Rust in Chromium

[TOC]

Why?

Handling untrustworthy data in non-trivial ways is a major source of security bugs, and it's therefore against Chromium's security policies to do it in the Browser or Gpu process unless you are working in a memory-safe language.

Rust provides a cross-platform, memory-safe language so that all platforms can handle untrustworthy data directly from a privileged process, without the performance overhead and complexity of a utility process.

Status

The Rust toolchain is enabled for and supports all platforms and development environments that are supported by the Chromium project. The first milestone to include full production-ready support was M119.

Rust can be used anywhere in the Chromium repository (not just //third_party) subject to current interop capabilities, however it is currently subject to a internal approval and FYI process. Googlers can view go/chrome-rust for details. New usages of Rust are documented at rust-fyi@chromium.org.

For questions or help, reach out to rust-dev@chromium.org, or #rust channel on the Chromium Slack, or (Google-internal, sorry) Chrome Rust chatroom.

If you use VSCode, we have additional advice below.

Adding a third-party Rust library

Third-party libraries are pulled from crates.io, but Chromium does not use Cargo as a build system.

Third-party review

All third-party libraries (not just Rust) need to go through third-party review. See //docs/adding_to_third_party.md for instructions.

Importing a crate from crates.io

Third-party crates (from crates.io) that Chromium depends on are described by two files:

• //third_party/rust/chromium_crates_io/Cargo.toml. This file defines the set of crates directly depended on from first-party code (from Chromium first-party code, but also from Pdfium, V8, etc.). Their transitive dependencies don't need to be listed, because they will be automatically identified and covered by tools like gnrt. The file is a standard Cargo.toml file, even though the crate itself is never built - it is only used to enable/disable crate features, specify crate versions, etc.
• //third_party/rust/chromium_crates_io/gnrt_config.toml. This file defines Chromium-specific, cargo-agnostic metadata like: - Configuring certain aspects of Chromium build (e.g. allow_unsafe, allow_unstable_features, extra_src_roots, group = "test", etc.) - Specifying licensing information when it can't be automatically inferred (e.g. pointing out license_files with non-standard filenames).

To import a third-party crate follow the steps below:

1. Change directory to the root src/ dir of Chromium.
2. Add the crate to //third_party/rust/chromium_crates_io/Cargo.toml:
   • vpython3 ./tools/crates/run_gnrt.py add foo to add the latest version of foo.
   • vpython3 ./tools/crates/run_gnrt.py add foo@1.2.3 to add a specific version of foo.
   • Or, edit //third_party/rust/chromium_crates_io/Cargo.toml by hand, finding the version you want from crates.io.
3. Download the crate's files:
   • ./tools/crates/run_gnrt.py vendor to download the new crate.
   • This will also apply any patches in //third_party/rust/chromium_crates_io/patches. See //third_party/rust/chromium_crates_io/patches/README.md for more details.
4. (optional) If the crate is only to be used by tests and tooling, then specify the "test" group in //third_party/rust/chromium_crates_io/gnrt_config.toml:

   [crate.foo]
   group = "test"
   

5. Generate the BUILD.gn file for the new crate:
   • vpython3 ./tools/crates/run_gnrt.py gen
6. Add the new files to git:
   • git add -f third_party/rust/chromium_crates_io/vendor. (The -f is important, as files may be skipped otherwise from a .gitignore inside the crate.)
   • git add third_party/rust
7. Upload the CL and get a review from //third_party/rust/OWNERS (check third_party/rust/OWNERS-review-checklist.md to see what to expect).

Note that at this point the new crate is still not seen by gn nor ninja, and is not covered by CQ. To make the new crate part of the build, you need to add a deps edge between an existing build target and the newly added //third_party/rust/some_crate/v123:lib target. This will allow autoninja -C out/Default third_party/rust/some_crate/v123:lib to work. Additionally, this will help CQ to prevent regressions when updating rustc or enabling new Rust warnings.

Security

If a shipping library needs security review (has any unsafe), and the review finds it's not satisfying the rule of 2, then move it to the "sandbox" group in //third_party/rust/chromium_crates_io/gnrt_config.toml to make it clear it can't be used in a privileged process:

[crate.foo]
group = "sandbox"

If a transitive dependency moves from "safe" to "sandbox" and causes a dependency chain across the groups, it will break the gnrt vendor step. You will need to fix the new crate so that it's deemed safe in unsafe review, or move the other dependent crates out of "safe" as well by setting their group in gnrt_config.toml.

Updating existing third-party crates

Third-party crates will get updated semi-automatically through the process described in ../tools/crates/create_update_cl.md. If you nevertheless need to manually update a crate to its latest minor or major version, then follow the steps below. To facilitate easier review, we recommend uploading separate patchsets for 1) manual changes, and 2) tool-driven, automated changes.

1. Change directory to the root src/ dir of Chromium.
2. Update the versions in //third_party/rust/chromium_crates_io/Cargo.toml.
   • vpython3 ./tools/crates/run_gnrt.py update <crate name>.
   • Under the hood this invokes cargo update and accepts the same command line parameters. In particular, you may need to specify --breaking when working on major version updates.
3. Download any updated crate's files:
   • ./tools/crates/run_gnrt.py vendor
4. Add the downloaded files to git:
   • git add -f third_party/rust/chromium_crates_io/vendor
   • The -f is important, as files may be skipped otherwise from a .gitignore inside the crate.
5. Generate the BUILD.gn files
   • vpython3 ./tools/crates/run_gnrt.py gen
   • Or, directly through (nightly) cargo: cargo run --release --manifest-path tools/crates/gnrt/Cargo.toml --target-dir out/gnrt gen
6. Add the generated files to git:
   • git add third_party/rust

Directory structure for third-party crates

The directory structure for a crate "foo" version 3.4.2 is:

//third_party/
    rust/
        foo/  (for the "foo" crate)
            v3/  (version 3.4.2 maps to the v3 epoch)
                BUILD.gn  (generated by gnrt gen)
                README.chromium  (generated by gnrt vendor)
        chromium_crates_io/
            vendor/
                foo-v3  (crate sources downloaded from crates.io)
            patches/
                foo/  (patches for the "foo" crate)
                    0001-Some-changes.diff
                    0002-Other-changes.diff
            Cargo.toml
            Cargo.lock
            gnrt_config.toml

Writing a wrapper for binding generation

Most Rust libraries will need a more C++-friendly API written on top of them in order to generate C++ bindings to them. The wrapper library can be placed in //third_party/rust/<cratename>/<epoch>/wrapper or at another single place that all C++ goes through to access the library. The CXX is used to generate bindings between C++ and Rust.

See //third_party/rust/serde_json_lenient/v0_1/wrapper/ and //components/qr_code_generator for examples.

Rust libraries should use the rust_static_library GN template (not the built-in rust_library) to integrate properly into the mixed-language Chromium build and get the correct compiler options applied to them.

See rust-ffi.md for information on C++/Rust FFI.

Unstable features

Unstable features are unsupported by default in Chromium. Any use of an unstable language or library feature should be agreed upon by the Rust toolchain team before enabling it. See tools/rust/unstable_rust_feature_usage.md for more details.

Logging

Use the log crate's macros in place of base LOG macros from C++. They do the same things. The debug! macro maps to DLOG(INFO), the info! macro maps to LOG(INFO), and warn! and error! map to LOG(WARNING) and LOG(ERROR) respectively. The additional trace! macro maps to DLOG(INFO) (but there is WIP to map it to DVLOG(INFO)).

Note that the standard library also includes a helpful dbg! macro which writes everything about a variable to stderr.

Logging may not yet work in component builds: crbug.com/374023535.

Tracing

TODO: crbug.com/377915495.

Using VSCode

1. Ensure you're using the rust-analyzer extension for VSCode, rather than earlier forms of Rust support.
2. Run gn with the --export-rust-project flag, such as: gn gen out/Release --export-rust-project.
3. ln -s out/Release/rust-project.json rust-project.json
4. When you run VSCode, or any other IDE that uses rust-analyzer it should detect the rust-project.json and use this to give you rich browsing, autocompletion, type annotations etc. for all the Rust within the Chromium codebase.
5. Point rust-analyzer to the rust toolchain in Chromium. Otherwise you will need to install Rustc in your system, and Chromium uses the nightly compiler, so you would need that to match. Add the following to .vscode/settings.json in the Chromium checkout:

   {
      // The rest of the settings...
   
      "rust-analyzer.cargo.extraEnv": {
        "PATH": "../../third_party/rust-toolchain/bin:$PATH",
      }
   }
   

   This assumes you are working with an output directory like out/Debug which has two levels; adjust the number of .. in the path according to your own setup.

Using cargo

If you are building a throwaway or experimental tool, you might like to use pure cargo tooling rather than gn and ninja. Even then, you may choose to restrict yourself to the toolchain and crates that are already approved for use in Chromium, by

• Using tools/crates/run_cargo.py (which will use Chromium's //third_party/rust-toolchain/bin/cargo)
• Configuring .cargo/config.toml to ask to use the crates vendored into Chromium's //third_party/rust/chromium_crates_io.

An example of how this can work can be found in https://crrev.com/c/6320795/5.