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 On top of that, Miri will also tell you about memory leaks: when there is memory
21 still allocated at the end of the execution, and that memory is not reachable
22 from a global `static`, Miri will raise an error. Note however that
23 [leak checking is currently disabled on Windows targets](https://github.com/rust-lang/miri/issues/1302).
25 Miri has already discovered some [real-world bugs](#bugs-found-by-miri). If you
26 found a bug with Miri, we'd appreciate if you tell us and we'll add it to the
29 However, be aware that Miri will **not catch all cases of undefined behavior**
30 in your program, and cannot run all programs:
32 * There are still plenty of open questions around the basic invariants for some
33 types and when these invariants even have to hold. Miri tries to avoid false
34 positives here, so if you program runs fine in Miri right now that is by no
35 means a guarantee that it is UB-free when these questions get answered.
37 In particular, Miri does currently not check that integers/floats are
38 initialized or that references point to valid data.
39 * If the program relies on unspecified details of how data is laid out, it will
40 still run fine in Miri -- but might break (including causing UB) on different
41 compiler versions or different platforms.
42 * Program execution is non-deterministic when it depends, for example, on where
43 exactly in memory allocations end up. Miri tests one of many possible
44 executions of your program. If your code is sensitive to allocation base
45 addresses or other non-deterministic data, try running Miri with different
46 values for `-Zmiri-seed` to test different executions.
47 * Miri runs the program as a platform-independent interpreter, so the program
48 has no access to most platform-specific APIs or FFI. A few APIs have been
49 implemented (such as printing to stdout) but most have not: for example, Miri
50 currently does not support SIMD or networking.
51 * Miri currently does not check for data-races and most other concurrency-related
54 [rust]: https://www.rust-lang.org/
55 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
56 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
57 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
58 [Stacked Borrows]: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md
63 Install Miri on Rust nightly via `rustup`:
66 rustup +nightly component add miri
69 If `rustup` says the `miri` component is unavailable, that's because not all
70 nightly releases come with all tools. Check out
71 [this website](https://rust-lang.github.io/rustup-components-history) to
72 determine a nightly version that comes with Miri and install that using
73 `rustup toolchain install nightly-YYYY-MM-DD`.
75 Now you can run your project in Miri:
77 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
78 dependencies to be compiled the right way, that would not happen if they have
79 previously already been compiled.
80 2. To run all tests in your project through Miri, use `cargo miri test`.
81 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
83 The first time you run Miri, it will perform some extra setup and install some
84 dependencies. It will ask you for confirmation before installing anything.
86 You can pass arguments to Miri after the first `--`, and pass arguments to the
87 interpreted program or test suite after the second `--`. For example, `cargo
88 miri run -- -Zmiri-disable-stacked-borrows` runs the program without checking
89 the aliasing of references. To filter the tests being run, use `cargo miri test
92 Miri supports cross-execution: if you want to run the program as if it was a
93 Linux program, you can do `cargo miri run --target x86_64-unknown-linux-gnu`.
94 This is particularly useful if you are using Windows, as the Linux target is
95 much better supported than Windows targets.
97 When compiling code via `cargo miri`, the `cfg(miri)` config flag is set. You
98 can use this to ignore test cases that fail under Miri because they do things
99 Miri does not support:
103 #[cfg_attr(miri, ignore)]
104 fn does_not_work_on_miri() {
105 std::thread::spawn(|| println!("Hello Thread!"))
111 There is no way to list all the infinite things Miri cannot do, but the
112 interpreter will explicitly tell you when it finds something unsupported:
115 error: unsupported operation: can't call foreign function: bind
117 = help: this is likely not a bug in the program; it indicates that the program \
118 performed an operation that the interpreter does not support
121 ### Running Miri on CI
123 To run Miri on CI, make sure that you handle the case where the latest nightly
124 does not ship the Miri component because it currently does not build. For
125 example, you can use the following snippet to always test with the latest
126 nightly that *does* come with Miri:
129 MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
130 echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
131 rustup set profile minimal
132 rustup default "$MIRI_NIGHTLY"
133 rustup component add miri
140 When using the above instructions, you may encounter a number of confusing compiler
143 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
145 Your build directory may contain artifacts from an earlier build that have/have
146 not been built for Miri. Run `cargo clean` before switching from non-Miri to
147 Miri builds and vice-versa.
149 #### "found crate `std` compiled by an incompatible version of rustc"
151 You may be running `cargo miri` with a different compiler version than the one
152 used to build the custom libstd that Miri uses, and Miri failed to detect that.
153 Try deleting `~/.cache/miri`.
155 #### "no mir for `std::rt::lang_start_internal`"
157 This means the sysroot you are using was not compiled with Miri in mind. This
158 should never happen when you use `cargo miri` because that takes care of setting
159 up the sysroot. If you are using `miri` (the Miri driver) directly, see the
160 [contributors' guide](CONTRIBUTING.md) for how to use `./miri` to best do that.
163 ## Miri `-Z` flags and environment variables
164 [miri-flags]: #miri--z-flags-and-environment-variables
166 Miri adds its own set of `-Z` flags:
168 * `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
169 can focus on other failures, but it means Miri can miss bugs in your program.
170 Using this flag is **unsound**.
171 * `-Zmiri-disable-stacked-borrows` disables checking the experimental
172 [Stacked Borrows] aliasing rules. This can make Miri run faster, but it also
173 means no aliasing violations will be detected. Using this flag is **unsound**
174 (but the affected soundness rules are experimental).
175 * `-Zmiri-disable-validation` disables enforcing validity invariants, which are
176 enforced by default. This is mostly useful to focus on other failures (such
177 as out-of-bounds accesses) first. Setting this flag means Miri can miss bugs
178 in your program. However, this can also help to make Miri run faster. Using
179 this flag is **unsound**.
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-env-exclude=<var>` keeps the `var` environment variable isolated from
184 the host so that it cannot be accessed by the program. Can be used multiple
185 times to exclude several variables. On Windows, the `TERM` environment
186 variable is excluded by default.
187 * `-Zmiri-ignore-leaks` disables the memory leak checker.
188 * `-Zmiri-seed=<hex>` configures the seed of the RNG that Miri uses to resolve
189 non-determinism. This RNG is used to pick base addresses for allocations.
190 When isolation is enabled (the default), this is also used to emulate system
191 entropy. The default seed is 0. **NOTE**: This entropy is not good enough
192 for cryptographic use! Do not generate secret keys in Miri or perform other
193 kinds of cryptographic operations that rely on proper random numbers.
194 * `-Zmiri-symbolic-alignment-check` makes the alignment check more strict. By
195 default, alignment is checked by casting the pointer to an integer, and making
196 sure that is a multiple of the alignment. This can lead to cases where a
197 program passes the alignment check by pure chance, because things "happened to
198 be" sufficiently aligned -- there is no UB in this execution but there would
199 be UB in others. To avoid such cases, the symbolic alignment check only takes
200 into account the requested alignment of the relevant allocation, and the
201 offset into that allocation. This avoids missing such bugs, but it also
202 incurs some false positives when the code does manual integer arithmetic to
203 ensure alignment. (The standard library `align_to` method works fine in both
204 modes; under symbolic alignment it only fills the middle slice when the
205 allocation guarantees sufficient alignment.)
206 * `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
207 being allocated or freed. This helps in debugging memory leaks and
209 * `-Zmiri-track-pointer-tag=<tag>` shows a backtrace when the given pointer tag
210 is popped from a borrow stack (which is where the tag becomes invalid and any
211 future use of it will error). This helps you in finding out why UB is
212 happening and where in your code would be a good place to look for it.
213 * `-Zmiri-track-call-id=<id>` shows a backtrace when the given call id is
214 assigned to a stack frame. This helps in debugging UB related to Stacked
215 Borrows "protectors".
217 Some native rustc `-Z` flags are also very relevant for Miri:
219 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
220 overrides the default to be `0`; be advised that using any higher level can
221 make Miri miss bugs in your program because they got optimized away.
222 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
223 functions. This is needed so that Miri can execute such functions, so Miri
224 sets this flag per default.
225 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
226 enables this per default because it is needed for [Stacked Borrows].
228 Moreover, Miri recognizes some environment variables:
230 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
231 Miri executions, also [see above][testing-miri].
232 * `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
233 indicates the sysroot to use. To do the same thing with `miri`
234 directly, use the `--sysroot` flag.
235 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
236 architecture to test against. `miri` and `cargo miri` accept the `--target`
237 flag for the same purpose.
238 * `MIRI_TEST_FLAGS` (recognized by the test suite) defines extra flags to be
241 The following environment variables are internal, but used to communicate between
242 different Miri binaries, and as such worth documenting:
244 * `MIRI_BE_RUSTC` when set to any value tells the Miri driver to actually not
245 interpret the code but compile it like rustc would. This is useful to be sure
246 that the compiled `rlib`s are compatible with Miri.
248 ## Miri `extern` functions
250 Miri provides some `extern` functions that programs can import to access
251 Miri-specific functionality:
256 /// Miri-provided extern function to mark the block `ptr` points to as a "root"
257 /// for some static memory. This memory and everything reachable by it is not
258 /// considered leaking even if it still exists when the program terminates.
260 /// `ptr` has to point to the beginning of an allocated block.
261 fn miri_static_root(ptr: *const u8);
263 /// Miri-provided extern function to begin unwinding with the given payload.
265 /// This is internal and unstable and should not be used; we give it here
266 /// just to be complete.
267 fn miri_start_panic(payload: *mut u8) -> !;
271 ## Contributing and getting help
273 If you want to contribute to Miri, great! Please check out our
274 [contribution guide](CONTRIBUTING.md).
276 For help with running Miri, you can open an issue here on
277 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
279 [Rust Zulip]: https://rust-lang.zulipchat.com
283 This project began as part of an undergraduate research course in 2015 by
284 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
285 [report] available from that project. In 2016, @oli-obk joined to prepare miri
286 for eventually being used as const evaluator in the Rust compiler itself
287 (basically, for `const` and `static` stuff), replacing the old evaluator that
288 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
289 and began developing miri towards a tool for detecting undefined behavior, and
290 also using miri as a way to explore the consequences of various possible
291 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
292 into the compiler finally came to completion in early 2018. Meanwhile, later
293 that year, @RalfJung did a second internship, developing miri further with
294 support for checking basic type invariants and verifying that references are
295 used according to their aliasing restrictions.
297 [usask]: https://www.usask.ca/
298 [slides]: https://solson.me/miri-slides.pdf
299 [report]: https://solson.me/miri-report.pdf
301 ## Bugs found by Miri
303 Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
307 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
308 * [`Vec::into_iter` doing an unaligned ZST read](https://github.com/rust-lang/rust/pull/53804)
309 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
310 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
311 * [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
312 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
313 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
314 * [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
315 * [The Unix allocator calling `posix_memalign` in an invalid way](https://github.com/rust-lang/rust/issues/62251)
316 * [`getrandom` calling the `getrandom` syscall in an invalid way](https://github.com/rust-random/getrandom/pull/73)
317 * [`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
318 * [`beef` leaking memory](https://github.com/maciejhirsz/beef/issues/12)
319 * [`EbrCell` using uninitialized memory incorrectly](https://github.com/Firstyear/concread/commit/b15be53b6ec076acb295a5c0483cdb4bf9be838f#diff-6282b2fc8e98bd089a1f0c86f648157cR229)
320 * [TiKV performing an unaligned pointer access](https://github.com/tikv/tikv/issues/7613)
321 * [`servo_arc` creating a dangling shared reference](https://github.com/servo/servo/issues/26357)
322 * [TiKV constructing out-of-bounds pointers (and overlapping mutable references)](https://github.com/tikv/tikv/pull/7751)
323 * [`encoding_rs` doing out-of-bounds pointer arithmetic](https://github.com/hsivonen/encoding_rs/pull/53)
325 Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows is currently just an experiment):
327 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
328 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
329 * [`LinkedList` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
330 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
331 * [`align_to_mut` violating uniqueness of mutable references](https://github.com/rust-lang/rust/issues/68549)
332 * [`sized-chunks` creating aliasing mutable references](https://github.com/bodil/sized-chunks/issues/8)
333 * [`String::push_str` invalidating existing references into the string](https://github.com/rust-lang/rust/issues/70301)
334 * [`ryu` using raw pointers outside their valid memory area](https://github.com/dtolnay/ryu/issues/24)
335 * [ink! creating overlapping mutable references](https://github.com/rust-lang/miri/issues/1364)
336 * [TiKV creating overlapping mutable reference and raw pointer](https://github.com/tikv/tikv/pull/7709)
337 * [Windows `Env` iterator creating `*const T` from `&T` to read memory outside of `T`](https://github.com/rust-lang/rust/pull/70479)
341 Licensed under either of
342 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
343 http://www.apache.org/licenses/LICENSE-2.0)
344 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
345 http://opensource.org/licenses/MIT) at your option.
349 Unless you explicitly state otherwise, any contribution intentionally submitted
350 for inclusion in the work by you shall be dual licensed as above, without any
351 additional terms or conditions.