its functions through the bindings.

Abstractions are not available for all the kernel internal APIs and concepts, but it is intended that coverage is expanded as time goes on. “Leaf” modules (e.g. drivers) should not use the C bindings directly. Instead, subsystems should provide as-safe-as-possible abstractions as needed.

                                                rust/bindings/
                                               (rust/helpers.c)

                                                   include/ -----+ <-+
                                                                 |   |
  drivers/              rust/kernel/              +----------+ <-+   |
    fs/                                           | bindgen  |       |
   .../            +-------------------+          +----------+ --+   |
                   |    Abstractions   |                         |   |
+---------+        | +------+ +------+ |          +----------+   |   |
| my_foo  | -----> | | foo  | | bar  | | -------> | Bindings | <-+   |
| driver  |  Safe  | | sub- | | sub- | |  Unsafe  |          |       |
+---------+        | |system| |system| |          | bindings | <-----+
     |             | +------+ +------+ |          |  crate   |       |
     |             |   kernel crate    |          +----------+       |
     |             +-------------------+                             |
     |                                                               |
     +------------------# FORBIDDEN #--------------------------------+

The main idea is to encapsulate all direct interaction with the kernel’s C APIs into carefully reviewed and documented abstractions. Then users of these abstractions cannot introduce undefined behavior (UB) as long as:

  1. The abstractions are correct (“sound”).

  2. Any unsafe blocks respect the safety contract necessary to call the operations inside the block. Similarly, any unsafe impls respect the safety contract necessary to implement the trait.

Bindings

By including a C header from include/ into rust/bindings/bindings_helper.h, the bindgen tool will auto-generate the bindings for the included subsystem. After building, see the *_generated.rs output files in the rust/bindings/ directory.

For parts of the C header that bindgen does not auto generate, e.g. C inline functions or non-trivial macros, it is acceptable to add a small wrapper function to rust/helpers.c to make it available for the Rust side as well.

Abstractions

Abstractions are the layer between the bindings and the in-kernel users. They are located in rust/kernel/ and their role is to encapsulate the unsafe access to the bindings into an as-safe-as-possible API that they expose to their users. Users of the abstractions include things like drivers or file systems written in Rust.

Besides the safety aspect, the abstractions are supposed to be “ergonomic”, in the sense that they turn the C interfaces into “idiomatic” Rust code. Basic examples are to turn the C resource acquisition and release into Rust constructors and destructors or C integer error codes into Rust’s Results.

Conditional compilation

Rust code has access to conditional compilation based on the kernel configuration:

#[cfg(CONFIG_X)]       // Enabled               (`y` or `m`)
#[cfg(CONFIG_X="y")]   // Enabled as a built-in (`y`)
#[cfg(CONFIG_X="m")]   // Enabled as a module   (`m`)
#[cfg(not(CONFIG_X))]  // Disabled