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# Miri
[![Actions build status][actions-badge]][actions-url]
[actions-badge]: https://github.com/rust-lang/miri/workflows/CI/badge.svg?branch=master
[actions-url]: https://github.com/rust-lang/miri/actions
An experimental interpreter for [Rust][rust]'s
[mid-level intermediate representation][mir] (MIR). It can run binaries and
test suites of cargo projects and detect certain classes of
[undefined behavior](https://doc.rust-lang.org/reference/behavior-considered-undefined.html),
for example:
* Out-of-bounds memory accesses and use-after-free
* Invalid use of uninitialized data
* Violation of intrinsic preconditions (an [`unreachable_unchecked`] being
reached, calling [`copy_nonoverlapping`] with overlapping ranges, ...)
* Not sufficiently aligned memory accesses and references
* Violation of *some* basic type invariants (a `bool` that is not 0 or 1, for example,
or an invalid enum discriminant)
* **Experimental**: Violations of the [Stacked Borrows] rules governing aliasing
for reference types
* **Experimental**: Data races (but no weak memory effects)
On top of that, Miri will also tell you about memory leaks: when there is memory
still allocated at the end of the execution, and that memory is not reachable
from a global `static`, Miri will raise an error.
You can use Miri to emulate programs on other targets, e.g. to ensure that
byte-level data manipulation works correctly both on little-endian and
big-endian systems. See
[cross-interpretation](#cross-interpretation-running-for-different-targets)
below.
Miri has already discovered some [real-world bugs](#bugs-found-by-miri). If you
found a bug with Miri, we'd appreciate if you tell us and we'll add it to the
list!
However, be aware that Miri will **not catch all cases of undefined behavior**
in your program, and cannot run all programs:
* There are still plenty of open questions around the basic invariants for some
types and when these invariants even have to hold. Miri tries to avoid false
positives here, so if your program runs fine in Miri right now that is by no
means a guarantee that it is UB-free when these questions get answered.
In particular, Miri does currently not check that integers/floats are
initialized or that references point to valid data.
* If the program relies on unspecified details of how data is laid out, it will
still run fine in Miri -- but might break (including causing UB) on different
compiler versions or different platforms.
* Program execution is non-deterministic when it depends, for example, on where
exactly in memory allocations end up, or on the exact interleaving of
concurrent threads. Miri tests one of many possible executions of your
program. You can alleviate this to some extent by running Miri with different
values for `-Zmiri-seed`, but that will still by far not explore all possible
executions.
* Miri runs the program as a platform-independent interpreter, so the program
has no access to most platform-specific APIs or FFI. A few APIs have been
implemented (such as printing to stdout) but most have not: for example, Miri
currently does not support SIMD or networking.
[rust]: https://www.rust-lang.org/
[mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
[`unreachable_unchecked`]: https://doc.rust-lang.org/stable/std/hint/fn.unreachable_unchecked.html
[`copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/ptr/fn.copy_nonoverlapping.html
[Stacked Borrows]: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md
## Using Miri
Install Miri on Rust nightly via `rustup`:
```sh
rustup +nightly component add miri
```
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 using
`rustup toolchain install nightly-YYYY-MM-DD`.
Now you can run your project in Miri:
1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
dependencies to be compiled the right way, that would not happen if they have
previously already been compiled.
2. To run all tests in your project through Miri, use `cargo miri test`.
3. If you have a binary project, you can run it through Miri using `cargo miri run`.
The first time you run Miri, it will perform some extra setup and install some
dependencies. It will ask you for confirmation before installing anything.
`cargo miri run/test` supports the exact same flags as `cargo run/test`. For
example, `cargo miri test filter` only runs the tests containing `filter` in
their name.
You can pass arguments to Miri via `MIRIFLAGS`. For example,
`MIRIFLAGS="-Zmiri-disable-stacked-borrows" cargo miri run` runs the program
without checking the aliasing of references.
When compiling code via `cargo miri`, the `cfg(miri)` config flag is set for code
that will be interpret under Miri. You can use this to ignore test cases that fail
under Miri because they do things Miri does not support:
```rust
#[test]
#[cfg_attr(miri, ignore)]
fn does_not_work_on_miri() {
tokio::run(futures::future::ok::<_, ()>(()));
}
```
There is no way to list all the infinite things Miri cannot do, but the
interpreter will explicitly tell you when it finds something unsupported:
```
error: unsupported operation: can't call foreign function: bind
...
= help: this is likely not a bug in the program; it indicates that the program \
performed an operation that the interpreter does not support
```
### Cross-interpretation: running for different targets
Miri can not only run a binary or test suite for your host target, it can also
perform cross-interpretation for arbitrary foreign targets: `cargo miri run
--target x86_64-unknown-linux-gnu` will run your program as if it was a Linux
program, no matter your host OS. This is particularly useful if you are using
Windows, as the Linux target is much better supported than Windows targets.
You can also use this to test platforms with different properties than your host
platform. For example `cargo miri test --target mips64-unknown-linux-gnuabi64`
will run your test suite on a big-endian target, which is useful for testing
endian-sensitive code.
### Running Miri on CI
To run Miri on CI, make sure that you handle the case where the latest nightly
does not ship the Miri component because it currently does not build. For
example, you can use the following snippet to always test with the latest
nightly that *does* come with Miri:
```sh
MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
rustup set profile minimal
rustup override set "$MIRI_NIGHTLY"
rustup component add miri
cargo miri test
```
### Common Problems
When using the above instructions, you may encounter a number of confusing compiler
errors.
### "note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace"
You may see this when trying to get Miri to display a backtrace. By default, Miri
doesn't expose any environment to the program, so running
`RUST_BACKTRACE=1 cargo miri test` will not do what you expect.
To get a backtrace, you need to disable isolation
[using `-Zmiri-disable-isolation`][miri-flags]:
```sh
RUST_BACKTRACE=1 MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test
```
#### "found possibly newer version of crate `std` which `<dependency>` depends on"
Your build directory may contain artifacts from an earlier build that have/have
not been built for Miri. Run `cargo clean` before switching from non-Miri to
Miri builds and vice-versa.
#### "found crate `std` compiled by an incompatible version of rustc"
You may be running `cargo miri` with a different compiler version than the one
used to build the custom libstd that Miri uses, and Miri failed to detect that.
Try deleting `~/.cache/miri`.
#### "no mir for `std::rt::lang_start_internal`"
This means the sysroot you are using was not compiled with Miri in mind. This
should never happen when you use `cargo miri` because that takes care of setting
up the sysroot. If you are using `miri` (the Miri driver) directly, see the
[contributors' guide](CONTRIBUTING.md) for how to use `./miri` to best do that.
## Miri `-Z` flags and environment variables
[miri-flags]: #miri--z-flags-and-environment-variables
Miri adds its own set of `-Z` flags, which are usually set via the `MIRIFLAGS`
environment variable:
* `-Zmiri-compare-exchange-weak-failure-rate=<rate>` changes the failure rate of
`compare_exchange_weak` operations. The default is `0.8` (so 4 out of 5 weak ops will fail).
You can change it to any value between `0.0` and `1.0`, where `1.0` means it
will always fail and `0.0` means it will never fail.
* `-Zmiri-disable-abi-check` disables checking [function ABI]. Using this flag
is **unsound**.
* `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
can focus on other failures, but it means Miri can miss bugs in your program.
Using this flag is **unsound**.
* `-Zmiri-disable-data-race-detector` disables checking for data races. Using
this flag is **unsound**.
* `-Zmiri-disable-stacked-borrows` disables checking the experimental
[Stacked Borrows] aliasing rules. This can make Miri run faster, but it also
means no aliasing violations will be detected. Using this flag is **unsound**
(but the affected soundness rules are experimental).
* `-Zmiri-disable-validation` disables enforcing validity invariants, which are
enforced by default. This is mostly useful to focus on other failures (such
as out-of-bounds accesses) first. Setting this flag means Miri can miss bugs
in your program. However, this can also help to make Miri run faster. Using
this flag is **unsound**.
* `-Zmiri-disable-isolation` disables host isolation. As a consequence,
the program has access to host resources such as environment variables, file
systems, and randomness.
* `-Zmiri-env-exclude=<var>` keeps the `var` environment variable isolated from
the host so that it cannot be accessed by the program. Can be used multiple
times to exclude several variables. On Windows, the `TERM` environment
variable is excluded by default.
* `-Zmiri-ignore-leaks` disables the memory leak checker.
* `-Zmiri-measureme=<name>` enables `measureme` profiling for the interpreted program.
This can be used to find which parts of your program are executing slowly under Miri.
The profile is written out to a file with the prefix `<name>`, and can be processed
using the tools in the repository https://github.com/rust-lang/measureme.
* `-Zmiri-seed=<hex>` configures the seed of the RNG that Miri uses to resolve
non-determinism. This RNG is used to pick base addresses for allocations.
When isolation is enabled (the default), this is also used to emulate system
entropy. The default seed is 0. **NOTE**: This entropy is not good enough
for cryptographic use! Do not generate secret keys in Miri or perform other
kinds of cryptographic operations that rely on proper random numbers.
* `-Zmiri-symbolic-alignment-check` makes the alignment check more strict. By
default, alignment is checked by casting the pointer to an integer, and making
sure that is a multiple of the alignment. This can lead to cases where a
program passes the alignment check by pure chance, because things "happened to
be" sufficiently aligned -- there is no UB in this execution but there would
be UB in others. To avoid such cases, the symbolic alignment check only takes
into account the requested alignment of the relevant allocation, and the
offset into that allocation. This avoids missing such bugs, but it also
incurs some false positives when the code does manual integer arithmetic to
ensure alignment. (The standard library `align_to` method works fine in both
modes; under symbolic alignment it only fills the middle slice when the
allocation guarantees sufficient alignment.)
* `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
being allocated or freed. This helps in debugging memory leaks and
use after free bugs.
* `-Zmiri-track-call-id=<id>` shows a backtrace when the given call id is
assigned to a stack frame. This helps in debugging UB related to Stacked
Borrows "protectors".
* `-Zmiri-track-pointer-tag=<tag>` shows a backtrace when the given pointer tag
is popped from a borrow stack (which is where the tag becomes invalid and any
future use of it will error). This helps you in finding out why UB is
happening and where in your code would be a good place to look for it.
* `-Zmiri-track-raw-pointers` makes Stacked Borrows track a pointer tag even for
raw pointers. This can make valid code fail to pass the checks, but also can
help identify latent aliasing issues in code that Miri accepts by default. You
can recognize false positives by `<untagged>` occurring in the message -- this
indicates a pointer that was cast from an integer, so Miri was unable to track
this pointer. Note that it is not currently guaranteed that code that works
with `-Zmiri-track-raw-pointers` also works without
`-Zmiri-track-raw-pointers`, but for the vast majority of code, this will be the case.
[function ABI]: https://doc.rust-lang.org/reference/items/functions.html#extern-function-qualifier
Some native rustc `-Z` flags are also very relevant for Miri:
* `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
overrides the default to be `0`; be advised that using any higher level can
make Miri miss bugs in your program because they got optimized away.
* `-Zalways-encode-mir` makes rustc dump MIR even for completely monomorphic
functions. This is needed so that Miri can execute such functions, so Miri
sets this flag per default.
* `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
enables this per default because it is needed for [Stacked Borrows].
Moreover, Miri recognizes some environment variables:
* `MIRI_LOG`, `MIRI_BACKTRACE` control logging and backtrace printing during
Miri executions, also [see "Testing the Miri driver" in `CONTRIBUTING.md`][testing-miri].
* `MIRIFLAGS` (recognized by `cargo miri` and the test suite) defines extra
flags to be passed to Miri.
* `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
indicates the sysroot to use. To do the same thing with `miri`
directly, use the `--sysroot` flag.
* `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
architecture to test against. `miri` and `cargo miri` accept the `--target`
flag for the same purpose.
The following environment variables are *internal* and must not be used by
anyone but Miri itself. They are used to communicate between different Miri
binaries, and as such worth documenting:
* `MIRI_BE_RUSTC` can be set to `host` or `target`. It tells the Miri driver to
actually not interpret the code but compile it like rustc would. With `target`, Miri sets
some compiler flags to prepare the code for interpretation; with `host`, this is not done.
This environment variable is useful to be sure that the compiled `rlib`s are compatible
with Miri.
* `MIRI_CALLED_FROM_XARGO` is set during the Miri-induced `xargo` sysroot build,
which will re-invoke `cargo-miri` as the `rustc` to use for this build.
* `MIRI_CALLED_FROM_RUSTDOC` when set to any value tells `cargo-miri` that it is
running as a child process of `rustdoc`, which invokes it twice for each doc-test
and requires special treatment, most notably a check-only build before interpretation.
This is set by `cargo-miri` itself when running as a `rustdoc`-wrapper.
* `MIRI_CWD` when set to any value tells the Miri driver to change to the given
directory after loading all the source files, but before commencing
interpretation. This is useful if the interpreted program wants a different
working directory at run-time than at build-time.
* `MIRI_VERBOSE` when set to any value tells the various `cargo-miri` phases to
perform verbose logging.
[testing-miri]: CONTRIBUTING.md#testing-the-miri-driver
## Miri `extern` functions
Miri provides some `extern` functions that programs can import to access
Miri-specific functionality:
```rust
#[cfg(miri)]
extern "Rust" {
/// Miri-provided extern function to mark the block `ptr` points to as a "root"
/// for some static memory. This memory and everything reachable by it is not
/// considered leaking even if it still exists when the program terminates.
///
/// `ptr` has to point to the beginning of an allocated block.
fn miri_static_root(ptr: *const u8);
/// Miri-provided extern function to obtain a backtrace of the current call stack.
/// This returns a boxed slice of pointers - each pointer is an opaque value
/// that is only useful when passed to `miri_resolve_frame`
/// The `flags` argument must be `0`.
fn miri_get_backtrace(flags: u64) -> Box<[*mut ()]>;
/// Miri-provided extern function to resolve a frame pointer obtained
/// from `miri_get_backtrace`. The `flags` argument must be `0`,
/// and `MiriFrame` should be declared as follows:
///
/// ```rust
/// #[repr(C)]
/// struct MiriFrame {
/// // The name of the function being executed, encoded in UTF-8
/// name: Box<[u8]>,
/// // The filename of the function being executed, encoded in UTF-8
/// filename: Box<[u8]>,
/// // The line number currently being executed in `filename`, starting from '1'.
/// lineno: u32,
/// // The column number currently being executed in `filename`, starting from '1'.
/// colno: u32,
/// // The function pointer to the function currently being executed.
/// // This can be compared against function pointers obtained by
/// // casting a function (e.g. `my_fn as *mut ()`)
/// fn_ptr: *mut ()
/// }
/// ```
///
/// The fields must be declared in exactly the same order as they appear in `MiriFrame` above.
/// This function can be called on any thread (not just the one which obtained `frame`).
fn miri_resolve_frame(frame: *mut (), flags: u64) -> MiriFrame;
/// Miri-provided extern function to begin unwinding with the given payload.
///
/// This is internal and unstable and should not be used; we give it here
/// just to be complete.
fn miri_start_panic(payload: *mut u8) -> !;
}
```
## Contributing and getting help
If you want to contribute to Miri, great! Please check out our
[contribution guide](CONTRIBUTING.md).
For help with running Miri, you can open an issue here on
GitHub or use the [Miri stream on the Rust Zulip][zulip].
[zulip]: https://rust-lang.zulipchat.com/#narrow/stream/269128-miri
## History
This project began as part of an undergraduate research course in 2015 by
@solson at the [University of Saskatchewan][usask]. There are [slides] and a
[report] available from that project. In 2016, @oli-obk joined to prepare miri
for eventually being used as const evaluator in the Rust compiler itself
(basically, for `const` and `static` stuff), replacing the old evaluator that
worked directly on the AST. In 2017, @RalfJung did an internship with Mozilla
and began developing miri towards a tool for detecting undefined behavior, and
also using miri as a way to explore the consequences of various possible
definitions for undefined behavior in Rust. @oli-obk's move of the miri engine
into the compiler finally came to completion in early 2018. Meanwhile, later
that year, @RalfJung did a second internship, developing miri further with
support for checking basic type invariants and verifying that references are
used according to their aliasing restrictions.
[usask]: https://www.usask.ca/
[slides]: https://solson.me/miri-slides.pdf
[report]: https://solson.me/miri-report.pdf
## Bugs found by Miri
Miri has already found a number of bugs in the Rust standard library and beyond, which we collect here.
Definite bugs found:
* [`Debug for vec_deque::Iter` accessing uninitialized memory](https://github.com/rust-lang/rust/issues/53566)
* [`Vec::into_iter` doing an unaligned ZST read](https://github.com/rust-lang/rust/pull/53804)
* [`From<&[T]> for Rc` creating a not sufficiently aligned reference](https://github.com/rust-lang/rust/issues/54908)
* [`BTreeMap` creating a shared reference pointing to a too small allocation](https://github.com/rust-lang/rust/issues/54957)
* [`Vec::append` creating a dangling reference](https://github.com/rust-lang/rust/pull/61082)
* [Futures turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/56319)
* [`str` turning a shared reference into a mutable one](https://github.com/rust-lang/rust/pull/58200)
* [`rand` performing unaligned reads](https://github.com/rust-random/rand/issues/779)
* [The Unix allocator calling `posix_memalign` in an invalid way](https://github.com/rust-lang/rust/issues/62251)
* [`getrandom` calling the `getrandom` syscall in an invalid way](https://github.com/rust-random/getrandom/pull/73)
* [`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
* [`beef` leaking memory](https://github.com/maciejhirsz/beef/issues/12)
* [`EbrCell` using uninitialized memory incorrectly](https://github.com/Firstyear/concread/commit/b15be53b6ec076acb295a5c0483cdb4bf9be838f#diff-6282b2fc8e98bd089a1f0c86f648157cR229)
* [TiKV performing an unaligned pointer access](https://github.com/tikv/tikv/issues/7613)
* [`servo_arc` creating a dangling shared reference](https://github.com/servo/servo/issues/26357)
* [TiKV constructing out-of-bounds pointers (and overlapping mutable references)](https://github.com/tikv/tikv/pull/7751)
* [`encoding_rs` doing out-of-bounds pointer arithmetic](https://github.com/hsivonen/encoding_rs/pull/53)
* [TiKV using `Vec::from_raw_parts` incorrectly](https://github.com/tikv/agatedb/pull/24)
* Incorrect doctests for [`AtomicPtr`](https://github.com/rust-lang/rust/pull/84052) and [`Box::from_raw_in`](https://github.com/rust-lang/rust/pull/84053)
Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows is currently just an experiment):
* [`VecDeque::drain` creating overlapping mutable references](https://github.com/rust-lang/rust/pull/56161)
* Various `BTreeMap` problems
* [`BTreeMap` iterators creating mutable references that overlap with shared references](https://github.com/rust-lang/rust/pull/58431)
* [`BTreeMap::iter_mut` creating overlapping mutable references](https://github.com/rust-lang/rust/issues/73915)
* [`BTreeMap` node insertion using raw pointers outside their valid memory area](https://github.com/rust-lang/rust/issues/78477)
* [`LinkedList` cursor insertion creating overlapping mutable references](https://github.com/rust-lang/rust/pull/60072)
* [`Vec::push` invalidating existing references into the vector](https://github.com/rust-lang/rust/issues/60847)
* [`align_to_mut` violating uniqueness of mutable references](https://github.com/rust-lang/rust/issues/68549)
* [`sized-chunks` creating aliasing mutable references](https://github.com/bodil/sized-chunks/issues/8)
* [`String::push_str` invalidating existing references into the string](https://github.com/rust-lang/rust/issues/70301)
* [`ryu` using raw pointers outside their valid memory area](https://github.com/dtolnay/ryu/issues/24)
* [ink! creating overlapping mutable references](https://github.com/rust-lang/miri/issues/1364)
* [TiKV creating overlapping mutable reference and raw pointer](https://github.com/tikv/tikv/pull/7709)
* [Windows `Env` iterator using a raw pointer outside its valid memory area](https://github.com/rust-lang/rust/pull/70479)
* [`VecDeque::iter_mut` creating overlapping mutable references](https://github.com/rust-lang/rust/issues/74029)
* [Various standard library aliasing issues involving raw pointers](https://github.com/rust-lang/rust/pull/78602)
* [`<[T]>::copy_within` using a loan after invalidating it](https://github.com/rust-lang/rust/pull/85610)
## License
Licensed under either of
* Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
http://www.apache.org/licenses/LICENSE-2.0)
* MIT license ([LICENSE-MIT](LICENSE-MIT) or
http://opensource.org/licenses/MIT)
at your option.
### Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you shall be dual licensed as above, without any
additional terms or conditions.
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