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 undefined behavior,
9 * Out-of-bounds memory accesses and use-after-free
10 * Invalid use of uninitialized data
11 * Violation of intrinsic preconditions (an [`unreachable_unchecked`] being
12 reached, calling [`copy_nonoverlapping`] with overlapping ranges, ...)
13 * Not sufficiently aligned memory accesses and references
14 * Violation of basic type invariants (a `bool` that is not 0 or 1, for example,
15 or an invalid enum discriminant)
16 * WIP: Violations of the rules governing aliasing for reference types
18 Miri has already discovered some [real-world bugs](#bugs-found-by-miri). If you
19 found a bug with Miri, we'd appreciate if you tell us and we'll add it to the
22 Be aware that Miri will not catch all possible errors in your program, and
23 cannot run all programs:
25 * There are still plenty of open questions around the basic invariants for some
26 types and when these invariants even have to hold, so if you program runs fine
27 in Miri right now that is by no means a guarantee that it is UB-free when
28 these questions get answered.
29 * If the program relies on unspecified details of how data is laid out, it will
30 still run fine in Miri -- but might break (including causing UB) on different
31 compiler versions or different platforms.
32 * Miri is fully deterministic and does not actually pick a base address in
33 virtual memory for the program's allocations. If program behavior depends on
34 the base address of an allocation, Miri will stop execution (with a few
35 exceptions to make some common pointer comparisons work).
36 * Miri runs the program as a platform-independent interpreter, so the program
37 has no access to any platform-specific APIs or FFI. A few APIs have been
38 implemented (such as printing to stdout) but most have not: for example, Miri
39 currently does not support concurrency, or networking, or file system access,
40 or gathering entropy from the system.
42 [rust]: https://www.rust-lang.org/
43 [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
44 [`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
45 [`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
48 ## Running Miri on your own project (and its test suite)
50 Install Miri via `rustup`:
53 rustup component add miri
56 If `rustup` says the `miri` component is unavailable, that's because not all nightly releases come with all tools. Check out [this website](https://rust-lang.github.io/rustup-components-history) to determine a nightly version that comes with Miri and install that, e.g. using `rustup install nightly-2019-03-28`.
58 Now you can run your project in Miri:
60 1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
61 dependencies to be compiled the right way, that would not happen if they have
62 previously already been compiled.
63 2. To run all tests in your project through Miri, use `cargo miri test`.
64 3. If you have a binary project, you can run it through Miri using `cargo miri run`.
66 The first time you run Miri, it will perform some extra setup and install some
67 dependencies. It will ask you for confirmation before installing anything. If
68 you run Miri on CI, run `cargo miri setup` to avoid getting interactive
71 You can pass arguments to Miri after the first `--`, and pass arguments to the
72 interpreted program or test suite after the second `--`. For example, `cargo
73 miri run -- -Zmiri-disable-validation` runs the program without validation of
74 basic type invariants and references. `cargo miri test -- -- -Zunstable-options
75 --exclude-should-panic` skips `#[should_panic]` tests, which is a good idea
76 because Miri does not support unwinding or catching panics.
78 When running code via `cargo miri`, the `miri` config flag is set. You can
79 use this to exclude test cases that will fail under Miri because they do things
80 Miri does not support:
85 fn does_not_work_on_miri() {
87 assert!(&x as *const _ as usize % 4 < 4);
93 When using the above instructions, you may encounter a number of confusing compiler
96 #### "found possibly newer version of crate `std` which `<dependency>` depends on"
98 Your build directory may contain artifacts from an earlier build that have/have
99 not been built for Miri. Run `cargo clean` before switching from non-Miri to
100 Miri builds and vice-versa.
102 #### "found crate `std` compiled by an incompatible version of rustc"
104 You may be running `cargo miri` with a different compiler version than the one
105 used to build the custom libstd that Miri uses, and Miri failed to detect that.
106 Try deleting `~/.cache/miri`.
108 #### "no mir for `std::rt::lang_start_internal`"
110 This means the sysroot you are using was not compiled with Miri in mind. This
111 should never happen when you use `cargo miri` because that takes care of setting
112 up the sysroot. If you are using `miri` (the Miri driver) directly, see
113 [below][testing-miri] for how to set up the sysroot.
115 ## Development and Debugging
117 If you want to hack on miri yourself, great! Here are some resources you might
120 ### Using a nightly rustc
122 Miri heavily relies on internal rustc interfaces to execute MIR. Still, some
123 things (like adding support for a new intrinsic or a shim for an external
124 function being called) can be done by working just on the Miri side.
126 To prepare, make sure you are using a nightly Rust compiler. Then you should be
127 able to just `cargo build` Miri.
129 In case this fails, your nightly might be incompatible with Miri master. The
130 `rust-version` file contains the commit hash of rustc that Miri is currently
131 tested against; you can use that to find a nightly that works or you might have
132 to wait for the next nightly to get released.
134 ### Testing the Miri driver
135 [testing-miri]: #testing-the-miri-driver
137 The Miri driver in the `miri` binary is the "heart" of Miri: it is basically a
138 version of `rustc` that, instead of compiling your code, runs it. It accepts
139 all the same flags as `rustc` (though the ones only affecting code generation
140 and linking obviously will have no effect) [and more][miri-flags].
142 To run the Miri driver, you need to have the `MIRI_SYSROOT` environment variable
143 set to an appropriate sysroot. You can generate such a sysroot with the
144 following incantation:
147 cargo run --bin cargo-miri -- miri setup
150 This basically runs the `cargo-miri` binary (which backs the `cargo miri`
151 subcommand) with `cargo`, and asks it to `setup`. It should in the end print
152 the directory where the libstd was built. In the following, we will assume it
153 is `~/.cache/miri/HOST`; you may have to adjust that if you are not using Linux.
155 Now you can run the driver directly using
158 MIRI_SYSROOT=~/.cache/miri/HOST cargo run tests/run-pass/format.rs # or whatever test you like
161 and you can run the test suite using
167 We recommend adding the `--release` flag to make tests run faster.
169 `cargo test --release FILTER` only runs those tests that contain `FILTER` in
170 their filename (including the base directory, e.g. `cargo test --release fail`
171 will run all compile-fail tests).
173 You can get a trace of which MIR statements are being executed by setting the
174 `MIRI_LOG` environment variable. For example:
177 MIRI_LOG=info cargo run tests/run-pass/vecs.rs
180 Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
181 the `rustc::mir::interpret` and `rustc_mir::interpret` modules in rustc. You
182 can also do more targeted configuration, e.g. the following helps debug the
183 stacked borrows implementation:
186 MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows cargo run tests/run-pass/vecs.rs
189 In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
190 evaluation error was originally raised.
192 ### Testing `cargo miri`
194 Working with the driver directly gives you full control, but you also lose all
195 the convenience provided by cargo. Once your test case depends on a crate, it
196 is probably easier to test it with the cargo wrapper. You can install your
197 development version of Miri using
200 cargo install --path . --force
203 and then you can use it as if it was installed by `rustup`. Make sure you use
204 the same toolchain when calling `cargo miri` that you used when installing Miri!
206 There's a test for the cargo wrapper in the `test-cargo-miri` directory; run
207 `./run-test.py` in there to execute it.
209 ### Using a locally built rustc
211 A big part of the Miri driver lives in rustc, so working on Miri will sometimes
212 require using a locally built rustc. The bug you want to fix may actually be on
213 the rustc side, or you just need to get more detailed trace of the execution
214 than what is possible with release builds -- in both cases, you should develop
215 miri against a rustc you compiled yourself, with debug assertions (and hence
218 The setup for a local rustc works as follows:
220 git clone https://github.com/rust-lang/rust/ rustc
222 cp config.toml.example config.toml
223 # Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
224 # The latter is important to build libstd with the right flags for miri.
225 # This step can take 30 minutes and more.
226 ./x.py build src/rustc
227 # If you change something, you can get a faster rebuild by doing
228 ./x.py --keep-stage 0 build src/rustc
229 # You may have to change the architecture in the next command
230 rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
231 # Now cd to your Miri directory, then configure rustup
232 rustup override set custom
235 With this, you should now have a working development setup! See
236 [above][testing-miri] for how to proceed working with the Miri driver. Notice
237 that rustc's sysroot is already built for Miri in this case, so you can set
238 `MIRI_SYSROOT=$(rustc --print sysroot)`.
240 Running `cargo miri` in this setup is a bit more complicated, because the Miri
241 binary you just created needs help to find the libraries it links against. On
242 Linux, you can set the rpath to make this "just work":
245 export RUSTFLAGS="-C link-args=-Wl,-rpath,$(rustc --print sysroot)/lib/rustlib/x86_64-unknown-linux-gnu/lib"
246 cargo install --path . --force
249 ### Miri `-Z` flags and environment variables
250 [miri-flags]: #miri--z-flags-and-environment-variables
252 Several `-Z` flags are relevant for Miri:
254 * `-Zmiri-seed=<hex>` is a custom `-Z` flag added by Miri. It enables the
255 interpreted program to seed an RNG with system entropy. Miri will keep an RNG
256 on its own that is seeded with the given seed, and use that to generate the
257 "system entropy" that seeds the RNG(s) in the interpreted program.
258 **NOTE**: This entropy is not good enough for cryptographic use! Do not
259 generate secret keys in Miri or perform other kinds of cryptographic
260 operations that rely on proper random numbers.
261 * `-Zmiri-disable-validation` disables enforcing the validity invariant, which
262 is enforced by default. This is mostly useful for debugging; it means Miri
263 will miss bugs in your program. However, this can also help to make Miri run
265 * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
266 overrides the default to be `0`; be advised that using any higher level can
267 make Miri miss bugs in your program because they got optimized away.
268 * `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
269 functions. This is needed so that Miri can execute such functions, so Miri
270 sets this flag per default.
272 Moreover, Miri recognizes some environment variables:
274 * `MIRI_SYSROOT` (recognized by `miri`, `cargo miri` and the test suite)
275 indicates the sysroot to use.
276 * `MIRI_TARGET` (recognized by the test suite) indicates which target
277 architecture to test against. `miri` and `cargo miri` accept the `--target`
278 flag for the same purpose.
280 ## Contributing and getting help
282 Check out the issues on this GitHub repository for some ideas. There's lots that
283 needs to be done that I haven't documented in the issues yet, however. For more
284 ideas or help with running or hacking on Miri, you can open an issue here on
285 GitHub or contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
287 [Rust Zulip]: https://rust-lang.zulipchat.com
291 This project began as part of an undergraduate research course in 2015 by
292 @solson at the [University of Saskatchewan][usask]. There are [slides] and a
293 [report] available from that project. In 2016, @oli-obk joined to prepare miri
294 for eventually being used as const evaluator in the Rust compiler itself
295 (basically, for `const` and `static` stuff), replacing the old evaluator that
296 worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
297 and began developing miri towards a tool for detecting undefined behavior, and
298 also using miri as a way to explore the consequences of various possible
299 definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
300 into the compiler finally came to completion in early 2018. Meanwhile, later
301 that year, @RalfJung did a second internship, developing miri further with
302 support for checking basic type invariants and verifying that references are
303 used according to their aliasing restrictions.
305 [usask]: https://www.usask.ca/
306 [slides]: https://solson.me/miri-slides.pdf
307 [report]: https://solson.me/miri-report.pdf
309 ## Bugs found by Miri
311 Miri has already found a number of bugs in the Rust standard library, which we collect here.
313 * [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
314 * [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
315 * [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
316 * [`VecDeque` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
317 * [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
318 * [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
319 * [`BTreeMap` creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
323 Licensed under either of
324 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
325 http://www.apache.org/licenses/LICENSE-2.0)
326 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
327 http://opensource.org/licenses/MIT) at your option.
331 Unless you explicitly state otherwise, any contribution intentionally submitted
332 for inclusion in the work by you shall be dual licensed as above, without any
333 additional terms or conditions.