Miri

An experimental interpreter for Rust's mid-level intermediate representation (MIR). It can run binaries and test suites of cargo projects and detect certain classes of undefined behavior, for example:

• Out-of-bounds memory accesses and use-after-free
• Invalid use of uninitialized data
• Violation of intrinsic preconditions (an unreachable_unchecked being reached, calling copy_nonoverlapping with overlapping ranges, ...)
• Not sufficiently aligned memory accesses and references
• Violation of basic type invariants (a bool that is not 0 or 1, for example, or an invalid enum discriminant)
• WIP: Violations of the rules governing aliasing for reference types

Building Miri

We recommend that you install rustup to obtain Rust. Then all you have to do is:

cargo +nightly build

This uses the very latest Rust version. If you experience any problem, refer to the rust-version file which contains a particular Rust nightly version that has been tested against the version of miri you are using. Make sure to use that particular nightly-YYYY-MM-DD whenever the instructions just say nightly.

To avoid repeating the nightly version all the time, you can use rustup override set nightly (or rustup override set nightly-YYYY-MM-DD), which means nightly Rust will automatically be used whenever you are working in this directory.

Running Miri

cargo +nightly run -- -Zmiri-disable-validation tests/run-pass/vecs.rs # Or whatever test you like.

We have to disable validation because that can lead to errors when libstd is not compiled the right way.

Running Miri with full libstd

Per default libstd does not contain the MIR of non-polymorphic functions, and also does not contain some extra MIR statements that miri needs for validation. When Miri hits a call to such a function, execution terminates, and even when the MIR is present, validation can fail. To fix this, it is possible to compile libstd with full MIR:

rustup component add --toolchain nightly rust-src
cargo +nightly install xargo
rustup run nightly xargo/build.sh

Now you can run Miri against the libstd compiled by xargo:

MIRI_SYSROOT=~/.xargo/HOST cargo +nightly run tests/run-pass-fullmir/hashmap.rs

Notice that you will have to re-run the last step of the preparations above (xargo/build.sh) when your toolchain changes (e.g., when you update the nightly).

Running Miri on your own project('s test suite)

Install Miri as a cargo subcommand with cargo +nightly install --all-features --path .. Be aware that if you used rustup override set to fix a particular Rust version for the miri directory, that will not apply to your own project directory! You have to use a consistent Rust version for building miri and your project for this to work, so remember to either always specify the nightly version manually, overriding it in your project directory as well, or use rustup default nightly (or rustup default nightly-YYYY-MM-DD) to globally make nightly the default toolchain.

We assume that you have prepared a MIR-enabled libstd as described above. Now compile your project and its dependencies against that libstd:

1. Run cargo clean to eliminate any cached dependencies that were built against the non-MIR libstd.
2. To run all tests in your project through, Miri, use MIRI_SYSROOT=~/.xargo/HOST cargo +nightly miri test. NOTE: This is currently broken, see the discussion in #479.
3. If you have a binary project, you can run it through Miri using MIRI_SYSROOT=~/.xargo/HOST cargo +nightly miri.

Common Problems

When using the above instructions, you may encounter a number of confusing compiler errors.

"constant evaluation error: no mir for <function>"

You may have forgotten to set MIRI_SYSROOT when calling cargo miri, and your program called into std or core. Be sure to set MIRI_SYSROOT=~/.xargo/HOST.

"found possibly newer version of crate std which <dependency> depends on"

Your build directory may contain artifacts from an earlier build that did/did not have MIRI_SYSROOT set. Run cargo clean before switching from non-Miri to Miri builds and vice-versa.

"found crate std compiled by an incompatible version of rustc"

You may be running cargo miri with a different compiler version than the one used to build the MIR-enabled std. Be sure to consistently use the same toolchain, which should be the toolchain specified in the rust-version file.

Miri -Z flags

Several -Z flags are relevant for miri:

• -Zmir-opt-level controls how many MIR optimizations are performed. miri overrides the default to be 0; be advised that using any higher level can make miri miss bugs in your program because they got optimized away.
• -Zalways-encode-mir makes rustc dump MIR even for completely monomorphic functions. This is needed so that miri can execute such functions, so miri sets this flag per default.
• -Zmiri-disable-validation is a custom -Z flag added by miri. It disables enforcing the validity invariant, which is enforced by default. This is mostly useful for debugging; it means miri will miss bugs in your program.

Development and Debugging

Since the heart of Miri (the main interpreter engine) lives in rustc, working on Miri will often require using a locally built rustc. This includes getting a trace of the execution, as distributed rustc has debug! and trace! disabled.

The first-time setup for a local rustc looks as follows:

git clone https://github.com/rust-lang/rust/ rustc
cd rustc
cp config.toml.example config.toml
# Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
# The latter is important to build libstd with the right flags for miri.
./x.py build src/rustc
# You may have to change the architecture in the next command
rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
# Now cd to your Miri directory
rustup override set custom

The build step can take 30 minutes and more.

Now you can cargo build Miri, and you can cargo test --release it. cargo test --release FILTER only runs those tests that contain FILTER in their filename (including the base directory, e.g. cargo test --release fail will run all compile-fail tests). We recommend using --release to make test running take less time.

Notice that the "fullmir" tests only run if you have MIRI_SYSROOT set, the test runner does not realized that your libstd comes with full MIR. The following will set it correctly:

MIRI_SYSROOT=$(rustc --print sysroot) cargo test --release

Moreover, you can now run Miri with a trace of all execution steps:

MIRI_LOG=debug cargo run tests/run-pass/vecs.rs

Setting MIRI_LOG like this will configure logging for miri itself as well as the rustc::mir::interpret and rustc_mir::interpret modules in rustc. You can also do more targeted configuration, e.g. to debug the stacked borrows implementation:

MIRI_LOG=rustc_mir::interpret=debug,miri::stacked_borrows cargo run tests/run-pass/vecs.rs

In addition, you can set MIRI_BACKTRACE=1 to get a backtrace of where an evaluation error was originally created.

If you changed something in rustc and want to re-build, run

./x.py --keep-stage 0 build src/rustc

This avoids rebuilding the entire stage 0, which can save a lot of time.

Contributing and getting help

Check out the issues on this GitHub repository for some ideas. There's lots that needs to be done that I haven't documented in the issues yet, however. For more ideas or help with running or hacking on Miri, you can contact me (scott) on Mozilla IRC in any of the Rust IRC channels (#rust, #rust-offtopic, etc).

History

This project began as part of an undergraduate research course in 2015 by @solson at the University of Saskatchewan. There are slides and a report available from that project. In 2016, @oli-obk joined to prepare miri for eventually being used as const evaluator in the Rust compiler itself (basically, for const and static stuff), replacing the old evaluator that worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla and began developing miri towards a tool for detecting undefined behavior, and also using miri as a way to explore the consequences of various possible definitions for undefined behavior in Rust. @oli-obk's move of the miri engine into the compiler finally came to completion in early 2018. Meanwhile, later that year, @RalfJung did a second internship, developing miri further with support for checking basic type invariants and verifying that references are used according to their aliasing restrictions.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT) at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you shall be dual licensed as above, without any additional terms or conditions.