Miri

An experimental interpreter for Rust's
mid-level intermediate representation (MIR). It can run binaries and
test suites of cargo projects and detect certain classes of
undefined behavior,
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 rules governing aliasing for reference types

Miri has already discovered some 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!

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 you 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 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. Miri tests one of many possible
  executions of your program. If your code is sensitive to allocation base
  addresses or other non-deterministic data, try running Miri with different
  values for -Zmiri-seed to test different 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 concurrency, or SIMD, or networking.

Using Miri

Install Miri on Rust nightly via rustup:

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 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.

You can pass arguments to Miri after the first --, and pass arguments to the
interpreted program or test suite after the second --. For example, cargo miri run -- -Zmiri-disable-validation runs the program without validation of
basic type invariants and without checking the aliasing of references.

When compiling code via cargo miri, the miri config flag is set. You can
use this to ignore test cases that will fail under Miri because they do things
Miri does not support:

#[test]
#[cfg_attr(miri, ignore)]
fn does_not_work_on_miri() {
    std::thread::spawn(, println!("Hello Thread!"))
        .join()
        .unwrap();
}

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:

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: $MIRI_NIGHTLY"
rustup set profile minimal
rustup default "$MIRI_NIGHTLY"

rustup component add miri
cargo miri setup

cargo miri test

We use cargo miri setup to avoid getting interactive questions about the extra
setup needed for Miri.

Common Problems

When using the above instructions, you may encounter a number of confusing compiler
errors.

"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
[below][testing-miri] for how to set up the sysroot.

Miri -Z flags and environment variables

Several -Z flags are relevant for Miri:

• -Zmiri-seed=<hex> is a custom -Z flag added by Miri. It 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-disable-validation disables enforcing validity invariants and
  reference aliasing rules, which are enforced by default. This is mostly
  useful for debugging. It means Miri will miss bugs in your program. However,
  this can also help to make Miri run faster.
• -Zmiri-disable-isolation disables host host isolation. As a consequence,
  the program has access to host resources such as environment variables, file
  systems, and randomness.
• -Zmiri-ignore-leaks disables the memory leak checker.
• -Zmiri-env-exclude=<var> keeps the var environment variable isolated from
  the host. Can be used multiple times to exclude several variables. The TERM
  environment variable is excluded by default.
• -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 validation.
• -Zmiri-track-pointer-tag=<tag> aborts interpretation with 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 anyway). This helps you
  in finding out why UB is happening and where in your code would be a good
  place to look for it.

Moreover, Miri recognizes some environment variables:

• MIRI_LOG, MIRI_BACKTRACE control logging and backtrace printing during
  Miri executions, also [see above][testing-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.

Contributing and getting help

If you want to contribute to Miri, great! Please check out our
contribution guide.

For help with running Miri, you can open an issue here on
GitHub or contact us (oli-obk and RalfJ) on the Rust Zulip.

History

This project began as part of an undergraduate research course in 2015 by
@solson at the University of Saskatchewan. 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.

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
• Vec::into_iter doing an unaligned ZST read
• From<&[T]> for Rc creating a not sufficiently aligned reference
• BTreeMap creating a shared reference pointing to a too small allocation
• Vec::append creating a dangling reference
• Futures turning a shared reference into a mutable one
• str turning a shared reference into a mutable one
• rand performing unaligned reads
• The Unix allocator calling posix_memalign in an invalid way
• getrandom calling the getrandom syscall in an invalid way

Violations of Stacked Borrows found that are likely bugs (but Stacked Borrows is currently just an experiment):

• VecDeque creating overlapping mutable references
• BTreeMap creating mutable references that overlap with shared references
• LinkedList creating overlapping mutable references
• Vec::push invalidating existing references into the vector

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or
  http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (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.