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 most 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.
46 [rust]: https://www.rust-lang.org/
47 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
48 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
49 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
54 Install Miri on Rust nightly via `rustup`:
57 rustup +nightly component add miri
60 If `rustup` says the `miri` component is unavailable, that's because not all
61 nightly releases come with all tools. Check out
62 [this website](https://rust-lang.github.io/rustup-components-history) to
63 determine a nightly version that comes with Miri and install that using
64 `rustup toolchain install nightly-YYYY-MM-DD`.
66 Now you can run your project in Miri:
68 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
69 dependencies to be compiled the right way, that would not happen if they have
70 previously already been compiled.
71 2. To run all tests in your project through Miri, use `cargo miri test`.
72 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
74 The first time you run Miri, it will perform some extra setup and install some
75 dependencies. It will ask you for confirmation before installing anything.
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 without checking the aliasing of references.
82 When compiling code via `cargo miri`, the `miri` config flag is set. You can
83 use this to ignore test cases that will fail under Miri because they do things
84 Miri does not support:
88 #[cfg_attr(miri, ignore)]
89 fn does_not_work_on_miri() {
90 std::thread::spawn(|| println!("Hello Thread!"))
96 ### Running Miri on CI
98 To run Miri on CI, make sure that you handle the case where the latest nightly
99 does not ship the Miri component because it currently does not build. For
100 example, you can use the following snippet to always test with the latest
101 nightly that *does* come with Miri:
104 MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
105 echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
106 rustup set profile minimal
107 rustup default "$MIRI_NIGHTLY"
109 rustup component add miri
115 We use `cargo miri setup` to avoid getting interactive questions about the extra
116 setup needed for Miri.
120 When using the above instructions, you may encounter a number of confusing compiler
123 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
125 Your build directory may contain artifacts from an earlier build that have/have
126 not been built for Miri. Run `cargo clean` before switching from non-Miri to
127 Miri builds and vice-versa.
129 #### "found crate `std` compiled by an incompatible version of rustc"
131 You may be running `cargo miri` with a different compiler version than the one
132 used to build the custom libstd that Miri uses, and Miri failed to detect that.
133 Try deleting `~/.cache/miri`.
135 #### "no mir for `std::rt::lang_start_internal`"
137 This means the sysroot you are using was not compiled with Miri in mind. This
138 should never happen when you use `cargo miri` because that takes care of setting
139 up the sysroot. If you are using `miri` (the Miri driver) directly, see
140 [below][testing-miri] for how to set up the sysroot.
143 ## Miri `-Z` flags and environment variables
144 [miri-flags]: #miri--z-flags-and-environment-variables
146 Several `-Z` flags are relevant for Miri:
148 * `-Zmiri-seed=<hex>` is a custom `-Z` flag added by Miri. It configures the
149 seed of the RNG that Miri uses to resolve non-determinism. This RNG is used
150 to pick base addresses for allocations, and when the interpreted program
151 requests system entropy. The default seed is 0.
152 **NOTE**: This entropy is not good enough for cryptographic use! Do not
153 generate secret keys in Miri or perform other kinds of cryptographic
154 operations that rely on proper random numbers.
155 * `-Zmiri-disable-validation` disables enforcing validity invariants and
156 reference aliasing rules, which are enforced by default. This is mostly
157 useful for debugging. It means Miri will miss bugs in your program. However,
158 this can also help to make Miri run faster.
159 * `-Zmiri-disable-isolation` disables host host isolation. As a consequence,
160 the program has access to host resources such as environment variables and
161 randomness (and, eventually, file systems and more).
162 * `-Zmiri-ignore-leaks` disables the memory leak checker.
163 * `-Zmiri-env-exclude=<var>` keeps the `var` environment variable isolated from
164 the host. Can be used multiple times to exclude several variables. The `TERM`
165 environment variable is excluded by default.
166 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
167 overrides the default to be `0`; be advised that using any higher level can
168 make Miri miss bugs in your program because they got optimized away.
169 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
170 functions. This is needed so that Miri can execute such functions, so Miri
171 sets this flag per default.
172 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
173 enables this per default because it is needed for validation.
174 * `-Zmiri-track-pointer-tag=<tag>` aborts interpretation with a backtrace when the
175 given pointer tag is popped from a borrow stack (which is where the tag
176 becomes invalid and any future use of it will error anyway). This helps you
177 in finding out why UB is happening and where in your code would be a good
178 place to look for it.
180 Moreover, Miri recognizes some environment variables:
182 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
183 Miri executions, also [see above][testing-miri].
184 * `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
185 indicates the sysroot to use. To do the same thing with `miri`
186 directly, use the `--sysroot` flag.
187 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
188 architecture to test against. `miri` and `cargo miri` accept the `--target`
189 flag for the same purpose.
191 ## Contributing and getting help
193 If you want to contribute to Miri, great! Please check out our
194 [contribution guide](CONTRIBUTING.md).
196 For help with running Miri, you can open an issue here on
197 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
199 [Rust Zulip]: https://rust-lang.zulipchat.com
203 This project began as part of an undergraduate research course in 2015 by
204 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
205 [report] available from that project. In 2016, @oli-obk joined to prepare miri
206 for eventually being used as const evaluator in the Rust compiler itself
207 (basically, for `const` and `static` stuff), replacing the old evaluator that
208 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
209 and began developing miri towards a tool for detecting undefined behavior, and
210 also using miri as a way to explore the consequences of various possible
211 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
212 into the compiler finally came to completion in early 2018. Meanwhile, later
213 that year, @RalfJung did a second internship, developing miri further with
214 support for checking basic type invariants and verifying that references are
215 used according to their aliasing restrictions.
217 [usask]: https://www.usask.ca/
218 [slides]: https://solson.me/miri-slides.pdf
219 [report]: https://solson.me/miri-report.pdf
221 ## Bugs found by Miri
223 Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
227 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
228 * [`Vec::into_iter` doing an unaligned ZST read](https://github.com/rust-lang/rust/pull/53804)
229 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
230 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
231 * [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
232 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
233 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
234 * [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
235 * [The Unix allocator calling `posix_memalign` in an invalid way](https://github.com/rust-lang/rust/issues/62251)
236 * [`getrandom` calling the `getrandom` syscall in an invalid way](https://github.com/rust-random/getrandom/pull/73)
238 Violations of Stacked Borrows found that are likely bugs (but Stacked Borrows is currently just an experiment):
240 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
241 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
242 * [`LinkedList` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
243 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
247 Licensed under either of
248 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
249 http://www.apache.org/licenses/LICENSE-2.0)
250 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
251 http://opensource.org/licenses/MIT) at your option.
255 Unless you explicitly state otherwise, any contribution intentionally submitted
256 for inclusion in the work by you shall be dual licensed as above, without any
257 additional terms or conditions.