1 # Miri [![Build Status](https://travis-ci.com/rust-lang/miri.svg?branch=master)](https://travis-ci.com/rust-lang/miri) [![Windows build status](https://ci.appveyor.com/api/projects/status/github/rust-lang/miri?svg=true)](https://ci.appveyor.com/project/rust-lang-libs/miri)
4 An experimental interpreter for [Rust][rust]'s
5 [mid-level intermediate representation][mir] (MIR). It can run binaries and
6 test suites of cargo projects and detect certain classes of undefined behavior,
9 * Out-of-bounds memory accesses and use-after-free
10 * Invalid use of uninitialized data
11 * Violation of intrinsic preconditions (an [`unreachable_unchecked`] being
12 reached, calling [`copy_nonoverlapping`] with overlapping ranges, ...)
13 * Not sufficiently aligned memory accesses and references
14 * Violation of basic type invariants (a `bool` that is not 0 or 1, for example,
15 or an invalid enum discriminant)
16 * WIP: Violations of the rules governing aliasing for reference types
18 Miri has already discovered some [real-world bugs](#bugs-found-by-miri). If you
19 found a bug with Miri, we'd appreciate if you tell us and we'll add it to the
22 [rust]: https://www.rust-lang.org/
23 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
24 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
25 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
28 ## Running Miri on your own project (and its test suite)
30 Install Miri via `rustup`:
33 rustup component add miri
36 Now you can run your project in Miri:
38 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
39 dependencies to be compiled the right way, that would not happen if they have
40 previously already been compiled.
41 2. To run all tests in your project through Miri, use `cargo miri test`.
42 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
44 The first time you run Miri, it will perform some extra setup and install some
45 dependencies. It will ask you for confirmation before installing anything. If
46 you run Miri on CI, run `cargo miri setup` to avoid getting interactive
49 You can pass arguments to Miri after the first `--`, and pass arguments to the
50 interpreted program or test suite after the second `--`. For example, `cargo
51 miri run -- -Zmiri-disable-validation` runs the program without validation of
52 basic type invariants and references. `cargo miri test -- -- -Zunstable-options
53 --exclude-should-panic` skips `#[should_panic]` tests, which is a good idea
54 because Miri does not support unwinding or catching panics.
56 When running code via `cargo miri`, the `miri` config flag is set. You can
57 use this to exclude test cases that will fail under Miri because they do things
58 Miri does not support:
63 fn does_not_work_on_miri() {
65 assert!(&x as *const _ as usize % 4 < 4);
71 When using the above instructions, you may encounter a number of confusing compiler
74 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
76 Your build directory may contain artifacts from an earlier build that have/have
77 not been built for Miri. Run `cargo clean` before switching from non-Miri to
78 Miri builds and vice-versa.
80 #### "found crate `std` compiled by an incompatible version of rustc"
82 You may be running `cargo miri` with a different compiler version than the one
83 used to build the custom libstd that Miri uses, and Miri failed to detect that.
84 Try deleting `~/.cache/miri`.
86 ## Development and Debugging
88 If you want to hack on miri yourself, great! Here are some resources you might
91 ### Using a nightly rustc
93 Miri heavily relies on internal rustc interfaces to execute MIR. Still, some
94 things (like adding support for a new intrinsic) can be done by working just on
97 To prepare, make sure you are using a nightly Rust compiler. The most
98 convenient way is to install Miri using cargo, then you can easily run it on
102 rustup component remove miri # avoid having Miri installed twice
103 cargo +nightly install --path "$DIR" --force # or the nightly in `rust-version`
104 cargo +nightly miri setup
107 (We are giving `+nightly` explicitly here all the time because it is important
108 that all of these commands get executed with the same toolchain.)
110 If you want to use a different libstd (not the one that comes with the
111 nightly), you can do that by running
114 XARGO_RUST_SRC=~/src/rust/rustc/src/ cargo +nightly miri setup
117 Either way, you can now do `cargo +nightly miri run` to run Miri with your
118 local changes on whatever project you are debugging.
120 `cargo miri setup` should end in printing the directory where the libstd was
121 built. For the next step to work, set that as your `MIRI_SYSROOT` environment
125 export MIRI_SYSROOT=~/.cache/miri/HOST # or whatever the previous command said
130 Instead of running an entire project using `cargo miri`, you can also use the
131 Miri "driver" directly to run just a single file. That can be easier during
135 cargo run tests/run-pass/format.rs # or whatever test you like
138 You can also run the test suite with `cargo test --release`. `cargo test
139 --release FILTER` only runs those tests that contain `FILTER` in their filename
140 (including the base directory, e.g. `cargo test --release fail` will run all
141 compile-fail tests). We recommend using `--release` to make test running take
144 Now you are set up! You can write a failing test case, and tweak miri until it
146 You can get a trace of which MIR statements are being executed by setting the
147 `MIRI_LOG` environment variable. For example:
150 MIRI_LOG=info cargo run tests/run-pass/vecs.rs
153 Setting `MIRI_LOG` like this will configure logging for miri itself as well as
154 the `rustc::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
155 can also do more targeted configuration, e.g. to debug the stacked borrows
158 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows cargo run tests/run-pass/vecs.rs
161 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
162 evaluation error was originally created.
165 ### Using a locally built rustc
167 Since the heart of Miri (the main interpreter engine) lives in rustc, working on
168 Miri will often require using a locally built rustc. The bug you want to fix
169 may actually be on the rustc side, or you just need to get more detailed trace
170 of the execution than what is possible with release builds -- in both cases, you
171 should develop miri against a rustc you compiled yourself, with debug assertions
172 (and hence tracing) enabled.
174 The setup for a local rustc works as follows:
176 git clone https://github.com/rust-lang/rust/ rustc
178 cp config.toml.example config.toml
179 # Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
180 # The latter is important to build libstd with the right flags for miri.
181 # This step can take 30 minutes and more.
182 ./x.py build src/rustc
183 # If you change something, you can get a faster rebuild by doing
184 ./x.py --keep-stage 0 build src/rustc
185 # You may have to change the architecture in the next command
186 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
187 # Now cd to your Miri directory, then configure rustup
188 rustup override set custom
191 With this, you should now have a working development setup! See
192 ["Testing Miri"](#testing-miri) above for how to proceed.
194 Running `cargo miri` in this setup is a bit more complicated, because the Miri
195 binary you just created does not actually run without some enviroment variables.
196 But you can contort cargo into calling `cargo miri` the right way for you:
199 # in some other project's directory, to run `cargo miri test`:
200 MIRI_SYSROOT=$(rustc +custom --print sysroot) cargo +custom run --manifest-path /path/to/miri/Cargo.toml --bin cargo-miri --release -- miri test
203 ### Miri `-Z` flags and environment variables
205 Several `-Z` flags are relevant for Miri:
207 * `-Zmir-opt-level` controls how many MIR optimizations are performed. miri
208 overrides the default to be `0`; be advised that using any higher level can
209 make miri miss bugs in your program because they got optimized away.
210 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
211 functions. This is needed so that miri can execute such functions, so miri
212 sets this flag per default.
213 * `-Zmiri-disable-validation` is a custom `-Z` flag added by miri. It disables
214 enforcing the validity invariant, which is enforced by default. This is
215 mostly useful for debugging; it means miri will miss bugs in your program.
217 Moreover, Miri recognizes some environment variables:
219 * `MIRI_SYSROOT` (recognized by `miri`, `cargo miri` and the test suite)
220 indicates the sysroot to use.
221 * `MIRI_TARGET` (recognized by the test suite) indicates which target
222 architecture to test against. `miri` and `cargo miri` accept the `--target`
223 flag for the same purpose.
225 ## Contributing and getting help
227 Check out the issues on this GitHub repository for some ideas. There's lots that
228 needs to be done that I haven't documented in the issues yet, however. For more
229 ideas or help with running or hacking on Miri, you can open an issue here on
230 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
232 [Rust Zulip]: https://rust-lang.zulipchat.com
236 This project began as part of an undergraduate research course in 2015 by
237 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
238 [report] available from that project. In 2016, @oli-obk joined to prepare miri
239 for eventually being used as const evaluator in the Rust compiler itself
240 (basically, for `const` and `static` stuff), replacing the old evaluator that
241 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
242 and began developing miri towards a tool for detecting undefined behavior, and
243 also using miri as a way to explore the consequences of various possible
244 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
245 into the compiler finally came to completion in early 2018. Meanwhile, later
246 that year, @RalfJung did a second internship, developing miri further with
247 support for checking basic type invariants and verifying that references are
248 used according to their aliasing restrictions.
250 [usask]: https://www.usask.ca/
251 [slides]: https://solson.me/miri-slides.pdf
252 [report]: https://solson.me/miri-report.pdf
254 ## Bugs found by Miri
256 Miri has already found a number of bugs in the Rust standard library, which we collect here.
258 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
259 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
260 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
261 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
262 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
263 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
264 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
268 Licensed under either of
269 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
270 http://www.apache.org/licenses/LICENSE-2.0)
271 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
272 http://opensource.org/licenses/MIT) at your option.
276 Unless you explicitly state otherwise, any contribution intentionally submitted
277 for inclusion in the work by you shall be dual licensed as above, without any
278 additional terms or conditions.