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benches | ||
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test-cargo-miri | ||
tests | ||
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LICENSE-APACHE | ||
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miri | ||
README.md | ||
rust-version | ||
travis.sh |
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, callingcopy_nonoverlapping
with overlapping ranges, ...) - Not sufficiently aligned memory accesses and references
- Violation of basic type invariants (a
bool
that is not 0 or 1, for example, or an invalid enum discriminant) - WIP: 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 possible errors 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.
- Miri is fully deterministic and does not actually pick a base address in virtual memory for the program's allocations. If program behavior depends on the base address of an allocation, Miri will stop execution (with a few exceptions to make some common pointer comparisons work).
- Miri runs the program as a platform-independent interpreter, so the program has no access to any 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, or file system access.
Running Miri on your own project (and its test suite)
Install Miri via rustup
:
rustup 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, e.g. using
rustup install nightly-2019-03-28
.
Now you can run your project in Miri:
- 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. - 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
.
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. If
you run Miri on CI, run cargo miri setup
to avoid getting interactive
questions.
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 references. cargo miri test -- -- -Zunstable-options --exclude-should-panic
skips #[should_panic]
tests, which is a good idea
because Miri does not support unwinding or catching panics.
When running code via cargo miri
, the miri
config flag is set. You can
use this to exclude test cases that will fail under Miri because they do things
Miri does not support:
#[cfg(not(miri))]
#[test]
fn does_not_work_on_miri() {
let x = 0u8;
assert!(&x as *const _ as usize % 4 < 4);
}
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: $MIRI_NIGHTLY"
rustup default "$MIRI_NIGHTLY"
rustup component add miri
cargo miri setup
cargo miri test -- -- -Zunstable-options --exclude-should-panic
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 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, and when the interpreted program requests 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 the validity invariant, which is 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.-Zmir-opt-level
controls how many MIR optimizations are performed. Miri overrides the default to be0
; 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 whetherRetag
statements are emitted. Miri enables this per default because it is needed for validation.
Moreover, Miri recognizes some environment variables:
MIRI_LOG
,MIRI_BACKTRACE
control logging and backtrace printing during Miri executions, also see above.MIRI_SYSROOT
(recognized bycargo miri
and the test suite) indicates the sysroot to use. To do the same thing withmiri
directly, use the--sysroot
flag.MIRI_TEST_TARGET
(recognized by the test suite) indicates which target architecture to test against.miri
andcargo miri
accept the--target
flag for the same purpose.
Development and Debugging
If you want to hack on miri yourself, great! Here are some resources you might find useful.
Using a nightly rustc
Miri heavily relies on internal rustc interfaces to execute MIR. Still, some things (like adding support for a new intrinsic or a shim for an external function being called) can be done by working just on the Miri side.
To prepare, make sure you are using a nightly Rust compiler. Then you should be
able to just cargo build
Miri.
In case this fails, your nightly might be incompatible with Miri master. The
rust-version
file contains the commit hash of rustc that Miri is currently
tested against; you can use that to find a nightly that works or you might have
to wait for the next nightly to get released. You can also use
rustup-toolchain-install-master
to install that exact version of rustc as a toolchain:
rustup-toolchain-install-master $(cat rust-version) -c rust-src
Another common problem is outdated dependencies: Miri does not come with a
lockfile (it cannot, due to how it gets embedded into the rustc build). So you
have to run cargo update
every now and then yourself to make sure you are
using the latest versions of everything (which is what gets tested on CI).
Testing the Miri driver
The Miri driver in the miri
binary is the "heart" of Miri: it is basically a
version of rustc
that, instead of compiling your code, runs it. It accepts
all the same flags as rustc
(though the ones only affecting code generation
and linking obviously will have no effect) and more.
Running the Miri driver requires some fiddling with environment variables, so
the miri
script helps you do that. For example, you can run the driver on a
particular file by doing
./miri run tests/run-pass/format.rs
./miri run tests/run-pass/hello.rs --target i686-unknown-linux-gnu
and you can run the test suite using:
./miri test
./miri test FILTER
only runs those tests that contain FILTER
in their
filename (including the base directory, e.g. ./miri test fail
will run all
compile-fail tests).
You can get a trace of which MIR statements are being executed by setting the
MIRI_LOG
environment variable. For example:
MIRI_LOG=info ./miri run tests/run-pass/vecs.rs
Setting MIRI_LOG
like this will configure logging for Miri itself as well as
the rustc::mir::interpret
and rustc_mir::interpret
modules in rustc. You
can also do more targeted configuration, e.g. the following helps debug the
stacked borrows implementation:
MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/run-pass/vecs.rs
In addition, you can set MIRI_BACKTRACE=1
to get a backtrace of where an
evaluation error was originally raised.
Testing cargo miri
Working with the driver directly gives you full control, but you also lose all the convenience provided by cargo. Once your test case depends on a crate, it is probably easier to test it with the cargo wrapper. You can install your development version of Miri using
./miri install
and then you can use it as if it was installed by rustup
. Make sure you use
the same toolchain when calling cargo miri
that you used when installing Miri!
There's a test for the cargo wrapper in the test-cargo-miri
directory; run
./run-test.py
in there to execute it.
Using a locally built rustc
A big part of the Miri driver lives in rustc, so working on Miri will sometimes require using a locally built rustc. The bug you want to fix may actually be on the rustc side, or you just need to get more detailed trace of the execution than what is possible with release builds -- in both cases, you should develop miri against a rustc you compiled yourself, with debug assertions (and hence tracing) enabled.
The setup for a local rustc works as follows:
git clone https://github.com/rust-lang/rust/ rustc
cd rustc
cp config.toml.example config.toml
# Now edit `config.toml` and set `debug-assertions = true` and `test-miri = true`.
# The latter is important to build libstd with the right flags for miri.
# This step can take 30 minutes and more.
./x.py build src/rustc
# If you change something, you can get a faster rebuild by doing
./x.py --keep-stage 0 build src/rustc
# You may have to change the architecture in the next command
rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
# Now cd to your Miri directory, then configure rustup
rustup override set custom
With this, you should now have a working development setup! See above for how to proceed working with the Miri driver.
Contributing and getting help
Check out the issues on this GitHub repository for some ideas. There's lots that
needs to be done that I haven't documented in the issues yet, however. For more
ideas or help with running or hacking on 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 memoryVec::into_iter
doing an unaligned ZST readFrom<&[T]> for Rc
creating a not sufficiently aligned referenceBTreeMap
creating a shared reference pointing to a too small allocationVec::append
creating a dangling reference- Futures turning a shared reference into a mutable one
str
turning a shared reference into a mutable onerand
performing unaligned reads- The Unix allocator calling
posix_memalign
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 referencesBTreeMap
creating mutable references that overlap with shared referencesLinkedList
creating overlapping mutable referencesVec::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.