haybale: Symbolic execution of LLVM IR, written in Rust

haybale is a general-purpose symbolic execution engine written in Rust. It operates on LLVM IR, which allows it to analyze programs written in C/C++, Rust, Swift, or any other language which compiles to LLVM IR. In this way, it may be compared to KLEE, as it has similar goals, except that haybale is written in Rust and makes some different design decisions. That said, haybale makes no claim of being at feature parity with KLEE.

Okay, but what is a symbolic execution engine?

A symbolic execution engine is a way of reasoning - rigorously and mathematically - about the behavior of a function or program. It can reason about all possible inputs to a function without literally brute-forcing every single one. For instance, a symbolic execution engine like haybale can answer questions like:

• Are there any inputs to (some function) that cause it to return 0? What are they?
• Is it possible for this loop to execute exactly 17 times?
• Can this pointer ever be NULL?

Symbolic execution engines answer these questions by converting each variable in the program or function into a mathematical expression which depends on the function or program inputs. Then they use an SMT solver to answer questions about these expressions, such as the questions listed above.

Getting started

1. Install

haybale is on crates.io, so you can simply add it as a dependency in your Cargo.toml:

[dependencies]
haybale = "0.1.1"

haybale also depends (indirectly) on the LLVM 8 and Boolector libraries, which must both be available on your system. See the llvm-sys or boolector-sys READMEs for more details and instructions.

2. Acquire bitcode to analyze

Since haybale operates on LLVM bitcode, you'll need some bitcode to get started. If the program or function you want to analyze is written in C, you can generate LLVM bitcode (*.bc files) with clang's -c and -emit-llvm flags:

clang -c -emit-llvm source.c -o source.bc

For debugging purposes, you may also want LLVM text-format (*.ll) files, which you can generate with clang's -S and -emit-llvm flags:

clang -S -emit-llvm source.c -o source.ll

If the program or function you want to analyze is written in Rust, you can likewise use rustc's --emit=llvm-bc and --emit=llvm-ir flags.

3. Create a Project

A haybale Project contains all of the code currently being analyzed, which may be one or more LLVM modules. To get started, simply create a Project from a single bitcode file:

let project = Project::from_bc_path(&Path::new("/path/to/file.bc"))?;

For more ways to create Projects, including analyzing entire libraries, see the Project documentation.

4. Use built-in analyses

haybale currently includes two simple built-in analyses: get_possible_return_values_of_func, which describes all the possible values a function could return for any input, and find_zero_of_func, which finds a set of inputs to a function such that it returns 0. These analyses are provided both because they may be of some use themselves, but also because they illustrate how to use haybale.

For an introductory example, let's suppose foo is the following C function:

int foo(int a, int b) {
    if (a > b) {
        return (a-1) * (b-1);
    } else {
        return (a + b) % 3 + 10;
    }
}

We can use find_zero_of_func to find inputs such that foo will return 0:

match find_zero_of_func("foo", &project, Config::default()) {
    None => println!("foo can never return 0"),
    Some(inputs) => println!("Inputs for which foo returns 0: {:?}", inputs),
}

Writing custom analyses

haybale can do much more than just describe possible function return values and find function zeroes. In this section, we'll walk through how we could find a zero of the function foo above without using the built-in find_zero_of_func. This will illustrate how to write a custom analysis using haybale.

ExecutionManager

All analyses will use an ExecutionManager to control the progress of the symbolic execution. In the code snippet below, we call symex_function to create an ExecutionManager which will analyze the function foo - it will start at the top of the function, and end when the function returns. In between, it will also analyze any functions called by foo, as necessary and depending on the Config settings.

let mut em = symex_function("foo", &project, Config::<BtorBackend>::default());

Here it was necessary to not only specify the default haybale configuration, as we did when calling find_zero_of_func, but also what "backend" we want to use. The default BtorBackend should be fine for most purposes.

Paths

The ExecutionManager acts like an Iterator over paths through the function foo. Each path is one possible sequence of control-flow decisions (e.g., which direction do we take at each if statement) leading to the function returning some value. The function foo in this example has two paths, one following the "true" branch and one following the "false" branch of the if.

Let's examine the first path through the function:

let retval = em.next().expect("Expected at least one path")?;

We're given the function return value, retval, as a Boolector BV (bitvector) wrapped in the ReturnValue enum. Since we know that foo isn't a void-typed function, we can simply unwrap the ReturnValue to get the BV:

let retval = match retval {
    ReturnValue::ReturnVoid => panic!("Function shouldn't return void"),
    ReturnValue::Return(r) => r,
};

States

Importantly, the ExecutionManager provides not only the final return value of the path as a BV, but also the final program State at the end of that path, either immutably with state() or mutably with mut_state(). (See the ExecutionManager documentation for more.)

let state = em.mut_state();  // the final program state along this path

To test whether retval can be equal to 0 in this State, we can use state.bvs_can_be_equal():

let zero = BV::zero(state.solver.clone(), 32);  // The 32-bit constant 0
if state.bvs_can_be_equal(&retval, &zero)? {
    println!("retval can be 0!");
}

Getting solutions for variables

If retval can be 0, let's find what values of the function parameters would cause that. First, we'll add a constraint to the State requiring that the return value must be 0:

retval._eq(&zero).assert();

and then we'll ask for solutions for each of the parameters, given this constraint:

// Get a possible solution for the first parameter.
// In this case, from looking at the text-format LLVM IR, we know the variable
// we're looking for is variable #0 in the function "foo".
let a = state.get_a_solution_for_irname(&String::from("foo"), Name::from(0))?
    .expect("Expected there to be a solution")
    .as_u64()
    .expect("Expected solution to fit in 64 bits");

// Likewise the second parameter, which is variable #1 in "foo"
let b = state.get_a_solution_for_irname(&String::from("foo"), Name::from(1))?
    .expect("Expected there to be a solution")
    .as_u64()
    .expect("Expected solution to fit in 64 bits");

println!("Parameter values for which foo returns 0: a = {}, b = {}", a, b);

Alternately, we could also have gotten the parameter BVs from the ExecutionManager like this:

let a_bv = em.param_bvs()[0].clone();
let b_bv = em.param_bvs()[1].clone();

let a = em.state().get_a_solution_for_bv(&a_bv)?
    .expect("Expected there to be a solution")
    .as_u64()
    .expect("Expected solution to fit in 64 bits");

let b = em.state().get_a_solution_for_bv(&b_bv)?
    .expect("Expected there to be a solution")
    .as_u64()
    .expect("Expected solution to fit in 64 bits");

println!("Parameter values for which foo returns 0: a = {}, b = {}", a, b);

Documentation

Full documentation for haybale can be found here, or of course you can generate local documentation with cargo doc --open.

Compatibility

Currently, haybale only supports LLVM 8.

haybale works on stable Rust, and requires Rust 1.36+.

Under the hood

haybale is built using the Rust llvm-ir crate and the Boolector SMT solver (via the Rust boolector crate).