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. Miri tries to avoid false
27 positives here, so if you program runs fine in Miri right now that is by no
28 means a guarantee that it is UB-free when these questions get answered. In
29 particular, Miri does currently not check that integers are initialized or
30 that references point to valid data.
31 * If the program relies on unspecified details of how data is laid out, it will
32 still run fine in Miri -- but might break (including causing UB) on different
33 compiler versions or different platforms.
34 * Miri is fully deterministic and does not actually pick a base address in
35 virtual memory for the program's allocations. If program behavior depends on
36 the base address of an allocation, Miri will stop execution (with a few
37 exceptions to make some common pointer comparisons work).
38 * Miri runs the program as a platform-independent interpreter, so the program
39 has no access to any platform-specific APIs or FFI. A few APIs have been
40 implemented (such as printing to stdout) but most have not: for example, Miri
41 currently does not support concurrency, or networking, or file system access,
42 or gathering entropy from the system.
44 [rust]: https://www.rust-lang.org/
45 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
46 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
47 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
50 ## Running Miri on your own project (and its test suite)
52 Install Miri via `rustup`:
55 rustup component add miri
58 If `rustup` says the `miri` component is unavailable, that's because not all
59 nightly releases come with all tools. Check out
60 [this website](https://rust-lang.github.io/rustup-components-history) to
61 determine a nightly version that comes with Miri and install that, e.g. using
62 `rustup install nightly-2019-03-28`.
64 Now you can run your project in Miri:
66 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
67 dependencies to be compiled the right way, that would not happen if they have
68 previously already been compiled.
69 2. To run all tests in your project through Miri, use `cargo miri test`.
70 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
72 The first time you run Miri, it will perform some extra setup and install some
73 dependencies. It will ask you for confirmation before installing anything. If
74 you run Miri on CI, run `cargo miri setup` to avoid getting interactive
77 You can pass arguments to Miri after the first `--`, and pass arguments to the
78 interpreted program or test suite after the second `--`. For example, `cargo
79 miri run -- -Zmiri-disable-validation` runs the program without validation of
80 basic type invariants and references. `cargo miri test -- -- -Zunstable-options
81 --exclude-should-panic` skips `#[should_panic]` tests, which is a good idea
82 because Miri does not support unwinding or catching panics.
84 When running code via `cargo miri`, the `miri` config flag is set. You can
85 use this to exclude test cases that will fail under Miri because they do things
86 Miri does not support:
91 fn does_not_work_on_miri() {
93 assert!(&x as *const _ as usize % 4 < 4);
99 When using the above instructions, you may encounter a number of confusing compiler
102 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
104 Your build directory may contain artifacts from an earlier build that have/have
105 not been built for Miri. Run `cargo clean` before switching from non-Miri to
106 Miri builds and vice-versa.
108 #### "found crate `std` compiled by an incompatible version of rustc"
110 You may be running `cargo miri` with a different compiler version than the one
111 used to build the custom libstd that Miri uses, and Miri failed to detect that.
112 Try deleting `~/.cache/miri`.
114 #### "no mir for `std::rt::lang_start_internal`"
116 This means the sysroot you are using was not compiled with Miri in mind. This
117 should never happen when you use `cargo miri` because that takes care of setting
118 up the sysroot. If you are using `miri` (the Miri driver) directly, see
119 [below][testing-miri] for how to set up the sysroot.
121 ## Development and Debugging
123 If you want to hack on miri yourself, great! Here are some resources you might
126 ### Using a nightly rustc
128 Miri heavily relies on internal rustc interfaces to execute MIR. Still, some
129 things (like adding support for a new intrinsic or a shim for an external
130 function being called) can be done by working just on the Miri side.
132 To prepare, make sure you are using a nightly Rust compiler. Then you should be
133 able to just `cargo build` Miri.
135 In case this fails, your nightly might be incompatible with Miri master. The
136 `rust-version` file contains the commit hash of rustc that Miri is currently
137 tested against; you can use that to find a nightly that works or you might have
138 to wait for the next nightly to get released. You can also use
139 [`rustup-toolchain-install-master`](https://github.com/kennytm/rustup-toolchain-install-master)
140 to install that exact version of rustc as a toolchain:
142 rustup-toolchain-install-master $(cat rust-version) -c rust-src
145 Another common problem is outdated dependencies: Miri does not come with a
146 lockfile (it cannot, due to how it gets embedded into the rustc build). So you
147 have to run `cargo update` every now and then yourself to make sure you are
148 using the latest versions of everything (which is what gets tested on CI).
150 ### Testing the Miri driver
151 [testing-miri]: #testing-the-miri-driver
153 The Miri driver in the `miri` binary is the "heart" of Miri: it is basically a
154 version of `rustc` that, instead of compiling your code, runs it. It accepts
155 all the same flags as `rustc` (though the ones only affecting code generation
156 and linking obviously will have no effect) [and more][miri-flags].
158 Running the Miri driver requires some fiddling with environment variables, so
159 the `miri` script helps you do that. For example, you can run the driver on a
160 particular file by doing
163 ./miri run tests/run-pass/format.rs
164 ./miri run tests/run-pass/hello.rs --target i686-unknown-linux-gnu
167 and you can run the test suite using:
173 `./miri test FILTER` only runs those tests that contain `FILTER` in their
174 filename (including the base directory, e.g. `./miri test fail` will run all
177 You can get a trace of which MIR statements are being executed by setting the
178 `MIRI_LOG` environment variable. For example:
181 MIRI_LOG=info ./miri run tests/run-pass/vecs.rs
184 Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
185 the `rustc::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
186 can also do more targeted configuration, e.g. the following helps debug the
187 stacked borrows implementation:
190 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/run-pass/vecs.rs
193 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
194 evaluation error was originally raised.
196 ### Testing `cargo miri`
198 Working with the driver directly gives you full control, but you also lose all
199 the convenience provided by cargo. Once your test case depends on a crate, it
200 is probably easier to test it with the cargo wrapper. You can install your
201 development version of Miri using
207 and then you can use it as if it was installed by `rustup`. Make sure you use
208 the same toolchain when calling `cargo miri` that you used when installing Miri!
210 There's a test for the cargo wrapper in the `test-cargo-miri` directory; run
211 `./run-test.py` in there to execute it.
213 ### Using a locally built rustc
215 A big part of the Miri driver lives in rustc, so working on Miri will sometimes
216 require using a locally built rustc. The bug you want to fix may actually be on
217 the rustc side, or you just need to get more detailed trace of the execution
218 than what is possible with release builds -- in both cases, you should develop
219 miri against a rustc you compiled yourself, with debug assertions (and hence
222 The setup for a local rustc works as follows:
224 git clone https://github.com/rust-lang/rust/ rustc
226 cp config.toml.example config.toml
227 # Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
228 # The latter is important to build libstd with the right flags for miri.
229 # This step can take 30 minutes and more.
230 ./x.py build src/rustc
231 # If you change something, you can get a faster rebuild by doing
232 ./x.py --keep-stage 0 build src/rustc
233 # You may have to change the architecture in the next command
234 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
235 # Now cd to your Miri directory, then configure rustup
236 rustup override set custom
239 With this, you should now have a working development setup! See
240 [above][testing-miri] for how to proceed working with the Miri driver.
242 ### Miri `-Z` flags and environment variables
243 [miri-flags]: #miri--z-flags-and-environment-variables
245 Several `-Z` flags are relevant for Miri:
247 * `-Zmiri-seed=<hex>` is a custom `-Z` flag added by Miri. It enables the
248 interpreted program to seed an RNG with system entropy. Miri will keep an RNG
249 on its own that is seeded with the given seed, and use that to generate the
250 "system entropy" that seeds the RNG(s) in the interpreted program.
251 **NOTE**: This entropy is not good enough for cryptographic use! Do not
252 generate secret keys in Miri or perform other kinds of cryptographic
253 operations that rely on proper random numbers.
254 * `-Zmiri-disable-validation` disables enforcing the validity invariant, which
255 is enforced by default. This is mostly useful for debugging; it means Miri
256 will miss bugs in your program. However, this can also help to make Miri run
258 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
259 overrides the default to be `0`; be advised that using any higher level can
260 make Miri miss bugs in your program because they got optimized away.
261 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
262 functions. This is needed so that Miri can execute such functions, so Miri
263 sets this flag per default.
265 Moreover, Miri recognizes some environment variables:
267 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
268 Miri executions, also [see above][testing-miri].
269 * `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
270 indicates the sysroot to use. To do the same thing with `miri`
271 directly, use the `--sysroot` flag.
272 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
273 architecture to test against. `miri` and `cargo miri` accept the `--target`
274 flag for the same purpose.
276 ## Contributing and getting help
278 Check out the issues on this GitHub repository for some ideas. There's lots that
279 needs to be done that I haven't documented in the issues yet, however. For more
280 ideas or help with running or hacking on Miri, you can open an issue here on
281 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
283 [Rust Zulip]: https://rust-lang.zulipchat.com
287 This project began as part of an undergraduate research course in 2015 by
288 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
289 [report] available from that project. In 2016, @oli-obk joined to prepare miri
290 for eventually being used as const evaluator in the Rust compiler itself
291 (basically, for `const` and `static` stuff), replacing the old evaluator that
292 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
293 and began developing miri towards a tool for detecting undefined behavior, and
294 also using miri as a way to explore the consequences of various possible
295 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
296 into the compiler finally came to completion in early 2018. Meanwhile, later
297 that year, @RalfJung did a second internship, developing miri further with
298 support for checking basic type invariants and verifying that references are
299 used according to their aliasing restrictions.
301 [usask]: https://www.usask.ca/
302 [slides]: https://solson.me/miri-slides.pdf
303 [report]: https://solson.me/miri-report.pdf
305 ## Bugs found by Miri
307 Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
311 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
312 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
313 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
314 * [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
315 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
316 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
317 * [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
319 Violations of Stacked Borrows found that are likely bugs (but Stacked Borrows is currently just an experiment):
321 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
322 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
323 * [`LinkedList` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
324 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
328 Licensed under either of
329 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
330 http://www.apache.org/licenses/LICENSE-2.0)
331 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
332 http://opensource.org/licenses/MIT) at your option.
336 Unless you explicitly state otherwise, any contribution intentionally submitted
337 for inclusion in the work by you shall be dual licensed as above, without any
338 additional terms or conditions.