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 SIMD, or networking, or file 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);
97 ### Running Miri on CI
99 To run Miri on CI, make sure that you handle the case where the latest nightly
100 does not ship the Miri component because it currently does not build. For
101 example, you can use the following snippet to always test with the latest
102 nightly that *does* come with Miri:
105 MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
106 echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
107 rustup default "$MIRI_NIGHTLY"
109 rustup component add miri
112 cargo miri test -- -- -Zunstable-options --exclude-should-panic
117 When using the above instructions, you may encounter a number of confusing compiler
120 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
122 Your build directory may contain artifacts from an earlier build that have/have
123 not been built for Miri. Run `cargo clean` before switching from non-Miri to
124 Miri builds and vice-versa.
126 #### "found crate `std` compiled by an incompatible version of rustc"
128 You may be running `cargo miri` with a different compiler version than the one
129 used to build the custom libstd that Miri uses, and Miri failed to detect that.
130 Try deleting `~/.cache/miri`.
132 #### "no mir for `std::rt::lang_start_internal`"
134 This means the sysroot you are using was not compiled with Miri in mind. This
135 should never happen when you use `cargo miri` because that takes care of setting
136 up the sysroot. If you are using `miri` (the Miri driver) directly, see
137 [below][testing-miri] for how to set up the sysroot.
139 ## Development and Debugging
141 If you want to hack on miri yourself, great! Here are some resources you might
144 ### Using a nightly rustc
146 Miri heavily relies on internal rustc interfaces to execute MIR. Still, some
147 things (like adding support for a new intrinsic or a shim for an external
148 function being called) can be done by working just on the Miri side.
150 To prepare, make sure you are using a nightly Rust compiler. Then you should be
151 able to just `cargo build` Miri.
153 In case this fails, your nightly might be incompatible with Miri master. The
154 `rust-version` file contains the commit hash of rustc that Miri is currently
155 tested against; you can use that to find a nightly that works or you might have
156 to wait for the next nightly to get released. You can also use
157 [`rustup-toolchain-install-master`](https://github.com/kennytm/rustup-toolchain-install-master)
158 to install that exact version of rustc as a toolchain:
160 rustup-toolchain-install-master $(cat rust-version) -c rust-src
163 Another common problem is outdated dependencies: Miri does not come with a
164 lockfile (it cannot, due to how it gets embedded into the rustc build). So you
165 have to run `cargo update` every now and then yourself to make sure you are
166 using the latest versions of everything (which is what gets tested on CI).
168 ### Testing the Miri driver
169 [testing-miri]: #testing-the-miri-driver
171 The Miri driver in the `miri` binary is the "heart" of Miri: it is basically a
172 version of `rustc` that, instead of compiling your code, runs it. It accepts
173 all the same flags as `rustc` (though the ones only affecting code generation
174 and linking obviously will have no effect) [and more][miri-flags].
176 Running the Miri driver requires some fiddling with environment variables, so
177 the `miri` script helps you do that. For example, you can run the driver on a
178 particular file by doing
181 ./miri run tests/run-pass/format.rs
182 ./miri run tests/run-pass/hello.rs --target i686-unknown-linux-gnu
185 and you can run the test suite using:
191 `./miri test FILTER` only runs those tests that contain `FILTER` in their
192 filename (including the base directory, e.g. `./miri test fail` will run all
195 You can get a trace of which MIR statements are being executed by setting the
196 `MIRI_LOG` environment variable. For example:
199 MIRI_LOG=info ./miri run tests/run-pass/vecs.rs
202 Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
203 the `rustc::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
204 can also do more targeted configuration, e.g. the following helps debug the
205 stacked borrows implementation:
208 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/run-pass/vecs.rs
211 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
212 evaluation error was originally raised.
214 ### Testing `cargo miri`
216 Working with the driver directly gives you full control, but you also lose all
217 the convenience provided by cargo. Once your test case depends on a crate, it
218 is probably easier to test it with the cargo wrapper. You can install your
219 development version of Miri using
225 and then you can use it as if it was installed by `rustup`. Make sure you use
226 the same toolchain when calling `cargo miri` that you used when installing Miri!
228 There's a test for the cargo wrapper in the `test-cargo-miri` directory; run
229 `./run-test.py` in there to execute it.
231 ### Using a locally built rustc
233 A big part of the Miri driver lives in rustc, so working on Miri will sometimes
234 require using a locally built rustc. The bug you want to fix may actually be on
235 the rustc side, or you just need to get more detailed trace of the execution
236 than what is possible with release builds -- in both cases, you should develop
237 miri against a rustc you compiled yourself, with debug assertions (and hence
240 The setup for a local rustc works as follows:
242 git clone https://github.com/rust-lang/rust/ rustc
244 cp config.toml.example config.toml
245 # Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
246 # The latter is important to build libstd with the right flags for miri.
247 # This step can take 30 minutes and more.
248 ./x.py build src/rustc
249 # If you change something, you can get a faster rebuild by doing
250 ./x.py --keep-stage 0 build src/rustc
251 # You may have to change the architecture in the next command
252 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
253 # Now cd to your Miri directory, then configure rustup
254 rustup override set custom
257 With this, you should now have a working development setup! See
258 [above][testing-miri] for how to proceed working with the Miri driver.
260 ### Miri `-Z` flags and environment variables
261 [miri-flags]: #miri--z-flags-and-environment-variables
263 Several `-Z` flags are relevant for Miri:
265 * `-Zmiri-seed=<hex>` is a custom `-Z` flag added by Miri. It enables the
266 interpreted program to seed an RNG with system entropy. Miri will keep an RNG
267 on its own that is seeded with the given seed, and use that to generate the
268 "system entropy" that seeds the RNG(s) in the interpreted program.
269 **NOTE**: This entropy is not good enough for cryptographic use! Do not
270 generate secret keys in Miri or perform other kinds of cryptographic
271 operations that rely on proper random numbers.
272 * `-Zmiri-disable-validation` disables enforcing the validity invariant, which
273 is enforced by default. This is mostly useful for debugging; it means Miri
274 will miss bugs in your program. However, this can also help to make Miri run
276 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
277 overrides the default to be `0`; be advised that using any higher level can
278 make Miri miss bugs in your program because they got optimized away.
279 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
280 functions. This is needed so that Miri can execute such functions, so Miri
281 sets this flag per default.
282 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
283 enables this per default because it is needed for validation.
285 Moreover, Miri recognizes some environment variables:
287 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
288 Miri executions, also [see above][testing-miri].
289 * `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
290 indicates the sysroot to use. To do the same thing with `miri`
291 directly, use the `--sysroot` flag.
292 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
293 architecture to test against. `miri` and `cargo miri` accept the `--target`
294 flag for the same purpose.
296 ## Contributing and getting help
298 Check out the issues on this GitHub repository for some ideas. There's lots that
299 needs to be done that I haven't documented in the issues yet, however. For more
300 ideas or help with running or hacking on Miri, you can open an issue here on
301 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
303 [Rust Zulip]: https://rust-lang.zulipchat.com
307 This project began as part of an undergraduate research course in 2015 by
308 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
309 [report] available from that project. In 2016, @oli-obk joined to prepare miri
310 for eventually being used as const evaluator in the Rust compiler itself
311 (basically, for `const` and `static` stuff), replacing the old evaluator that
312 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
313 and began developing miri towards a tool for detecting undefined behavior, and
314 also using miri as a way to explore the consequences of various possible
315 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
316 into the compiler finally came to completion in early 2018. Meanwhile, later
317 that year, @RalfJung did a second internship, developing miri further with
318 support for checking basic type invariants and verifying that references are
319 used according to their aliasing restrictions.
321 [usask]: https://www.usask.ca/
322 [slides]: https://solson.me/miri-slides.pdf
323 [report]: https://solson.me/miri-report.pdf
325 ## Bugs found by Miri
327 Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
331 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
332 * [`Vec::into_iter` doing an unaligned ZST read](https://github.com/rust-lang/rust/pull/53804)
333 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
334 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
335 * [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
336 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
337 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
338 * [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
339 * [The Unix allocator calling `posix_memalign` in an invalid way](https://github.com/rust-lang/rust/issues/62251)
341 Violations of Stacked Borrows found that are likely bugs (but Stacked Borrows is currently just an experiment):
343 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
344 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
345 * [`LinkedList` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
346 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
350 Licensed under either of
351 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
352 http://www.apache.org/licenses/LICENSE-2.0)
353 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
354 http://opensource.org/licenses/MIT) at your option.
358 Unless you explicitly state otherwise, any contribution intentionally submitted
359 for inclusion in the work by you shall be dual licensed as above, without any
360 additional terms or conditions.