1 # Miri [![Build Status](https://travis-ci.org/solson/miri.svg?branch=master)](https://travis-ci.org/solson/miri) [![Windows build status](https://ci.appveyor.com/api/projects/status/github/solson/miri?svg=true)](https://ci.appveyor.com/project/solson63299/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 [rust]: https://www.rust-lang.org/
19 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
20 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
21 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
24 ## Running Miri on your own project('s test suite)
26 Install Miri as a cargo subcommand:
29 cargo +nightly install --force --git https://github.com/solson/miri miri
32 If this does not work, try using the nightly version given in
33 [this file](https://raw.githubusercontent.com/solson/miri/master/rust-version). CI
34 should ensure that this nightly always works.
36 You have to use a consistent Rust version for building miri and your project, so
37 remember to either always specify the nightly version manually (like in the
38 example above), overriding it in your project directory as well, or use `rustup
39 default nightly` (or `rustup default nightly-YYYY-MM-DD`) to globally make
40 `nightly` the default toolchain.
42 Now you can run your project in Miri:
44 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
45 dependencies to be compiled the right way, that would not happen if they have
46 previously already been compiled.
47 2. To run all tests in your project through Miri, use `cargo +nightly miri test`.
48 3. If you have a binary project, you can run it through Miri using `cargo
51 When running code via `cargo miri`, the `cargo-miri` feature is set. You can
52 use this to exclude test cases that will fail under Miri because they do things
53 Miri does not support:
56 #[cfg(not(feature = "cargo-miri"))]
58 fn does_not_work_on_miri() {
60 assert!(&x as *const _ as usize % 4 < 4);
66 When using the above instructions, you may encounter a number of confusing compiler
69 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
71 Your build directory may contain artifacts from an earlier build that have/have
72 not been built for Miri. Run `cargo clean` before switching from non-Miri to
73 Miri builds and vice-versa.
75 #### "found crate `std` compiled by an incompatible version of rustc"
77 You may be running `cargo miri` with a different compiler version than the one
78 used to build the custom libstd that Miri uses, and Miri failed to detect that.
79 Try deleting `~/.cache/miri`.
81 ## Development and Debugging
83 If you want to hack on miri yourself, great! Here are some resources you might
86 ### Using a nightly rustc
88 Miri heavily relies on internal rustc interfaces to execute MIR. Still, some
89 things (like adding support for a new intrinsic) can be done by working just on
92 To prepare, make sure you are using a nightly Rust compiler. The most
93 convenient way is to install Miri using cargo, then you can easily run it on
97 cargo +nightly install --path "$DIR" --force # or the nightly in `rust-version`
98 cargo +nightly miri setup
101 If you want to use a different libstd (not the one that comes with the
102 nightly), you can do that by running
105 XARGO_RUST_SRC=~/src/rust/rustc/src/ cargo +nightly miri setup
108 Either way, you can now do `cargo +nightly miri run` to run Miri with your
109 local changes on whatever project you are debugging.
111 (We are giving `+nightly` explicitly here all the time because it is important
112 that all of these commands get executed with the same toolchain.)
116 Instead of running an entire project using `cargo miri`, you can also use the
117 Miri "driver" directly to run just a single file. That can be easier during
120 `cargo miri setup` should end in printing the directory where the libstd was
121 built. Set that as your `MIRI_SYSROOT` environment variable:
124 export MIRI_SYSROOT=~/.cache/miri/HOST # or whatever the previous command said
127 Now you can run Miri directly, without going through `cargo miri`:
130 cargo run tests/run-pass/format.rs # or whatever test you like
133 You can also run the test suite with `cargo test --release`. `cargo test
134 --release FILTER` only runs those tests that contain `FILTER` in their filename
135 (including the base directory, e.g. `cargo test --release fail` will run all
136 compile-fail tests). We recommend using `--release` to make test running take
139 Now you are set up! You can write a failing test case, and tweak miri until it
141 You can get a trace of which MIR statements are being executed by setting the
142 `MIRI_LOG` environment variable. For example:
145 MIRI_LOG=info cargo run tests/run-pass/vecs.rs
148 Setting `MIRI_LOG` like this will configure logging for miri itself as well as
149 the `rustc::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
150 can also do more targeted configuration, e.g. to debug the stacked borrows
153 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows cargo run tests/run-pass/vecs.rs
156 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
157 evaluation error was originally created.
160 ### Using a locally built rustc
162 Since the heart of Miri (the main interpreter engine) lives in rustc, working on
163 Miri will often require using a locally built rustc. The bug you want to fix
164 may actually be on the rustc side, or you just need to get more detailed trace
165 of the execution than what is possible with release builds -- in both cases, you
166 should develop miri against a rustc you compiled yourself, with debug assertions
167 (and hence tracing) enabled.
169 The setup for a local rustc works as follows:
171 git clone https://github.com/rust-lang/rust/ rustc
173 cp config.toml.example config.toml
174 # Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
175 # The latter is important to build libstd with the right flags for miri.
176 # This step can take 30 minutes and more.
177 ./x.py build src/rustc
178 # If you change something, you can get a faster rebuild by doing
179 ./x.py --keep-stage 0 build src/rustc
180 # You may have to change the architecture in the next command
181 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
182 # Now cd to your Miri directory, then configure rustup
183 rustup override set custom
186 With this, you should now have a working development setup! See
187 ["Testing Miri"](#testing-miri) above for how to proceed.
189 ### Miri `-Z` flags and environment variables
191 Several `-Z` flags are relevant for Miri:
193 * `-Zmir-opt-level` controls how many MIR optimizations are performed. miri
194 overrides the default to be `0`; be advised that using any higher level can
195 make miri miss bugs in your program because they got optimized away.
196 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
197 functions. This is needed so that miri can execute such functions, so miri
198 sets this flag per default.
199 * `-Zmiri-disable-validation` is a custom `-Z` flag added by miri. It disables
200 enforcing the validity invariant, which is enforced by default. This is
201 mostly useful for debugging; it means miri will miss bugs in your program.
203 Moreover, Miri recognizes some environment variables:
205 * `MIRI_SYSROOT` (recognized by `miri`, `cargo miri` and the test suite)
206 indicates the sysroot to use.
207 * `MIRI_TARGET` (recognized by the test suite) indicates which target
208 architecture to test against. `miri` and `cargo miri` accept the `--target`
209 flag for the same purpose.
211 ## Contributing and getting help
213 Check out the issues on this GitHub repository for some ideas. There's lots that
214 needs to be done that I haven't documented in the issues yet, however. For more
215 ideas or help with running or hacking on Miri, you can open an issue here on
216 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
218 [Rust Zulip]: https://rust-lang.zulipchat.com
222 This project began as part of an undergraduate research course in 2015 by
223 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
224 [report] available from that project. In 2016, @oli-obk joined to prepare miri
225 for eventually being used as const evaluator in the Rust compiler itself
226 (basically, for `const` and `static` stuff), replacing the old evaluator that
227 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
228 and began developing miri towards a tool for detecting undefined behavior, and
229 also using miri as a way to explore the consequences of various possible
230 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
231 into the compiler finally came to completion in early 2018. Meanwhile, later
232 that year, @RalfJung did a second internship, developing miri further with
233 support for checking basic type invariants and verifying that references are
234 used according to their aliasing restrictions.
236 [usask]: https://www.usask.ca/
237 [slides]: https://solson.me/miri-slides.pdf
238 [report]: https://solson.me/miri-report.pdf
242 Licensed under either of
243 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
244 http://www.apache.org/licenses/LICENSE-2.0)
245 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
246 http://opensource.org/licenses/MIT) at your option.
250 Unless you explicitly state otherwise, any contribution intentionally submitted
251 for inclusion in the work by you shall be dual licensed as above, without any
252 additional terms or conditions.