Use Rhai in Dynamic Libraries

Sometimes functions registered into a Rhai Engine come from dynamic libraries (a.k.a. shared libraries in Linux or DLL’s in Windows), which are compiled separately from the main binary, not statically linked but loaded dynamically at runtime.

rhai-dylib

The project rhai-dylib demonstrates an API for creating dynamically-loadable libraries for use with a Rhai Engine.

Problem Symptom

The symptom is usually Function Not Found errors even though the relevant functions (within the dynamic library) have already been registered into the Engine.

This usually happens when a mutable reference to the Engine is passed into an entry-point function exposed by the dynamic library and the Engine’s function registration API is called inside the dynamic library.

Problem Cause

To counter DOS attacks, the hasher used by Rhai, ahash, automatically generates a different seed for hashing during each compilation and execution run.

This means that hash values generated by the hasher will not be stable – they change during each compile, and during each run.

This creates hash mismatches between the main binary and the loaded dynamic library because, as they are not linked together at compile time, two independent copies of the hasher reside in them, resulting in different hashes for even the same function signature.

Solution

Use static hashing for force predictable hashes.

Warning: Safety considerations

Static hashing allows dynamic libraries with Rhai code to be loaded and used with an Engine in the main binary.

However, a fixed seed enlarges the attack surface of Rhai to malicious intent (e.g. DOS attacks).