mirror of
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184 lines
6.2 KiB
Markdown
184 lines
6.2 KiB
Markdown
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Hacking on Rust in Tor
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========================
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Getting Started
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-----------------
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Please read or review our documentation on Rust coding standards
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(`.../doc/HACKING/CodingStandardsRust.md`) before doing anything.
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Please also read
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[the Rust Code of Conduct](https://www.rust-lang.org/en-US/conduct.html). We
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aim to follow the good example set by the Rust community and be
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excellent to one another. Let's be careful with each other, so we can
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be memory-safe together!
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Next, please contact us before rewriting anything! Rust in Tor is still
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an experiment. It is an experiment that we very much want to see
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succeed, so we're going slowly and carefully. For the moment, it's also
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a completely volunteer-driven effort: while many, if not most, of us are
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paid to work on Tor, we are not yet funded to write Rust code for Tor.
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Please be patient with the other people who are working on getting more
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Rust code into Tor, because they are graciously donating their free time
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to contribute to this effort.
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Resources for learning Rust
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-----------------------------
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**Beginning resources**
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The primary resource for learning Rust is
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[The Book](https://doc.rust-lang.org/book/). If you'd like to start writing
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Rust immediately, without waiting for anything to install, there is
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[an interactive browser-based playground](https://play.rust-lang.org/).
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**Advanced resources**
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If you're interested in playing with various Rust compilers and viewing
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a very nicely displayed output of the generated assembly, there is
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[the Godbolt compiler explorer](https://rust.godbolt.org/)
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For learning how to write unsafe Rust, read
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[The Rustonomicon](https://doc.rust-lang.org/nomicon/).
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For learning everything you ever wanted to know about Rust macros, there
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is
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[The Little Book of Rust Macros](https://danielkeep.github.io/tlborm/book/index.html).
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For learning more about FFI and Rust, see Jake Goulding's
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[Rust FFI Omnibus](http://jakegoulding.com/rust-ffi-omnibus/).
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Compiling Tor with Rust enabled
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---------------------------------
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You will need to run the `configure` script with the `--enable-rust`
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flag to explicitly build with Rust. Additionally, you will need to
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specify where to fetch Rust dependencies, as we allow for either
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fetching dependencies from Cargo or specifying a local directory.
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**Fetch dependencies from Cargo**
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./configure --enable-rust --enable-cargo-online-mode
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**Using a local dependency cache**
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You'll need the following Rust dependencies (as of this writing):
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libc==0.2.39
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We vendor our Rust dependencies in a separate repo using
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[cargo-vendor](https://github.com/alexcrichton/cargo-vendor). To use
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them, do:
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git submodule init
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git submodule update
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To specify the local directory containing the dependencies, (assuming
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you are in the top level of the repository) configure tor with:
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TOR_RUST_DEPENDENCIES='path_to_dependencies_directory' ./configure --enable-rust
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(Note that TOR_RUST_DEPENDENCIES must be the full path to the directory; it
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cannot be relative.)
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Assuming you used the above `git submodule` commands and you're in the
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topmost directory of the repository, this would be:
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TOR_RUST_DEPENDENCIES=`pwd`/src/ext/rust/crates ./configure --enable-rust
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Identifying which modules to rewrite
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======================================
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The places in the Tor codebase that are good candidates for porting to
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Rust are:
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1. loosely coupled to other Tor submodules,
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2. have high test coverage, and
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3. would benefit from being implemented in a memory safe language.
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Help in either identifying places such as this, or working to improve
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existing areas of the C codebase by adding regression tests and
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simplifying dependencies, would be really helpful.
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Furthermore, as submodules in C are implemented in Rust, this is a good
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opportunity to refactor, add more tests, and split modules into smaller
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areas of responsibility.
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A good first step is to build a module-level callgraph to understand how
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interconnected your target module is.
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git clone https://git.torproject.org/user/nickm/calltool.git
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cd tor
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CFLAGS=0 ./configure
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../calltool/src/main.py module_callgraph
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The output will tell you each module name, along with a set of every module that
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the module calls. Modules which call fewer other modules are better targets.
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Writing your Rust module
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==========================
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Strive to change the C API as little as possible.
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We are currently targetting Rust stable. (See CodingStandardsRust.md for more
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details.)
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It is on our TODO list to try to cultivate good
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standing with various distro maintainers of `rustc` and `cargo`, in
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order to ensure that whatever version we solidify on is readily
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available.
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If parts of your Rust code needs to stay in sync with C code (such as
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handling enums across the FFI boundary), annonotate these places in a
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comment structured as follows:
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/// C_RUST_COUPLED: <path_to_file> `<name_of_c_object>`
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Where <name_of_c_object> can be an enum, struct, constant, etc. Then,
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do the same in the C code, to note that rust will need to be changed
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when the C does.
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Adding your Rust module to Tor's build system
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-----------------------------------------------
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0. Your translation of the C module should live in its own crate(s)
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in the `.../tor/src/rust/` directory.
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1. Add your crate to `.../tor/src/rust/Cargo.toml`, in the
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`[workspace.members]` section.
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2. Add your crate's files to src/rust/include.am
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If your crate should be available to C (rather than just being included as a
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dependency of other Rust modules):
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0. Declare the crate as a dependency of tor_rust in
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`src/rust/tor_util/Cargo.toml` and include it in
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`src/rust/tor_rust/lib.rs`
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How to test your Rust code
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----------------------------
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Everything should be tested full stop. Even non-public functionality.
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Be sure to edit `.../tor/src/test/test_rust.sh` to add the name of your
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crate to the `crates` variable! This will ensure that `cargo test` is
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run on your crate.
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Configure Tor's build system to build with Rust enabled:
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./configure --enable-fatal-warnings --enable-rust --enable-cargo-online-mode
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Tor's test should be run by doing:
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make check
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Tor's integration tests should also pass:
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make test-stem
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Submitting a patch
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=====================
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Please follow the instructions in `.../doc/HACKING/GettingStarted.md`.
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