A host function is a Java function provided to a Wasm module to fulfill one of its imports. Because host functions run in the JVM, they are unrestricted and can perform any operation (like file I/O or network calls), effectively allowing the Wasm module to "escape the sandbox."
Security Warning: Host functions act as a security boundary. If the Wasm code is untrusted, implement host functions carefully to prevent unauthorized access to the host environment.
To define a HostFunction, you must provide:
- Namespace and Name: The module's import identifier (e.g.,
console and log). - Function Type: The Wasm type signature (argument types and return types).
- Implementation: A lambda or function that executes when the Wasm module calls the import. This implementation receives an
Instance object, which provides access to the Wasm module's linear memory.
import com.dylibso.chicory.runtime.Instance;
import com.dylibso.chicory.runtime.HostFunction;
import com.dylibso.chicory.wasm.types.ValType;
import com.dylibso.chicory.wasm.types.FunctionType;
import java.util.List;
var func = new HostFunction(
"console",
"log",
FunctionType.of(
List.of(ValType.I32, ValType.I32), // Arguments: length and offset
List.of() // Return type: void
),
(Instance instance, long... args) -> {
var len = (int) args[0];
var offset = (int) args[1];
// Use instance.memory() to read data from the Wasm module's linear memory
var message = instance.memory().readString(offset, len);
System.out.println(message);
return null;
});
