Miri

Miri is an Undefined Behavior detection tool for Rust. It can run binaries and test suites of cargo projects and detect unsafe code that fails to uphold its safety requirements. For instance:

• Out-of-bounds memory accesses and use-after-free
• Invalid use of uninitialized data
• Violation of intrinsic preconditions (an [unreachable_unchecked] being

• Not sufficiently aligned memory accesses and references
• Violation of basic type invariants (a bool that is not 0 or 1, for example,

• Experimental: Violations of the [Stacked Borrows] rules governing aliasing

• Experimental: Violations of the [Tree Borrows] aliasing rules, as an optional

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 below.

Miri has already discovered many real-world bugs. If you found a bug with Miri, we'd appreciate if you tell us and we'll add it to the list!

By default, Miri ensures a fully deterministic execution and isolates the program from the host system. Some APIs that would usually access the host, such as gathering entropy for random number generators, environment variables, and clocks, are replaced by deterministic "fake" implementations. Set MIRIFLAGS="-Zmiri-disable-isolation" to access the real system APIs instead. (In particular, the "fake" system RNG APIs make Miri **not suited for cryptographic use**! Do not generate keys using Miri.)

All that said, be aware that Miri does not catch every violation of the Rust specification in your program, not least because there is no such specification. Miri uses its own approximation of what is and is not Undefined Behavior in Rust. To the best of our knowledge, all Undefined Behavior that has the potential to affect a program's correctness is being detected by Miri (modulo bugs), but you should consult the Reference for the official definition of Undefined Behavior. Miri will be updated with the Rust compiler to protect against UB as it is understood by the current compiler, but it makes no promises about future versions of rustc.

Further caveats that Miri users should be aware of:

• If the program relies on unspecified details of how data is laid out, it will

• Program execution is non-deterministic when it depends, for example, on where

• Miri runs the program as a platform-independent interpreter, so the program

• Weak memory emulation is not complete: there are legal behaviors that Miri will never produce.

Moreover, Miri fundamentally cannot ensure that your code is sound. [Soundness] is the property of never causing undefined behavior when invoked from arbitrary safe code, even in combination with other sound code. In contrast, Miri can just tell you if *a particular way of interacting with your code (e.g., a test suite) causes any undefined behavior in a particular execution* (of which there may be many, e.g. when concurrency or other forms of non-determinism are involved). When Miri finds UB, your code is definitely unsound, but when Miri does not find UB, then you may just have to test more inputs or more possible non-deterministic choices.

Using Miri

Install Miri on Rust nightly via rustup:

rustup +nightly component add miri

All the following commands assume the nightly toolchain is pinned via rustup override set nightly. Alternatively, use cargo +nightly for each of the following commands.

Now you can run your project in Miri:

• To run all tests in your project through Miri, use cargo miri test.
• If you have a binary project, you can run it through Miri using cargo miri run.

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 flags 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 interpreted under Miri. You can use this to ignore test cases that fail under Miri because they do things Miri does not support:

#[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 Miri 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 s390x-unknown-linux-gnu will run your test suite on a big-endian target, which is useful for testing endian-sensitive code.

Testing multiple different executions

Certain parts of the execution are picked randomly by Miri, such as the exact base address allocations are stored at and the interleaving of concurrently executing threads. Sometimes, it can be useful to explore multiple different execution, e.g. to make sure that your code does not depend on incidental "super-alignment" of new allocations and to test different thread interleavings. This can be done with the -Zmiri-many-seeds flag:

MIRIFLAGS="-Zmiri-many-seeds" cargo miri test # tries the seeds in 0..64
MIRIFLAGS="-Zmiri-many-seeds=0..16" cargo miri test

The default of 64 different seeds can be quite slow, so you often want to specify a smaller range.

Running Miri on CI

When running Miri on CI, use the following snippet to install a nightly toolchain with the Miri component:

rustup toolchain install nightly --component miri
rustup override set nightly

cargo miri test

Here is an example job for GitHub Actions:

miri:
    name: "Miri"
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Install Miri
        run: |
          rustup toolchain install nightly --component miri
          rustup override set nightly
          cargo miri setup
      - name: Test with Miri
        run: cargo miri test

The explicit cargo miri setup helps to keep the output of the actual test step clean.

Supported targets

Miri does not support all targets supported by Rust. The good news, however, is that no matter your host OS/platform, it is easy to run code for any target using --target!

The following targets are tested on CI and thus should always work (to the degree documented below):

• All Rust Tier 1 targets are supported by

• s390x-unknown-linux-gnu is supported as our "big-endian target of choice".
• For every other target with OS linux, macos, or windows, Miri should generally work, but we

• We have unofficial support (not maintained by the Miri team itself) for some further operating systems.

• solaris / illumos: maintained by @devnexen. Supports the entire test suite.

• freebsd: maintained by @YohDeadfall and @LorrensP-2158466. Supports the entire test suite.

• android: maintainer wanted. Supports the entire test suite.
• For targets on other operating systems, Miri might fail before even reaching the main function.

Running tests in parallel

Though it implements Rust threading, Miri itself is a single-threaded interpreter (it works like a multi-threaded OS on a single-core CPU). This means that when running cargo miri test, you will probably see a dramatic increase in the amount of time it takes to run your whole test suite due to the inherent interpreter slowdown and a loss of parallelism.

You can get your test suite's parallelism back by running cargo miri nextest run -jN (note that you will need cargo-nextest installed). This works because cargo-nextest collects a list of all tests then launches a separate cargo miri run for each test. For more information about nextest, see the cargo-nextest Miri documentation.

Note: This one-test-per-process model means that cargo miri test is able to detect data races where two tests race on a shared resource, but cargo miri nextest run will not detect such races.

Note: cargo-nextest does not support doctests, see https://github.com/nextest-rs/nextest/issues/16

Directly invoking the miri driver

The recommended way to invoke Miri is via cargo miri. Directly invoking the underlying miri driver is not supported, which is why that binary is not even installed into the PATH. However, if you need to run Miri on many small tests and want to invoke it directly like you would invoke rustc, that is still possible with a bit of extra effort:

# one-time setup
cargo +nightly miri setup
SYSROOT=$(cargo +nightly miri setup --print-sysroot)
# per file
~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/miri --sysroot "$SYSROOT" file.rs

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:

RUST_BACKTRACE=1 MIRIFLAGS="-Zmiri-disable-isolation" cargo miri test

"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 running cargo miri clean.

Miri -Z flags and environment variables

Miri adds its own set of -Z flags, which are usually set via the MIRIFLAGS environment variable. We first document the most relevant and most commonly used flags:

• -Zmiri-backtrace=<0|1|full> configures how Miri prints backtraces: 1 is the default,

• -Zmiri-deterministic-concurrency makes Miri's concurrency-related behavior fully deterministic.

• -Zmiri-disable-isolation disables host isolation. As a consequence,

• -Zmiri-disable-leak-backtraces disables backtraces reports for memory leaks. By default, a

• -Zmiri-env-forward=<var> forwards the var environment variable to the interpreted program. Can

• -Zmiri-env-set=<var>=<value> sets the var environment variable to value in the interpreted program.

• -Zmiri-ignore-leaks disables the memory leak checker, and also allows some

• -Zmiri-isolation-error=<action> configures Miri's response to operations

• -Zmiri-many-seeds=[<from>]..<to> runs the program multiple times with different seeds for Miri's

• -Zmiri-many-seeds-keep-going tells Miri to really try all the seeds in the given range, even if

• -Zmiri-max-extra-rounding-error tells Miri to always apply the maximum error to float operations

• -Zmiri-no-extra-rounding-error stops Miri from adding extra rounding errors to float operations

• -Zmiri-no-short-fd-operations stops Miri from artificially forcing read/write operations

• -Zmiri-num-cpus states the number of available CPUs to be reported by miri. By default, the

• -Zmiri-permissive-provenance disables the warning for integer-to-pointer casts and

• -Zmiri-report-progress makes Miri print the current stacktrace every now and then, so you can

• -Zmiri-seed=<num> configures the seed of the RNG that Miri uses to resolve non-determinism. This

• -Zmiri-strict-provenance enables strict

• -Zmiri-symbolic-alignment-check makes the alignment check more strict. By default, alignment is

• -Zmiri-user-relevant-crates=<crate>,<crate>,... extends the list of crates that Miri considers

The remaining flags are for advanced use only, and more likely to change or be removed. Some of these are unsound, which means they can lead to Miri failing to detect cases of undefined behavior in a program.

-Zmiri-address-reuse-rate=<rate> changes the probability that a freed non-stack* allocation will be added to the pool for address reuse, and the probability that a new non-stack allocation will be taken from the pool. Stack allocations never get added to or taken from the pool. The default is 0.5.

• -Zmiri-address-reuse-cross-thread-rate=<rate> changes the probability that an allocation which

• -Zmiri-compare-exchange-weak-failure-rate=<rate> changes the failure rate of

• -Zmiri-deterministic-floats makes Miri's floating-point behavior fully deterministic. This means

• -Zmiri-disable-alignment-check disables checking pointer alignment, so you

• -Zmiri-disable-data-race-detector disables checking for data races. Using

• -Zmiri-disable-stacked-borrows disables checking the experimental

• -Zmiri-disable-validation disables enforcing validity invariants, which are

• -Zmiri-disable-weak-memory-emulation disables the emulation of some C++11 weak

• -Zmiri-fixed-schedule disables preemption (like -Zmiri-preemption-rate=0.0) and furthermore

• -Zmiri-force-intrinsic-fallback forces the use of the "fallback" body for all intrinsics that

• -Zmiri-native-lib=<path to a shared object file or folder> is an experimental flag for providing

• -Zmiri-native-lib-enable-tracing enables the WIP detailed tracing mode for invoking native code.

• -Zmiri-measureme=<name> enables measureme profiling for the interpreted program.

• -Zmiri-mute-stdout-stderr silently ignores all writes to stdout and stderr,

• -Zmiri-preemption-rate configures the probability that at the end of a basic block, the active

• -Zmiri-provenance-gc=<blocks> configures how often the pointer provenance garbage collector runs.

• -Zmiri-track-alloc-accesses show not only allocation and free events for tracked allocations,

• -Zmiri-track-alloc-id=<id1>,<id2>,... shows a backtrace when the given allocations are

• -Zmiri-track-pointer-tag=<tag1>,<tag2>,... shows a backtrace when a given pointer tag

• -Zmiri-track-weak-memory-loads shows a backtrace when weak memory emulation returns an outdated

• -Zmiri-tree-borrows-no-precise-interior-mut makes Tree Borrows

• -Zmiri-force-page-size=<num> overrides the default page size for an architecture, in multiples of 1k.

Some native rustc -Z flags are also very relevant for Miri:

• -Zmir-opt-level controls how many MIR optimizations are performed. Miri

• -Zalways-encode-mir makes rustc dump MIR even for completely monomorphic

• -Zmir-emit-retag controls whether Retag statements are emitted. Miri

Moreover, Miri recognizes some environment variables:

• MIRIFLAGS defines extra flags to be passed to Miri.
• MIRI_LIB_SRC defines the directory where Miri expects the sources of the standard library that

• MIRI_SYSROOT indicates the sysroot to use. When using cargo miri test/cargo miri run, this skips the automatic

• MIRI_NO_STD makes sure that the target's sysroot is built without libstd. This allows testing

Miri extern functions

Miri provides some extern functions that programs can import to access Miri-specific functionality. They are declared in /tests/utils/miri\extern.rs.

#[cfg(miri)]
#[unsafe(no_mangle)]
fn miri_start(argc: isize, argv: *const *const u8) -> isize {
    // Call the actual start function that your project implements, based on your target's conventions.
}