Fiu

Pronounced like "Phew". This interpreter aims to provide a decently fast and reliable way of executing Luau bytecode without the use of loadstring.

Fiu does not taint the environment if you pass a table of functions or a wrapper around getfenv that does not taint Fiu's thread. Note that only an interpreter is provided, and compiled code must be obtained from an external source. While Fiu is in a working state, bugs and side effects can be encountered! Open an issue if you encounter any issues.

Usage

Fiu can be downloaded from the releases page and the experimental version from here.

• luau_load(module | bytecode, env, settings?)
  Accepts a luau module or bytecode. Returns the main prototype wrapped and a luau_close function to kill the interpreter if needed.
• luau_deserialize(bytecode, settings?)
  Used to deserialise bytecode.
• luau_newsettings()
  Used to create a table of default settings.
• luau_validatesettings(settings)
  Used to validate the passed settings.

Settings

vectorCtor - Vector constructor function
vectorSize - Vector size
useNativeNamecall - Boolean to indicate if namecallHandler should be used
namecallHandler - Function handler for native namecalling
extensions - Table of injectable globals callHooks - VM callback behaviour emulators
errorHandling - Error handling by the VM for line information and opcode information
generalizedIteration - Use generalized iteration in the VM

Vector constant support

Vector constants can be supported by setting vectorCtor and vectorSize. vectorSize can be 3 or 4, vectorCtor on Roblox would be Vector3.new.

Native namecall support

Namecalling is impossible to support dynamically so in environments where only __namecall is implemented without __index you can define a namecallHandler and set useNativeNamecall to true. In the namecall handler you must return a boolean as the first return value.

Example:

luau_settings.namecallHandler = function(namecallMethod, self, ...)
    if namecallMethod == "FindFirstChild" then
        return true, self:FindFirstChild(...)
    end 

    return false
end

Stack frame, running thread and errors

The interpreter can be accurate 1:1 to existing stack frames if errorHandling is disabled.

Generalized iteration can also cause issues because it requires a new thread to be created so anything that gets iterated will go deeper in the C stack depth limit and __iter and __call will have a different thread when coroutine.running is called. If generialized iteration is disabled with generializedIteration __call will still function but __iter and iterating over tables without an iterator will no longer work.

Fiu cannot provide perfect errors, if errorHandling is enabled all errors are processed by Fiu and will be turned into strings if they are not. If you want to be able to error a table and then grab the table enable allowProxyErrors which will error the errored object again. Note that when errorHandling is disabled this behaviour is done by default because Fiu will not interfere with errors.

Injectable globals

The vm allows globals to be injected into the VM with extensions, these injected globals can never be deleted by the interpreter and any attempts will just result in environment globals being set to nil.

Example:

luau_settings.extensions["foo"] = function()
    print('hello world!')
end

VM callback behaviour

The Luau VM provides simple hooks to see into the VM at certain points, the Fiu VM emulates 4 of these callbacks.

breakHook(stack, debugging, proto, module, upvals) - When a LOP_BREAK/breakpoint is encountered breakHook will be called
interruptHook(stack, debugging, proto, module, upvals) - When a vm interrupt occurs the interruptHook will be called
panicHook(message, stack, debugging, proto, module, upvals) - When an unprotected error occurs within the VM the panicHook will be called if errorHandling is enabled
stepHook(stack, debugging, proto, module, upvals) - When a VM step (In Fiu, when the PC increases) occurs stepHook will be called

Example:

settings.callHooks.stepHook = function(stack, debugging)
	print('step occured', debugging.name, debugging.pc)	
end
settings.callHooks.panicHook = function(message, stack, debugging)
	print(debugging.name, message)
	for i,v in stack do 
		print(i,v)
	end
end
settings.callHooks.interruptHook = function(stack, debugging)
	print(debugging.name, "interrupted!")
end

Contributing

• Testing using GitHub Actions
  • Fork this repository, and commit your changes.
  • Commiting changes should automatically run tests with GitHub Actions.
  • Results would show for the commit.
• Testing locally(optional)
  • fiu-tests must be compiled using CMake, this is needed to run and debug tests.
    • Setup cmake tree with cmake ./tests or cmake -DCMAKE_BUILD_TYPE=Release ./tests for release tree.
    • Compile cmake tree with cmake --build . --target=fiu-tests.
  • Tests must be ran with fiu-tests executable compiled from cmake.
    • For specific test use -t <file path> or test the whole directory with -tf <directory path>, the paths are relative to (workspaceRoot)/tests/.

Luau updates often and Fiu will need to be updated and have working releases added for every new bytecode version.

Contributors

TheGreatSageEqualToHeaven
Rerumu
Green
SnorlaxAssist
Someon1e
arcturus-prime
SnowyShiro