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 Be aware that Miri will not catch all possible errors in your program, and
23 cannot run all programs:
25 * There are still plenty of open questions around the basic invariants for some
26 types and when these invariants even have to hold, so if you program runs fine
27 in Miri right now that is by no means a guarantee that it is UB-free when
28 these questions get answered.
29 * If the program relies on unspecified details of how data is laid out, it will
30 still run fine in Miri -- but might break (including causing UB) on different
31 compiler versions or different platforms.
32 * Miri is fully deterministic and does not actually pick a base address in
33 virtual memory for the program's allocations. If program behavior depends on
34 the base address of an allocation, Miri will stop execution (with a few
35 exceptions to make some common pointer comparisons work).
36 * Miri runs the program as a platform-independent interpreter, so the program
37 has no access to any platform-specific APIs or FFI. A few APIs have been
38 implemented (such as printing to stdout) but most have not: for example, Miri
39 currently does not support concurrency, or networking, or file system access,
40 or gathering entropy from the system.
42 [rust]: https://www.rust-lang.org/
43 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
44 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
45 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
48 ## Running Miri on your own project (and its test suite)
50 Install Miri via `rustup`:
53 rustup component add miri
56 If `rustup` says the `miri` component is unavailable, that's because not all nightly releases come with all tools. Check out [this website](https://rust-lang.github.io/rustup-components-history) to determine a nightly version that comes with Miri and install that, e.g. using `rustup install nightly-2019-03-28`.
58 Now you can run your project in Miri:
60 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
61 dependencies to be compiled the right way, that would not happen if they have
62 previously already been compiled.
63 2. To run all tests in your project through Miri, use `cargo miri test`.
64 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
66 The first time you run Miri, it will perform some extra setup and install some
67 dependencies. It will ask you for confirmation before installing anything. If
68 you run Miri on CI, run `cargo miri setup` to avoid getting interactive
71 You can pass arguments to Miri after the first `--`, and pass arguments to the
72 interpreted program or test suite after the second `--`. For example, `cargo
73 miri run -- -Zmiri-disable-validation` runs the program without validation of
74 basic type invariants and references. `cargo miri test -- -- -Zunstable-options
75 --exclude-should-panic` skips `#[should_panic]` tests, which is a good idea
76 because Miri does not support unwinding or catching panics.
78 When running code via `cargo miri`, the `miri` config flag is set. You can
79 use this to exclude test cases that will fail under Miri because they do things
80 Miri does not support:
85 fn does_not_work_on_miri() {
87 assert!(&x as *const _ as usize % 4 < 4);
93 When using the above instructions, you may encounter a number of confusing compiler
96 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
98 Your build directory may contain artifacts from an earlier build that have/have
99 not been built for Miri. Run `cargo clean` before switching from non-Miri to
100 Miri builds and vice-versa.
102 #### "found crate `std` compiled by an incompatible version of rustc"
104 You may be running `cargo miri` with a different compiler version than the one
105 used to build the custom libstd that Miri uses, and Miri failed to detect that.
106 Try deleting `~/.cache/miri`.
108 ## Development and Debugging
110 If you want to hack on miri yourself, great! Here are some resources you might
113 ### Using a nightly rustc
115 Miri heavily relies on internal rustc interfaces to execute MIR. Still, some
116 things (like adding support for a new intrinsic) can be done by working just on
119 To prepare, make sure you are using a nightly Rust compiler. The most
120 convenient way is to install Miri using cargo, then you can easily run it on
124 rustup component remove miri # avoid having Miri installed twice
125 cargo +nightly install --path "$DIR" --force # or the nightly in `rust-version`
126 cargo +nightly miri setup
129 (We are giving `+nightly` explicitly here all the time because it is important
130 that all of these commands get executed with the same toolchain.)
132 If you want to use a different libstd (not the one that comes with the
133 nightly), you can do that by running
136 XARGO_RUST_SRC=~/src/rust/rustc/src/ cargo +nightly miri setup
139 Either way, you can now do `cargo +nightly miri run` to run Miri with your
140 local changes on whatever project you are debugging.
142 `cargo miri setup` should end in printing the directory where the libstd was
143 built. For the next step to work, set that as your `MIRI_SYSROOT` environment
147 export MIRI_SYSROOT=~/.cache/miri/HOST # or whatever the previous command said
152 Instead of running an entire project using `cargo miri`, you can also use the
153 Miri "driver" directly to run just a single file. That can be easier during
157 cargo run tests/run-pass/format.rs # or whatever test you like
160 You can also run the test suite with `cargo test --release`. `cargo test
161 --release FILTER` only runs those tests that contain `FILTER` in their filename
162 (including the base directory, e.g. `cargo test --release fail` will run all
163 compile-fail tests). We recommend using `--release` to make test running take
166 Now you are set up! You can write a failing test case, and tweak miri until it
168 You can get a trace of which MIR statements are being executed by setting the
169 `MIRI_LOG` environment variable. For example:
172 MIRI_LOG=info cargo run tests/run-pass/vecs.rs
175 Setting `MIRI_LOG` like this will configure logging for miri itself as well as
176 the `rustc::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
177 can also do more targeted configuration, e.g. to debug the stacked borrows
180 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows cargo run tests/run-pass/vecs.rs
183 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
184 evaluation error was originally created.
187 ### Using a locally built rustc
189 Since the heart of Miri (the main interpreter engine) lives in rustc, working on
190 Miri will often require using a locally built rustc. The bug you want to fix
191 may actually be on the rustc side, or you just need to get more detailed trace
192 of the execution than what is possible with release builds -- in both cases, you
193 should develop miri against a rustc you compiled yourself, with debug assertions
194 (and hence tracing) enabled.
196 The setup for a local rustc works as follows:
198 git clone https://github.com/rust-lang/rust/ rustc
200 cp config.toml.example config.toml
201 # Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
202 # The latter is important to build libstd with the right flags for miri.
203 # This step can take 30 minutes and more.
204 ./x.py build src/rustc
205 # If you change something, you can get a faster rebuild by doing
206 ./x.py --keep-stage 0 build src/rustc
207 # You may have to change the architecture in the next command
208 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
209 # Now cd to your Miri directory, then configure rustup
210 rustup override set custom
213 With this, you should now have a working development setup! See
214 ["Testing Miri"](#testing-miri) above for how to proceed.
216 Running `cargo miri` in this setup is a bit more complicated, because the Miri
217 binary you just created does not actually run without some environment variables.
218 But you can contort cargo into calling `cargo miri` the right way for you:
221 # in some other project's directory, to run `cargo miri test`:
222 MIRI_SYSROOT=$(rustc +custom --print sysroot) cargo +custom run --manifest-path /path/to/miri/Cargo.toml --bin cargo-miri --release -- miri test
225 ### Miri `-Z` flags and environment variables
227 Several `-Z` flags are relevant for Miri:
229 * `-Zmir-opt-level` controls how many MIR optimizations are performed. miri
230 overrides the default to be `0`; be advised that using any higher level can
231 make miri miss bugs in your program because they got optimized away.
232 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
233 functions. This is needed so that miri can execute such functions, so miri
234 sets this flag per default.
235 * `-Zmiri-disable-validation` is a custom `-Z` flag added by miri. It disables
236 enforcing the validity invariant, which is enforced by default. This is
237 mostly useful for debugging; it means miri will miss bugs in your program.
239 Moreover, Miri recognizes some environment variables:
241 * `MIRI_SYSROOT` (recognized by `miri`, `cargo miri` and the test suite)
242 indicates the sysroot to use.
243 * `MIRI_TARGET` (recognized by the test suite) indicates which target
244 architecture to test against. `miri` and `cargo miri` accept the `--target`
245 flag for the same purpose.
247 ## Contributing and getting help
249 Check out the issues on this GitHub repository for some ideas. There's lots that
250 needs to be done that I haven't documented in the issues yet, however. For more
251 ideas or help with running or hacking on Miri, you can open an issue here on
252 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
254 [Rust Zulip]: https://rust-lang.zulipchat.com
258 This project began as part of an undergraduate research course in 2015 by
259 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
260 [report] available from that project. In 2016, @oli-obk joined to prepare miri
261 for eventually being used as const evaluator in the Rust compiler itself
262 (basically, for `const` and `static` stuff), replacing the old evaluator that
263 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
264 and began developing miri towards a tool for detecting undefined behavior, and
265 also using miri as a way to explore the consequences of various possible
266 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
267 into the compiler finally came to completion in early 2018. Meanwhile, later
268 that year, @RalfJung did a second internship, developing miri further with
269 support for checking basic type invariants and verifying that references are
270 used according to their aliasing restrictions.
272 [usask]: https://www.usask.ca/
273 [slides]: https://solson.me/miri-slides.pdf
274 [report]: https://solson.me/miri-report.pdf
276 ## Bugs found by Miri
278 Miri has already found a number of bugs in the Rust standard library, which we collect here.
280 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
281 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
282 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
283 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
284 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
285 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
286 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
290 Licensed under either of
291 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
292 http://www.apache.org/licenses/LICENSE-2.0)
293 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
294 http://opensource.org/licenses/MIT) at your option.
298 Unless you explicitly state otherwise, any contribution intentionally submitted
299 for inclusion in the work by you shall be dual licensed as above, without any
300 additional terms or conditions.