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 * **Experimental**: Violations of the [Stacked Borrows] rules governing aliasing
20 Miri has already discovered some [real-world bugs](#bugs-found-by-miri). If you
21 found a bug with Miri, we'd appreciate if you tell us and we'll add it to the
24 Be aware that Miri will **not catch all cases of undefined behavior** in your
25 program, and cannot run all programs:
27 * There are still plenty of open questions around the basic invariants for some
28 types and when these invariants even have to hold. Miri tries to avoid false
29 positives here, so if you program runs fine in Miri right now that is by no
30 means a guarantee that it is UB-free when these questions get answered.
32 In particular, Miri does currently not check that integers/floats are
33 initialized or that references point to valid data.
34 * If the program relies on unspecified details of how data is laid out, it will
35 still run fine in Miri -- but might break (including causing UB) on different
36 compiler versions or different platforms.
37 * Program execution is non-deterministic when it depends, for example, on where
38 exactly in memory allocations end up. Miri tests one of many possible
39 executions of your program. If your code is sensitive to allocation base
40 addresses or other non-deterministic data, try running Miri with different
41 values for `-Zmiri-seed` to test different executions.
42 * Miri runs the program as a platform-independent interpreter, so the program
43 has no access to most platform-specific APIs or FFI. A few APIs have been
44 implemented (such as printing to stdout) but most have not: for example, Miri
45 currently does not support concurrency, or SIMD, or networking.
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
51 [Stacked Borrows]: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md
56 Install Miri on Rust nightly via `rustup`:
59 rustup +nightly component add miri
62 If `rustup` says the `miri` component is unavailable, that's because not all
63 nightly releases come with all tools. Check out
64 [this website](https://rust-lang.github.io/rustup-components-history) to
65 determine a nightly version that comes with Miri and install that using
66 `rustup toolchain install nightly-YYYY-MM-DD`.
68 Now you can run your project in Miri:
70 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
71 dependencies to be compiled the right way, that would not happen if they have
72 previously already been compiled.
73 2. To run all tests in your project through Miri, use `cargo miri test`.
74 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
76 The first time you run Miri, it will perform some extra setup and install some
77 dependencies. It will ask you for confirmation before installing anything.
79 Miri supports cross-execution: if you want to run the program as if it was a
80 Linux program, you can do `cargo miri run --target x86_64-unknown-linux-gnu`.
81 This is particularly useful if you are using Windows, as the Linux target is
82 much better supported than Windows targets.
84 You can pass arguments to Miri after the first `--`, and pass arguments to the
85 interpreted program or test suite after the second `--`. For example, `cargo
86 miri run -- -Zmiri-disable-validation` runs the program without validation of
87 basic type invariants and without checking the aliasing of references.
89 When compiling code via `cargo miri`, the `miri` config flag is set. You can
90 use this to ignore test cases that will fail under Miri because they do things
91 Miri does not support:
95 #[cfg_attr(miri, ignore)]
96 fn does_not_work_on_miri() {
97 std::thread::spawn(|| println!("Hello Thread!"))
103 An exhaustive list of what `miri` does not support is not available, as this could be
104 an unbounded set with FFI and more. However `miri` will explicitly tell you when it finds
105 something unsupported with an error, containing a message such as:
108 error: unsupported operation: can't call foreign function: mach_timebase_info
110 = help: this is likely not a bug in the program; it indicates that the program \
111 performed an operation that the interpreter does not support
114 ### Running Miri on CI
116 To run Miri on CI, make sure that you handle the case where the latest nightly
117 does not ship the Miri component because it currently does not build. For
118 example, you can use the following snippet to always test with the latest
119 nightly that *does* come with Miri:
122 MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
123 echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
124 rustup set profile minimal
125 rustup default "$MIRI_NIGHTLY"
127 rustup component add miri
133 We use `cargo miri setup` to avoid getting interactive questions about the extra
134 setup needed for Miri.
138 When using the above instructions, you may encounter a number of confusing compiler
141 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
143 Your build directory may contain artifacts from an earlier build that have/have
144 not been built for Miri. Run `cargo clean` before switching from non-Miri to
145 Miri builds and vice-versa.
147 #### "found crate `std` compiled by an incompatible version of rustc"
149 You may be running `cargo miri` with a different compiler version than the one
150 used to build the custom libstd that Miri uses, and Miri failed to detect that.
151 Try deleting `~/.cache/miri`.
153 #### "no mir for `std::rt::lang_start_internal`"
155 This means the sysroot you are using was not compiled with Miri in mind. This
156 should never happen when you use `cargo miri` because that takes care of setting
157 up the sysroot. If you are using `miri` (the Miri driver) directly, see
158 [below][testing-miri] for how to set up the sysroot.
161 ## Miri `-Z` flags and environment variables
162 [miri-flags]: #miri--z-flags-and-environment-variables
164 Several `-Z` flags are relevant for Miri:
166 * `-Zmiri-seed=<hex>` is a custom `-Z` flag added by Miri. It configures the
167 seed of the RNG that Miri uses to resolve non-determinism. This RNG is used
168 to pick base addresses for allocations. When isolation is enabled (the default),
169 this is also used to emulate system entropy. The default seed is 0.
170 **NOTE**: This entropy is not good enough for cryptographic use! Do not
171 generate secret keys in Miri or perform other kinds of cryptographic
172 operations that rely on proper random numbers.
173 * `-Zmiri-disable-validation` disables enforcing validity invariants, which are
174 enforced by default. This is mostly useful for debugging. It means Miri will
175 miss bugs in your program. However, this can also help to make Miri run
177 * `-Zmiri-disable-stacked-borrows` disables checking the experimental
178 [Stacked Borrows] aliasing rules. This can make Miri run faster, but it also
179 means no aliasing violations will be detected.
180 * `-Zmiri-disable-isolation` disables host isolation. As a consequence,
181 the program has access to host resources such as environment variables, file
182 systems, and randomness.
183 * `-Zmiri-ignore-leaks` disables the memory leak checker.
184 * `-Zmiri-env-exclude=<var>` keeps the `var` environment variable isolated from
185 the host. Can be used multiple times to exclude several variables. The `TERM`
186 environment variable is excluded by default.
187 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
188 overrides the default to be `0`; be advised that using any higher level can
189 make Miri miss bugs in your program because they got optimized away.
190 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
191 functions. This is needed so that Miri can execute such functions, so Miri
192 sets this flag per default.
193 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
194 enables this per default because it is needed for validation.
195 * `-Zmiri-track-pointer-tag=<tag>` shows a backtrace when the given pointer tag
196 is popped from a borrow stack (which is where the tag becomes invalid and any
197 future use of it will error). This helps you in finding out why UB is
198 happening and where in your code would be a good place to look for it.
199 * `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
200 being allocated. This helps in debugging memory leaks.
202 Moreover, Miri recognizes some environment variables:
204 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
205 Miri executions, also [see above][testing-miri].
206 * `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
207 indicates the sysroot to use. To do the same thing with `miri`
208 directly, use the `--sysroot` flag.
209 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
210 architecture to test against. `miri` and `cargo miri` accept the `--target`
211 flag for the same purpose.
212 * `MIRI_TEST_FLAGS` (recognized by the test suite) defines extra flags to be
215 ## Contributing and getting help
217 If you want to contribute to Miri, great! Please check out our
218 [contribution guide](CONTRIBUTING.md).
220 For help with running Miri, you can open an issue here on
221 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
223 [Rust Zulip]: https://rust-lang.zulipchat.com
227 This project began as part of an undergraduate research course in 2015 by
228 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
229 [report] available from that project. In 2016, @oli-obk joined to prepare miri
230 for eventually being used as const evaluator in the Rust compiler itself
231 (basically, for `const` and `static` stuff), replacing the old evaluator that
232 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
233 and began developing miri towards a tool for detecting undefined behavior, and
234 also using miri as a way to explore the consequences of various possible
235 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
236 into the compiler finally came to completion in early 2018. Meanwhile, later
237 that year, @RalfJung did a second internship, developing miri further with
238 support for checking basic type invariants and verifying that references are
239 used according to their aliasing restrictions.
241 [usask]: https://www.usask.ca/
242 [slides]: https://solson.me/miri-slides.pdf
243 [report]: https://solson.me/miri-report.pdf
245 ## Bugs found by Miri
247 Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
251 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
252 * [`Vec::into_iter` doing an unaligned ZST read](https://github.com/rust-lang/rust/pull/53804)
253 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
254 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
255 * [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
256 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
257 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
258 * [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
259 * [The Unix allocator calling `posix_memalign` in an invalid way](https://github.com/rust-lang/rust/issues/62251)
260 * [`getrandom` calling the `getrandom` syscall in an invalid way](https://github.com/rust-random/getrandom/pull/73)
261 * [`Vec`](https://github.com/rust-lang/rust/issues/69770) and [`BTreeMap`](https://github.com/rust-lang/rust/issues/69769) leaking memory under some (panicky) conditions
262 * [Memory leak in `beef`](https://github.com/maciejhirsz/beef/issues/12)
264 Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows is currently just an experiment):
266 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
267 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
268 * [`LinkedList` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
269 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
270 * [`align_to_mut` violating uniqueness of mutable references](https://github.com/rust-lang/rust/issues/68549)
271 * [Aliasing mutable references in `sized-chunks`](https://github.com/bodil/sized-chunks/issues/8)
275 Licensed under either of
276 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
277 http://www.apache.org/licenses/LICENSE-2.0)
278 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
279 http://opensource.org/licenses/MIT) at your option.
283 Unless you explicitly state otherwise, any contribution intentionally submitted
284 for inclusion in the work by you shall be dual licensed as above, without any
285 additional terms or conditions.