Computicles – Unsafe, but fun, code injection

Computicles are capable of receiving bits of code to execute. That is in fact one way in which they communicate with each. From .
one context I can simple do: comp.someValue = 100, and that will set “someValue = 100” in another context. Well, great. How about sending something across that’s more substantial than a simple variable setting?

Well, with the Computicle.exec() method, which the previous example is based on, you can really send across any bit of code to be executed. It works, and you can even send across the body of a function like this:

function hello()
  print("Hello, World!");

That works fine, and you have full access to all stuff that’s running in that context. It’s a bit clunky though because the entirety of your piece of code is wrapped up in a string value. This is very similar to executing SQL code on some server. The bulk of your code, if not in stored procedures”, ends up in these opaque strings. Hard to catch errors, hard to debug, etc. How about the following:

comp.hello = function()
  print("Hello, World!");

What that you say? Basically, assigning a function as a variable to the computicle. Yes, this can work. There is a slight modification to turn a function into a string, and it looks like this:

Computicle.datumToString = function(self, data, name)
	local dtype = type(data);
	local datastr = tostring(nil);

--print("DATUM TYPE: ", name, dtype);

	if type(data) == "cdata" then
		-- If it is a cdata type that easily converts to 
		-- a number, then convert to a number and assign to string
		if tonumber(data) then
			datastr = tostring(tonumber(data));
			-- if not easily converted to number, then just assign the pointer
			datastr = string.format("TINNThread:StringToPointer(%s);", 
	elseif dtype == "table" then
		if getmetatable(data) == Computicle_mt then
			-- package up a computicle
			-- get a json string representation of the table
			datastr = string.format("[[ %s ]]",JSON.encode(data, {indent=true}));

			--print("=== JSON ===");
	elseif dtype == "function" then
		datastr = "loadstring([["..string.dump(data).."]])";
	elseif dtype == "string" then
		datastr = string.format("[[%s]]", data);
		datastr = tostring(data);

	return datastr;

Notice the part that starts: ‘elseif dtype == “function()” then’.

Basically, there is a string function that turns anything into bytecode which can later be hydrated using loadstring later. It is placed in ‘loadstring([[…]])’, because this is the only way to preserve the actual 8-bit values. If you use string.format(“%s”), it will chop out the non-ascii stuff.

Then, this code can be executed in the remote context just the same as any other little bit of code. There are some restrictions to what kinds of functions you can do this with though. No ‘upvalues’, meaning, everything must be contained within the function itself, no calling out to global variables and the like.

Of course, that just gets the code over to the other side in a convenient way. What about actually executing the code?


Same as ever, just make the function call using “exec()”. Of course, this little bit could be wrapped up in some other convenient override, like using the ‘:’ notation, but there’s some more work to be done before making that a reality.

What other functions can be dealt with? Well, the loop running within the computicle looks like the following now:

-- comp_msgpump.lua

local ffi = require("ffi");

-- This is a basic message pump

-- default to 15 millisecond timeout
gIdleTimeout = gIdleTimeout or 15

local idlecount = 0;

while true do
  local msg, err = SELFICLE:getMessage(gIdleTimeout);
  -- false, WAIT_TIMEOUT == timed out
  --print("MSG: ", msg, err);

  if not msg then
    if err == WAIT_TIMEOUT then
      --print("about to idle")
      idlecount = idlecount + 1;
      if OnIdle then
  	local msgFullyHandled = false;

    if OnMessage then
      msgFullyHandled = OnMessage(msg);

    if not msgFullyHandled then
      msg = ffi.cast("ComputicleMsg *", msg);
      local Message = msg.Message;
      --print("Message: ", Message, msg.Param1, msg.Param2);
      if Message == Computicle.Messages.QUIT then

      if Message == Computicle.Messages.CODE then
        local len = msg.Param2;
        local codePtr = ffi.cast("const char *", msg.Param1);
        if codePtr ~= nil and len > 0 then
          local code = ffi.string(codePtr, len);

          SELFICLE:freeData(ffi.cast("void *",codePtr));

          local func = loadstring(code);

This new message pump has a couple of interesting new features. The first feature has to do with the getMessage(). It will timeout after some number of milliseconds have passed and a message hasn’t come in. If there is an ‘OnIdle()’ function defined, it will be called, passing that function the current count. If that function does not exist, then nothing will happen.

The second change has to do with message processing. If there is a “OnMessage()” function defined, it will be called, and the message will be handed to it for processing. If that function does not exist, then it will perform some default actions, such as reading and code, and possibly handling a “QUIT”.

So, how about that OnIdle? that’s a function that takes a parameter. Can I inject that? Sure can:

comp.OnIdle = function(count)
  print("IDLE", count)

Just the the hello() case, except this one can take a parameter. This does not violate the “no upvalues” rule, so it’s just fine.

In this case, I don’t have to actually execute the code, because the loop within the computicle will pick up on it and execute it. And if you wanted to remove this idling funcition, simple do: ‘comp.OnIdle = nil’

And then what?

Well, that’s pretty cool. Now I can easily set simple variables, and I can set table values, and I can even set functions to be called. I have a generic message pumping loop, which has embedded “OnIdle”, and “OnMessage” functions.

The last little bit of work to be done here is to deal with Asynchronous IO in a relatively seamless way using continuations, just like I had in the pre-computicle world. That’s basically a marriage of the core of the EventScheduler and the main even loop here.

Throw some authentication on top of it all and suddenly you have a system that is very cool, secure, and useful.


One Comment on “Computicles – Unsafe, but fun, code injection”

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