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
7 [undefined behavior](https://doc.rust-lang.org/reference/behavior-considered-undefined.html),
10 * Out-of-bounds memory accesses and use-after-free
11 * Invalid use of uninitialized data
12 * Violation of intrinsic preconditions (an [`unreachable_unchecked`] being
13 reached, calling [`copy_nonoverlapping`] with overlapping ranges, ...)
14 * Not sufficiently aligned memory accesses and references
15 * Violation of *some* basic type invariants (a `bool` that is not 0 or 1, for example,
16 or an invalid enum discriminant)
17 * WIP: Violations of the rules governing aliasing for reference types
19 Miri has already discovered some [real-world bugs](#bugs-found-by-miri). If you
20 found a bug with Miri, we'd appreciate if you tell us and we'll add it to the
23 Be aware that Miri will not catch all cases of undefined behavior in your
24 program, and cannot run all programs:
26 * There are still plenty of open questions around the basic invariants for some
27 types and when these invariants even have to hold. Miri tries to avoid false
28 positives here, so if you program runs fine in Miri right now that is by no
29 means a guarantee that it is UB-free when these questions get answered.
31 In particular, Miri does currently not check that integers are initialized
32 or that references point to valid data.
33 * If the program relies on unspecified details of how data is laid out, it will
34 still run fine in Miri -- but might break (including causing UB) on different
35 compiler versions or different platforms.
36 * Program execution is non-deterministic when it depends, for example, on where
37 exactly in memory allocations end up. Miri tests one of many possible
38 executions of your program. If your code is sensitive to allocation base
39 addresses or other non-deterministic data, try running Miri with different
40 values for `-Zmiri-seed` to test different executions.
41 * Miri runs the program as a platform-independent interpreter, so the program
42 has no access to any platform-specific APIs or FFI. A few APIs have been
43 implemented (such as printing to stdout) but most have not: for example, Miri
44 currently does not support concurrency, or SIMD, or networking, or file system
47 [rust]: https://www.rust-lang.org/
48 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
49 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
50 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
53 ## Running Miri on your own project (and its test suite)
55 Install Miri via `rustup`:
58 rustup component add miri
61 If `rustup` says the `miri` component is unavailable, that's because not all
62 nightly releases come with all tools. Check out
63 [this website](https://rust-lang.github.io/rustup-components-history) to
64 determine a nightly version that comes with Miri and install that, e.g. using
65 `rustup install nightly-2019-03-28`.
67 Now you can run your project in Miri:
69 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
70 dependencies to be compiled the right way, that would not happen if they have
71 previously already been compiled.
72 2. To run all tests in your project through Miri, use `cargo miri test`.
73 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
75 The first time you run Miri, it will perform some extra setup and install some
76 dependencies. It will ask you for confirmation before installing anything.
78 You can pass arguments to Miri after the first `--`, and pass arguments to the
79 interpreted program or test suite after the second `--`. For example, `cargo
80 miri run -- -Zmiri-disable-validation` runs the program without validation of
81 basic type invariants and references. `cargo miri test -- -- -Zunstable-options
82 --exclude-should-panic` skips `#[should_panic]` tests, which is a good idea
83 because Miri does not support unwinding or catching panics.
85 When running code via `cargo miri`, the `miri` config flag is set. You can
86 use this to exclude test cases that will fail under Miri because they do things
87 Miri does not support:
92 fn does_not_work_on_miri() {
94 assert!(&x as *const _ as usize % 4 < 4);
98 ### Running Miri on CI
100 To run Miri on CI, make sure that you handle the case where the latest nightly
101 does not ship the Miri component because it currently does not build. For
102 example, you can use the following snippet to always test with the latest
103 nightly that *does* come with Miri:
106 MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
107 echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
108 rustup default "$MIRI_NIGHTLY"
110 rustup component add miri
113 cargo miri test -- -- -Zunstable-options --exclude-should-panic
116 We use `cargo miri setup` to avoid getting interactive questions about the extra
117 setup needed for Miri.
121 When using the above instructions, you may encounter a number of confusing compiler
124 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
126 Your build directory may contain artifacts from an earlier build that have/have
127 not been built for Miri. Run `cargo clean` before switching from non-Miri to
128 Miri builds and vice-versa.
130 #### "found crate `std` compiled by an incompatible version of rustc"
132 You may be running `cargo miri` with a different compiler version than the one
133 used to build the custom libstd that Miri uses, and Miri failed to detect that.
134 Try deleting `~/.cache/miri`.
136 #### "no mir for `std::rt::lang_start_internal`"
138 This means the sysroot you are using was not compiled with Miri in mind. This
139 should never happen when you use `cargo miri` because that takes care of setting
140 up the sysroot. If you are using `miri` (the Miri driver) directly, see
141 [below][testing-miri] for how to set up the sysroot.
144 ## Miri `-Z` flags and environment variables
145 [miri-flags]: #miri--z-flags-and-environment-variables
147 Several `-Z` flags are relevant for Miri:
149 * `-Zmiri-seed=<hex>` is a custom `-Z` flag added by Miri. It configures the
150 seed of the RNG that Miri uses to resolve non-determinism. This RNG is used
151 to pick base addresses for allocations, and when the interpreted program
152 requests system entropy. The default seed is 0.
153 **NOTE**: This entropy is not good enough for cryptographic use! Do not
154 generate secret keys in Miri or perform other kinds of cryptographic
155 operations that rely on proper random numbers.
156 * `-Zmiri-disable-validation` disables enforcing the validity invariant, which
157 is enforced by default. This is mostly useful for debugging; it means Miri
158 will miss bugs in your program. However, this can also help to make Miri run
160 * `-Zmiri-enable-communication` enables communication between the host
161 environment and Miri, i.e., all the host environment variables are available
163 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
164 overrides the default to be `0`; be advised that using any higher level can
165 make Miri miss bugs in your program because they got optimized away.
166 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
167 functions. This is needed so that Miri can execute such functions, so Miri
168 sets this flag per default.
169 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
170 enables this per default because it is needed for validation.
172 Moreover, Miri recognizes some environment variables:
174 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
175 Miri executions, also [see above][testing-miri].
176 * `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
177 indicates the sysroot to use. To do the same thing with `miri`
178 directly, use the `--sysroot` flag.
179 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
180 architecture to test against. `miri` and `cargo miri` accept the `--target`
181 flag for the same purpose.
183 ## Development and Debugging
185 If you want to hack on miri yourself, great! Here are some resources you might
188 ### Using a nightly rustc
190 Miri heavily relies on internal rustc interfaces to execute MIR. Still, some
191 things (like adding support for a new intrinsic or a shim for an external
192 function being called) can be done by working just on the Miri side.
194 To prepare, make sure you are using a nightly Rust compiler. Then you should be
195 able to just `cargo build` Miri.
197 In case this fails, your nightly might be incompatible with Miri master. The
198 `rust-version` file contains the commit hash of rustc that Miri is currently
199 tested against; you can use that to find a nightly that works or you might have
200 to wait for the next nightly to get released. You can also use
201 [`rustup-toolchain-install-master`](https://github.com/kennytm/rustup-toolchain-install-master)
202 to install that exact version of rustc as a toolchain:
204 rustup-toolchain-install-master $(cat rust-version) -c rust-src
207 Another common problem is outdated dependencies: Miri does not come with a
208 lockfile (it cannot, due to how it gets embedded into the rustc build). So you
209 have to run `cargo update` every now and then yourself to make sure you are
210 using the latest versions of everything (which is what gets tested on CI).
212 ### Testing the Miri driver
213 [testing-miri]: #testing-the-miri-driver
215 The Miri driver in the `miri` binary is the "heart" of Miri: it is basically a
216 version of `rustc` that, instead of compiling your code, runs it. It accepts
217 all the same flags as `rustc` (though the ones only affecting code generation
218 and linking obviously will have no effect) [and more][miri-flags].
220 Running the Miri driver requires some fiddling with environment variables, so
221 the `miri` script helps you do that. For example, you can run the driver on a
222 particular file by doing
225 ./miri run tests/run-pass/format.rs
226 ./miri run tests/run-pass/hello.rs --target i686-unknown-linux-gnu
229 and you can run the test suite using:
235 `./miri test FILTER` only runs those tests that contain `FILTER` in their
236 filename (including the base directory, e.g. `./miri test fail` will run all
239 You can get a trace of which MIR statements are being executed by setting the
240 `MIRI_LOG` environment variable. For example:
243 MIRI_LOG=info ./miri run tests/run-pass/vecs.rs
246 Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
247 the `rustc::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
248 can also do more targeted configuration, e.g. the following helps debug the
249 stacked borrows implementation:
252 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/run-pass/vecs.rs
255 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
256 evaluation error was originally raised.
258 ### Testing `cargo miri`
260 Working with the driver directly gives you full control, but you also lose all
261 the convenience provided by cargo. Once your test case depends on a crate, it
262 is probably easier to test it with the cargo wrapper. You can install your
263 development version of Miri using
269 and then you can use it as if it was installed by `rustup`. Make sure you use
270 the same toolchain when calling `cargo miri` that you used when installing Miri!
272 There's a test for the cargo wrapper in the `test-cargo-miri` directory; run
273 `./run-test.py` in there to execute it.
275 ### Using a locally built rustc
277 A big part of the Miri driver lives in rustc, so working on Miri will sometimes
278 require using a locally built rustc. The bug you want to fix may actually be on
279 the rustc side, or you just need to get more detailed trace of the execution
280 than what is possible with release builds -- in both cases, you should develop
281 miri against a rustc you compiled yourself, with debug assertions (and hence
284 The setup for a local rustc works as follows:
286 git clone https://github.com/rust-lang/rust/ rustc
288 cp config.toml.example config.toml
289 # Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
290 # The latter is important to build libstd with the right flags for miri.
291 # This step can take 30 minutes and more.
292 ./x.py build src/rustc
293 # If you change something, you can get a faster rebuild by doing
294 ./x.py --keep-stage 0 build src/rustc
295 # You may have to change the architecture in the next command
296 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
297 # Now cd to your Miri directory, then configure rustup
298 rustup override set custom
301 With this, you should now have a working development setup! See
302 [above][testing-miri] for how to proceed working with the Miri driver.
304 ## Contributing and getting help
306 Check out the issues on this GitHub repository for some ideas. There's lots that
307 needs to be done that I haven't documented in the issues yet, however. For more
308 ideas or help with running or hacking on Miri, you can open an issue here on
309 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
311 [Rust Zulip]: https://rust-lang.zulipchat.com
315 This project began as part of an undergraduate research course in 2015 by
316 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
317 [report] available from that project. In 2016, @oli-obk joined to prepare miri
318 for eventually being used as const evaluator in the Rust compiler itself
319 (basically, for `const` and `static` stuff), replacing the old evaluator that
320 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
321 and began developing miri towards a tool for detecting undefined behavior, and
322 also using miri as a way to explore the consequences of various possible
323 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
324 into the compiler finally came to completion in early 2018. Meanwhile, later
325 that year, @RalfJung did a second internship, developing miri further with
326 support for checking basic type invariants and verifying that references are
327 used according to their aliasing restrictions.
329 [usask]: https://www.usask.ca/
330 [slides]: https://solson.me/miri-slides.pdf
331 [report]: https://solson.me/miri-report.pdf
333 ## Bugs found by Miri
335 Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
339 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
340 * [`Vec::into_iter` doing an unaligned ZST read](https://github.com/rust-lang/rust/pull/53804)
341 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
342 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
343 * [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
344 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
345 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
346 * [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
347 * [The Unix allocator calling `posix_memalign` in an invalid way](https://github.com/rust-lang/rust/issues/62251)
348 * [`getrandom` calling the `getrandom` syscall in an invalid way](https://github.com/rust-random/getrandom/pull/73)
350 Violations of Stacked Borrows found that are likely bugs (but Stacked Borrows is currently just an experiment):
352 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
353 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
354 * [`LinkedList` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
355 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
359 Licensed under either of
360 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
361 http://www.apache.org/licenses/LICENSE-2.0)
362 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
363 http://opensource.org/licenses/MIT) at your option.
367 Unless you explicitly state otherwise, any contribution intentionally submitted
368 for inclusion in the work by you shall be dual licensed as above, without any
369 additional terms or conditions.