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10 An experimental interpreter for [Rust][rust]'s
11 [mid-level intermediate representation][mir] (MIR). It can run binaries and
12 test suites of cargo projects and detect certain classes of
13 [undefined behavior](https://doc.rust-lang.org/reference/behavior-considered-undefined.html),
16 * Out-of-bounds memory accesses and use-after-free
17 * Invalid use of uninitialized data
18 * Violation of intrinsic preconditions (an [`unreachable_unchecked`] being
19 reached, calling [`copy_nonoverlapping`] with overlapping ranges, ...)
20 * Not sufficiently aligned memory accesses and references
21 * Violation of *some* basic type invariants (a `bool` that is not 0 or 1, for example,
22 or an invalid enum discriminant)
23 * **Experimental**: Violations of the [Stacked Borrows] rules governing aliasing
26 On top of that, Miri will also tell you about memory leaks: when there is memory
27 still allocated at the end of the execution, and that memory is not reachable
28 from a global `static`, Miri will raise an error.
30 Miri has already discovered some [real-world bugs](#bugs-found-by-miri). If you
31 found a bug with Miri, we'd appreciate if you tell us and we'll add it to the
34 However, be aware that Miri will **not catch all cases of undefined behavior**
35 in your program, and cannot run all programs:
37 * There are still plenty of open questions around the basic invariants for some
38 types and when these invariants even have to hold. Miri tries to avoid false
39 positives here, so if you program runs fine in Miri right now that is by no
40 means a guarantee that it is UB-free when these questions get answered.
42 In particular, Miri does currently not check that integers/floats are
43 initialized or that references point to valid data.
44 * If the program relies on unspecified details of how data is laid out, it will
45 still run fine in Miri -- but might break (including causing UB) on different
46 compiler versions or different platforms.
47 * Program execution is non-deterministic when it depends, for example, on where
48 exactly in memory allocations end up. Miri tests one of many possible
49 executions of your program. If your code is sensitive to allocation base
50 addresses or other non-deterministic data, try running Miri with different
51 values for `-Zmiri-seed` to test different executions.
52 * Miri runs the program as a platform-independent interpreter, so the program
53 has no access to most platform-specific APIs or FFI. A few APIs have been
54 implemented (such as printing to stdout) but most have not: for example, Miri
55 currently does not support SIMD or networking.
56 * Miri currently does not check for data-races and most other concurrency-related
59 [rust]: https://www.rust-lang.org/
60 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
61 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
62 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
63 [Stacked Borrows]: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md
68 Install Miri on Rust nightly via `rustup`:
71 rustup +nightly component add miri
74 If `rustup` says the `miri` component is unavailable, that's because not all
75 nightly releases come with all tools. Check out
76 [this website](https://rust-lang.github.io/rustup-components-history) to
77 determine a nightly version that comes with Miri and install that using
78 `rustup toolchain install nightly-YYYY-MM-DD`.
80 Now you can run your project in Miri:
82 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
83 dependencies to be compiled the right way, that would not happen if they have
84 previously already been compiled.
85 2. To run all tests in your project through Miri, use `cargo miri test`.
86 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
88 The first time you run Miri, it will perform some extra setup and install some
89 dependencies. It will ask you for confirmation before installing anything.
91 `cargo miri run/test` supports the exact same flags as `cargo run/test`. You
92 can pass arguments to Miri via `MIRIFLAGS`. For example,
93 `MIRIFLAGS="-Zmiri-disable-stacked-borrows" cargo miri run` runs the program
94 without checking the aliasing of references.
96 Miri supports cross-execution: if you want to run the program as if it was a
97 Linux program, you can do `cargo miri run --target x86_64-unknown-linux-gnu`.
98 This is particularly useful if you are using Windows, as the Linux target is
99 much better supported than Windows targets.
101 When compiling code via `cargo miri`, the `cfg(miri)` config flag is set. You
102 can use this to ignore test cases that fail under Miri because they do things
103 Miri does not support:
107 #[cfg_attr(miri, ignore)]
108 fn does_not_work_on_miri() {
109 std::thread::spawn(|| println!("Hello Thread!"))
115 There is no way to list all the infinite things Miri cannot do, but the
116 interpreter will explicitly tell you when it finds something unsupported:
119 error: unsupported operation: can't call foreign function: bind
121 = help: this is likely not a bug in the program; it indicates that the program \
122 performed an operation that the interpreter does not support
125 ### Running Miri on CI
127 To run Miri on CI, make sure that you handle the case where the latest nightly
128 does not ship the Miri component because it currently does not build. For
129 example, you can use the following snippet to always test with the latest
130 nightly that *does* come with Miri:
133 MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
134 echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
135 rustup set profile minimal
136 rustup default "$MIRI_NIGHTLY"
137 rustup component add miri
144 When using the above instructions, you may encounter a number of confusing compiler
147 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
149 Your build directory may contain artifacts from an earlier build that have/have
150 not been built for Miri. Run `cargo clean` before switching from non-Miri to
151 Miri builds and vice-versa.
153 #### "found crate `std` compiled by an incompatible version of rustc"
155 You may be running `cargo miri` with a different compiler version than the one
156 used to build the custom libstd that Miri uses, and Miri failed to detect that.
157 Try deleting `~/.cache/miri`.
159 #### "no mir for `std::rt::lang_start_internal`"
161 This means the sysroot you are using was not compiled with Miri in mind. This
162 should never happen when you use `cargo miri` because that takes care of setting
163 up the sysroot. If you are using `miri` (the Miri driver) directly, see the
164 [contributors' guide](CONTRIBUTING.md) for how to use `./miri` to best do that.
167 ## Miri `-Z` flags and environment variables
168 [miri-flags]: #miri--z-flags-and-environment-variables
170 Miri adds its own set of `-Z` flags, which are usually set via the `MIRIFLAGS`
171 environment variable:
173 * `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
174 can focus on other failures, but it means Miri can miss bugs in your program.
175 Using this flag is **unsound**.
176 * `-Zmiri-disable-stacked-borrows` disables checking the experimental
177 [Stacked Borrows] aliasing rules. This can make Miri run faster, but it also
178 means no aliasing violations will be detected. Using this flag is **unsound**
179 (but the affected soundness rules are experimental).
180 * `-Zmiri-disable-validation` disables enforcing validity invariants, which are
181 enforced by default. This is mostly useful to focus on other failures (such
182 as out-of-bounds accesses) first. Setting this flag means Miri can miss bugs
183 in your program. However, this can also help to make Miri run faster. Using
184 this flag is **unsound**.
185 * `-Zmiri-disable-isolation` disables host isolation. As a consequence,
186 the program has access to host resources such as environment variables, file
187 systems, and randomness.
188 * `-Zmiri-env-exclude=<var>` keeps the `var` environment variable isolated from
189 the host so that it cannot be accessed by the program. Can be used multiple
190 times to exclude several variables. On Windows, the `TERM` environment
191 variable is excluded by default.
192 * `-Zmiri-ignore-leaks` disables the memory leak checker.
193 * `-Zmiri-seed=<hex>` configures the seed of the RNG that Miri uses to resolve
194 non-determinism. This RNG is used to pick base addresses for allocations.
195 When isolation is enabled (the default), this is also used to emulate system
196 entropy. The default seed is 0. **NOTE**: This entropy is not good enough
197 for cryptographic use! Do not generate secret keys in Miri or perform other
198 kinds of cryptographic operations that rely on proper random numbers.
199 * `-Zmiri-symbolic-alignment-check` makes the alignment check more strict. By
200 default, alignment is checked by casting the pointer to an integer, and making
201 sure that is a multiple of the alignment. This can lead to cases where a
202 program passes the alignment check by pure chance, because things "happened to
203 be" sufficiently aligned -- there is no UB in this execution but there would
204 be UB in others. To avoid such cases, the symbolic alignment check only takes
205 into account the requested alignment of the relevant allocation, and the
206 offset into that allocation. This avoids missing such bugs, but it also
207 incurs some false positives when the code does manual integer arithmetic to
208 ensure alignment. (The standard library `align_to` method works fine in both
209 modes; under symbolic alignment it only fills the middle slice when the
210 allocation guarantees sufficient alignment.)
211 * `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
212 being allocated or freed. This helps in debugging memory leaks and
214 * `-Zmiri-track-pointer-tag=<tag>` shows a backtrace when the given pointer tag
215 is popped from a borrow stack (which is where the tag becomes invalid and any
216 future use of it will error). This helps you in finding out why UB is
217 happening and where in your code would be a good place to look for it.
218 * `-Zmiri-track-call-id=<id>` shows a backtrace when the given call id is
219 assigned to a stack frame. This helps in debugging UB related to Stacked
220 Borrows "protectors".
222 Some native rustc `-Z` flags are also very relevant for Miri:
224 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
225 overrides the default to be `0`; be advised that using any higher level can
226 make Miri miss bugs in your program because they got optimized away.
227 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
228 functions. This is needed so that Miri can execute such functions, so Miri
229 sets this flag per default.
230 * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
231 enables this per default because it is needed for [Stacked Borrows].
233 Moreover, Miri recognizes some environment variables:
235 * `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
236 Miri executions, also [see above][testing-miri].
237 * `MIRIFLAGS` (recognized by `cargo miri` and the test suite) defines extra
238 flags to be passed to Miri.
239 * `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
240 indicates the sysroot to use. To do the same thing with `miri`
241 directly, use the `--sysroot` flag.
242 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
243 architecture to test against. `miri` and `cargo miri` accept the `--target`
244 flag for the same purpose.
246 The following environment variables are internal, but used to communicate between
247 different Miri binaries, and as such worth documenting:
249 * `MIRI_BE_RUSTC` when set to any value tells the Miri driver to actually not
250 interpret the code but compile it like rustc would. This is useful to be sure
251 that the compiled `rlib`s are compatible with Miri.
252 When set while running `cargo-miri`, it indicates that we are part of a sysroot
253 build (for which some crates need special treatment).
254 * `MIRI_CWD` when set to any value tells the Miri driver to change to the given
255 directory after loading all the source files, but before commencing
256 interpretation. This is useful if the interpreted program wants a different
257 working directory at run-time than at build-time.
259 ## Miri `extern` functions
261 Miri provides some `extern` functions that programs can import to access
262 Miri-specific functionality:
267 /// Miri-provided extern function to mark the block `ptr` points to as a "root"
268 /// for some static memory. This memory and everything reachable by it is not
269 /// considered leaking even if it still exists when the program terminates.
271 /// `ptr` has to point to the beginning of an allocated block.
272 fn miri_static_root(ptr: *const u8);
274 /// Miri-provided extern function to obtain a backtrace of the current call stack.
275 /// This returns a boxed slice of pointers - each pointer is an opaque value
276 /// that is only useful when passed to `miri_resolve_frame`
277 /// The `flags` argument must be `0`.
278 fn miri_get_backtrace(flags: u64) -> Box<[*mut ()]>;
280 /// Miri-provided extern function to resolve a frame pointer obtained
281 /// from `miri_get_backtrace`. The `flags` argument must be `0`,
282 /// and `MiriFrame` should be declared as follows:
286 /// struct MiriFrame {
287 /// // The name of the function being executed, encoded in UTF-8
289 /// // The filename of the function being executed, encoded in UTF-8
290 /// filename: Box<[u8]>,
291 /// // The line number currently being executed in `filename`, starting from '1'.
293 /// // The column number currently being executed in `filename`, starting from '1'.
295 /// // The function pointer to the function currently being executed.
296 /// // This can be compared against function pointers obtained by
297 /// // casting a function (e.g. `my_fn as *mut ()`)
302 /// The fields must be declared in exactly the same order as they appear in `MiriFrame` above.
303 /// This function can be called on any thread (not just the one which obtained `frame`).
304 fn miri_resolve_frame(frame: *mut (), flags: u64) -> MiriFrame;
306 /// Miri-provided extern function to begin unwinding with the given payload.
308 /// This is internal and unstable and should not be used; we give it here
309 /// just to be complete.
310 fn miri_start_panic(payload: *mut u8) -> !;
314 ## Contributing and getting help
316 If you want to contribute to Miri, great! Please check out our
317 [contribution guide](CONTRIBUTING.md).
319 For help with running Miri, you can open an issue here on
320 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
322 [Rust Zulip]: https://rust-lang.zulipchat.com
326 This project began as part of an undergraduate research course in 2015 by
327 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
328 [report] available from that project. In 2016, @oli-obk joined to prepare miri
329 for eventually being used as const evaluator in the Rust compiler itself
330 (basically, for `const` and `static` stuff), replacing the old evaluator that
331 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
332 and began developing miri towards a tool for detecting undefined behavior, and
333 also using miri as a way to explore the consequences of various possible
334 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
335 into the compiler finally came to completion in early 2018. Meanwhile, later
336 that year, @RalfJung did a second internship, developing miri further with
337 support for checking basic type invariants and verifying that references are
338 used according to their aliasing restrictions.
340 [usask]: https://www.usask.ca/
341 [slides]: https://solson.me/miri-slides.pdf
342 [report]: https://solson.me/miri-report.pdf
344 ## Bugs found by Miri
346 Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
350 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
351 * [`Vec::into_iter` doing an unaligned ZST read](https://github.com/rust-lang/rust/pull/53804)
352 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
353 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
354 * [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
355 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
356 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
357 * [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
358 * [The Unix allocator calling `posix_memalign` in an invalid way](https://github.com/rust-lang/rust/issues/62251)
359 * [`getrandom` calling the `getrandom` syscall in an invalid way](https://github.com/rust-random/getrandom/pull/73)
360 * [`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
361 * [`beef` leaking memory](https://github.com/maciejhirsz/beef/issues/12)
362 * [`EbrCell` using uninitialized memory incorrectly](https://github.com/Firstyear/concread/commit/b15be53b6ec076acb295a5c0483cdb4bf9be838f#diff-6282b2fc8e98bd089a1f0c86f648157cR229)
363 * [TiKV performing an unaligned pointer access](https://github.com/tikv/tikv/issues/7613)
364 * [`servo_arc` creating a dangling shared reference](https://github.com/servo/servo/issues/26357)
365 * [TiKV constructing out-of-bounds pointers (and overlapping mutable references)](https://github.com/tikv/tikv/pull/7751)
366 * [`encoding_rs` doing out-of-bounds pointer arithmetic](https://github.com/hsivonen/encoding_rs/pull/53)
368 Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows is currently just an experiment):
370 * [`VecDeque::drain` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
371 * [`BTreeMap` iterators creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
372 * [`LinkedList` cursor insertion creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
373 * [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
374 * [`align_to_mut` violating uniqueness of mutable references](https://github.com/rust-lang/rust/issues/68549)
375 * [`sized-chunks` creating aliasing mutable references](https://github.com/bodil/sized-chunks/issues/8)
376 * [`String::push_str` invalidating existing references into the string](https://github.com/rust-lang/rust/issues/70301)
377 * [`ryu` using raw pointers outside their valid memory area](https://github.com/dtolnay/ryu/issues/24)
378 * [ink! creating overlapping mutable references](https://github.com/rust-lang/miri/issues/1364)
379 * [TiKV creating overlapping mutable reference and raw pointer](https://github.com/tikv/tikv/pull/7709)
380 * [Windows `Env` iterator creating `*const T` from `&T` to read memory outside of `T`](https://github.com/rust-lang/rust/pull/70479)
381 * [`BTreeMap::iter_mut` creating overlapping mutable references](https://github.com/rust-lang/rust/issues/73915)
382 * [`VecDeque::iter_mut` creating overlapping mutable references](https://github.com/rust-lang/rust/issues/74029)
386 Licensed under either of
387 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
388 http://www.apache.org/licenses/LICENSE-2.0)
389 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
390 http://opensource.org/licenses/MIT) at your option.
394 Unless you explicitly state otherwise, any contribution intentionally submitted
395 for inclusion in the work by you shall be dual licensed as above, without any
396 additional terms or conditions.