5 replies to this topic

[tenshae]

I've seen a few examples of this that are either dated or just completely untested and absolutely none that I've seen even work a bit (at least with this version of CC). I was wondering, and yes I have searched on google, how you would do read-only tables in Lua.
I want to guard the OS and FS apis from userspace code, so that they can't modify it and break some things/break out of the sandbox. Could anyone help?

[KingofGamesYami]

You'll want to look at metatables for this. Here's a tutorial on how to use metatables.

[natedogith1]

function readOnly(tbl)
  return setmetatable({}, __index = tbl, __newindex =
	function(self,key,value)
	  error("not modifyable")
	end,__metatable={})
end

'__index' defines where/how to look up values
'__newindex' defines where/how to set values
'__metatable' prevents me from using getmetatable and modifying it that way
if you're interested in more about metatables, you might like to look at http://www.lua.org/m...manual.html#2.8

There are a few issues with this though, like the fact that 'pairs' doesn't work, though it does do a better job of read-only than the CC's OS

[tenshae]

natedogith1, on 16 September 2014 - 03:22 AM, said:

--snippeth--

I guess I should try to override setmetatable so that people can't "unreadonly-ize" the tables, right?
Is there anything else I should watch out for?

[natedogith1]

ashnwill, on 16 September 2014 - 03:34 AM, said:

natedogith1, on 16 September 2014 - 03:22 AM, said:

--snippeth--

I guess I should try to override setmetatable so that people can't "unreadonly-ize" the tables, right?
Is there anything else I should watch out for?

Actually, since the metatable has a '__metatable' field, setmetatable will throw an error; however you should maybe watch out for 'rawset'.

[TsarN]

If you want to also block rawset here the code to put into "startup" (I'm using this in UberOS):

local absoluteReadOnly = {}
function applyreadonly(table)
  local tmp = {}
  setmetatable(tmp, {
	__index = table,
	__newindex = function(table, key, value)
	  error("Attempt to modify read-only table")
	end,
	__metatable = false
  })
  absoluteReadOnly[#absoluteReadOnly + 1] = tmp
  return tmp
end
local oldrawset = rawset
rawset = function(table, index, value)
  for i = 1, #absoluteReadOnly do
	if (table == absoluteReadOnly[i]) or (index == absoluteReadOnly[i]) then
	  error("Attempt to modify read-only table")
	  return
	end
  end
  oldrawset(table, index, value)
end

Then, to make table "a" read-only, do the following:

a = applyreadonly(a)

Edited by TsarN, 26 January 2015 - 05:35 AM.