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Kani Rust Verifier

The Kani Rust Verifier is a bit-precise model checker for Rust. Kani is designed to prove safety properties in your code as well as the absence of some forms of undefined behavior. It uses model checking under the hood to ensure that Rust programs adhere to user specified properties.

You can find more information about how to install in the installation section of the Kani book.

Usage

Consider a Rust function that takes an integer and returns its absolute value and a Kani proof that invokes the function that you want to verify

fn abs_diff(a: i32, b: i32) -> i32 {
    if a > b {
        a - b
    } else {
        b - a
    }
}

#[cfg(kani)]
#[kani::proof]
fn harness() {
    let a = kani::any::<i32>();
    let b = kani::any::<i32>();
    let result = abs_diff(a, b);
    kani::assert(result >= 0, "Result should always be more than 0");
}

Running the command cargo kani in your cargo crate will give the result

RESULTS:
Check 1: abs_diff.assertion.1
         - Status: FAILURE
         - Description: "attempt to subtract with overflow"
         - Location: src/main.rs:5:9 in function abs_diff

... Other properties that might have failed or succeeded.

Summary:
Verification failed for - harness
Complete - 0 successfully verified harnesses, 1 failures, 1 total.

For a more detailed tutorial, you can refer to the tutorial section of the Kani book.

Using Kani to verify the Rust Standard Library

Here is a short tutorial of how to use Kani to verify proofs for the standard library.

Step 1 - Add some proofs to your copy of the model-checking std

Create a local copy of the model-checking fork of the Rust Standard Library. The fork comes with Kani configured, so all you'll need to do is to call Kani's building-block APIs (such as assert, assume, proof and function-contracts such as modifies, requires and ensures) directly.

For example, insert this module into an existing file in the core library, like library/core/src/hint.rs or library/core/src/error.rs in your copy of the library. This is just for the purpose of getting started, so you can insert in any existing file in the core library if you have other preferences.

#[cfg(kani)]
#[unstable(feature = "kani", issue = "none")]
pub mod verify {
    use crate::kani;

    #[kani::proof]
    pub fn harness_introduction() {
        kani::assert(true, "yay");
    }

    #[kani::proof_for_contract(trivial_function)]
    fn dummy_proof() {
        trivial_function(true);
    }

    #[kani::requires(x == true)]
    fn trivial_function(x: bool) -> bool {
        x
    }
}

Step 2 - Run the Kani verify-std subcommand

To aid the Rust Standard Library verification effort, Kani provides a sub-command out of the box to help you get started. Run the following command in your local terminal (Replace "/path/to/library" and "/path/to/target" with your local paths) from the verify repository root:

kani verify-std -Z unstable-options "/path/to/library" --target-dir "/path/to/target" -Z function-contracts -Z mem-predicates

The command kani verify-std is a sub-command of the kani. This specific sub-command is used to verify the Rust Standard Library with the following arguments.

  • "path/to/library": This argument specifies the path to the modified Rust Standard Library that was prepared earlier in the script. For example, ./library or /home/ubuntu/verify-rust-std/library
  • --target-dir "path/to/target": This optional argument sets the target directory where Kani will store its output and intermediate files. For example, /tmp or /tmp/verify-std

Apart from these, you can use your regular kani-args such as -Z function-contracts, -Z stubbing and -Z mem-predicates depending on your verification needs. If you run into kani error that says Use of unstable feature, add the corresponding feature with -Z to the command. For more details on Kani's features, refer to the features section in the Kani Book

Step 3 - Check verification result

After running the command, you can expect an output that looks like this:

SUMMARY:
 ** 0 of 1 failed

VERIFICATION:- SUCCESSFUL
Verification Time: 0.017101772s

Complete - 2 successfully verified harnesses, 0 failures, 2 total.

Running on a specific harness

You can specify a specific harness to be verified using the --harness flag.

For example, in your local copy of the verify repo, run the following command.

kani verify-std --harness harness_introduction -Z unstable-options "./library" --target-dir "/tmp" -Z function-contracts -Z mem-predicates

This gives you the verification result for just harness_introduction from the aforementioned blob.

RESULTS:
Check 1: verify::harness_introduction.assertion.1
         - Status: SUCCESS
         - Description: "yay"
         - Location: library/core/src/lib.rs:479:9 in function verify::harness_introduction


SUMMARY:
 ** 0 of 1 failed

VERIFICATION:- SUCCESSFUL
Verification Time: 0.01885804s

Complete - 1 successfully verified harnesses, 0 failures, 1 total.

Now you can write proof harnesses to verify specific functions in the library. The current convention is to keep proofs in the same module file of the verification target. To run kani for individual proof, use --harness [harness_function_name]. Note that Kani will batch run all proofs in the library folder if you do not supply the --harness flag. If kani returns the error no harnesses matched the harness filter, you can give the fullname of the harness. For example, to run the proof harness named check_new in library/core/src/ptr/unique.rs, use --harness ptr::unique::verify::check_new. To run all proofs in unique.rs, use --harness ptr::unique::verify. To find the fullname of a harness, check the kani output and find the line starting with Checking harness [harness fullname].

More details

You can find more information about how to install and how you can customize your use of Kani in the Kani book.