Add a crate to interface with the WebAssembly spec interpreter

The WebAssembly spec interpreter is written in OCaml and the new crate
uses `ocaml-interop` along with a small OCaml wrapper to interpret Wasm
modules in-process. The build process for this crate is currently
Linux-specific: it requires several OCaml packages (e.g. `apt install -y
ocaml-nox ocamlbuild`) as well as `make`, `cp`, and `ar`.

crates/fuzzing/wasm-spec-interpreter/src/lib.rs

@@ -0,0 +1,33 @@
+//! This library provides a way to interpret Wasm functions in the official Wasm
+//! specification interpreter, written in OCaml, from Rust.
+//!
+//! In order to not break Wasmtime's build, this library will always compile. It
+//! does depend on certain tools (see `README.md`) that may or may not be
+//! available in the environment:
+//!  - when the tools are available, we build and link to an OCaml static
+//!    library (see `with_library` module)
+//!  - when the tools are not available, this library will panic at runtime (see
+//!    `without_library` module).
+
+/// Enumerate the kinds of Wasm values.
+#[derive(Clone, Debug, PartialEq)]
+pub enum Value {
+    I32(i32),
+    I64(i64),
+    F32(i32),
+    F64(i64),
+}
+
+#[cfg(feature = "has-libinterpret")]
+mod with_library;
+#[cfg(feature = "has-libinterpret")]
+pub use with_library::*;
+
+#[cfg(not(feature = "has-libinterpret"))]
+mod without_library;
+#[cfg(not(feature = "has-libinterpret"))]
+pub use without_library::*;
+
+// If the user is fuzzing`, we expect the OCaml library to have been built.
+#[cfg(all(fuzzing, not(feature = "has-libinterpret")))]
+compile_error!("The OCaml library was not built.");

crates/fuzzing/wasm-spec-interpreter/src/with_library.rs

@@ -0,0 +1,98 @@
+//! Interpret WebAssembly modules using the OCaml spec interpreter.
+//! ```
+//! # use wasm_spec_interpreter::{Value, interpret};
+//! let module = wat::parse_file("tests/add.wat").unwrap();
+//! let parameters = vec![Value::I32(42), Value::I32(1)];
+//! let results = interpret(&module, parameters).unwrap();
+//! assert_eq!(results, &[Value::I32(43)]);
+//! ```
+use crate::Value;
+use lazy_static::lazy_static;
+use ocaml_interop::{OCamlRuntime, ToOCaml};
+use std::sync::Mutex;
+
+lazy_static! {
+    static ref INTERPRET: Mutex<()> = Mutex::new(());
+}
+
+/// Interpret the first function in the passed WebAssembly module (in Wasm form,
+/// currently, not WAT) with the given parameters.
+pub fn interpret(module: &[u8], parameters: Vec<Value>) -> Result<Vec<Value>, String> {
+    // The OCaml runtime is not re-entrant
+    // (https://ocaml.org/manual/intfc.html#ss:parallel-execution-long-running-c-code).
+    // We need  to make sure that only one Rust thread is executing at a time
+    // (using this lock) or we can observe `SIGSEGV` failures while running
+    // `cargo test`.
+    let _lock = INTERPRET.lock().unwrap();
+    // Here we use an unsafe approach to initializing the `OCamlRuntime` based
+    // on the discussion in https://github.com/tezedge/ocaml-interop/issues/35.
+    // This was the recommendation to resolve seeing errors like `boxroot is not
+    // setup` followed by a `SIGSEGV`; this is similar to the testing approach
+    // in
+    // https://github.com/tezedge/ocaml-interop/blob/master/testing/rust-caller/src/lib.rs
+    // and is only as safe as the OCaml code running underneath.
+    OCamlRuntime::init_persistent();
+    let ocaml_runtime = unsafe { OCamlRuntime::recover_handle() };
+    // Parse and execute, returning results converted to Rust.
+    let module = module.to_boxroot(ocaml_runtime);
+    let parameters = parameters.to_boxroot(ocaml_runtime);
+    let results = ocaml_bindings::interpret(ocaml_runtime, &module, &parameters);
+    results.to_rust(ocaml_runtime)
+}
+
+// Here we declare which functions we will use from the OCaml library. See
+// https://docs.rs/ocaml-interop/0.8.4/ocaml_interop/index.html#example.
+mod ocaml_bindings {
+    use super::*;
+    use ocaml_interop::{
+        impl_conv_ocaml_variant, ocaml, OCamlBytes, OCamlInt32, OCamlInt64, OCamlList,
+    };
+
+    // Using this macro converts the enum both ways: Rust to OCaml and OCaml to
+    // Rust. See
+    // https://docs.rs/ocaml-interop/0.8.4/ocaml_interop/macro.impl_conv_ocaml_variant.html.
+    impl_conv_ocaml_variant! {
+        Value {
+            Value::I32(i: OCamlInt32),
+            Value::I64(i: OCamlInt64),
+            Value::F32(i: OCamlInt32),
+            Value::F64(i: OCamlInt64),
+        }
+    }
+
+    // These functions must be exposed from OCaml with:
+    //   `Callback.register "interpret" interpret`
+    //
+    // In Rust, this function becomes:
+    //   `pub fn interpret(_: &mut OCamlRuntime, ...: OCamlRef<...>) -> BoxRoot<...>;`
+    ocaml! {
+        pub fn interpret(module: OCamlBytes, params: OCamlList<Value>) -> Result<OCamlList<Value>, String>;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn multiple() {
+        let module = wat::parse_file("tests/add.wat").unwrap();
+        let parameters = vec![Value::I32(42), Value::I32(1)];
+        let results1 = interpret(&module, parameters.clone()).unwrap();
+        let results2 = interpret(&module, parameters.clone()).unwrap();
+        assert_eq!(results1, results2);
+        let results3 = interpret(&module, parameters).unwrap();
+        assert_eq!(results2, results3);
+    }
+
+    #[test]
+    fn oob() {
+        let module = wat::parse_file("tests/oob.wat").unwrap();
+        let parameters = vec![];
+        let results = interpret(&module, parameters);
+        assert_eq!(
+            results,
+            Err("Error(_, \"out of bounds memory access\")".to_string())
+        );
+    }
+}

crates/fuzzing/wasm-spec-interpreter/src/without_library.rs

@@ -0,0 +1,17 @@
+//! Panic when interpreting WebAssembly modules; see the rationale for this in
+//! `lib.rs`.
+//!
+//! ```should_panic
+//! # use wasm_spec_interpreter::interpret;
+//! let _ = interpret(&[], vec![]);
+//! ```
+
+use crate::Value;
+
+#[allow(dead_code)]
+pub fn interpret(_module: &[u8], _parameters: Vec<Value>) -> Result<Vec<Value>, String> {
+    panic!(
+        "wasm-spec-interpreter was built without its Rust-to-OCaml shim \
+        library; re-compile with the dependencies listed in its README.md."
+    );
+}