Sandboxing Embedded Lua in 5.2 / Set Envirenment for Functions from lua.file

Question

Lets say i have at least two lua script files.

test1.lua test2.lua

both define an init function and other functions with similar names.

How can i load each script file using c++/c into a separate environment using Lua 5.2 so that the same function names will not clash - i found a sample code for 5.1 which does not work for me (because setenv is gone and lua_setuservalue does not seem to work)

Sample here Calling lua functions from .lua's using handles?

Basically if i replace setenv with setuservalue - i get an access violation.

• Rook: This is one of the reasons why I've stuck with lua 5.1 in my own projects for now. I believe that the v5.2 load and loadfile functions let you specify an environment; they are probably the best place to start searching for a solution.
• Steve: thanks but i read about load and loadfile however i could not find a solution
• Rook: setuservalue is definitely not the correct function to use when trying to manipulate the environment. The lua docs seem somewhat unclear on what you should be doing, however. lua_load says that a single upvalue associated with a loaded chunk is set as its environment, but does not mention how you go about associating an upvalue with the chunk from C. If you were loading the chunk from within lua, the lua version of the load function looks like it should set the environment correctly.
• Steve: thanks - i looked into the upvalues - and found this post on the list lua-list.2524044.n2.nabble.com/… basically it is said you should call if (luaL_loadfile(L, filename)) return lua_error(L); lua_getglobal(L, "environ); lua_setupvalue(L, -2, 1); lua_call(L, 0, 0); however - when i do this i get the error "PANIC: unprotected error in call to Lua API (test1.lua:1: attempt to index upvaue '_ENV' (a nil value))"
• Rook: I'd be suspicious of people suggesting you call lua_setupvalue. It is in the docs as part of the debug API, and as such you should not need to touch it for normal use of lua.

Answer 1

The unofficial Lua FAQ has an entry about sandboxing in Lua. My guess is that you can transpose that logic easily enough to your C/C++ code.

See also LuaFiveTo on the lua-users wiki.

Correction

It's indeed not as trivial as it seemed. But in the end the point is simple: load your chunk, push the _ENV table, use lua_setupvalue(L,-2,1). The important is that the table should be at the top of the stack.

As a small example, using 2 environments defaulting to _G for reading stuff via metatables:

#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
int main(void){
 lua_State *L = luaL_newstate();
 char *file1 = "file1.lua";
 char *file2 = "file2.lua";
 luaL_openlibs(L);
 luaL_loadfile(L,file2); // S: 1
 luaL_loadfile(L,file1); // S: 2
 lua_newtable(L); // ENV for file 1: S: 321
 lua_newtable(L); // ENV for file 2: S: 4321
 //lets have each function have its metatable, where missed lookups are
 //instead looked up in the global table _G
 lua_newtable(L); // metatable S: 54321
 lua_getglobal(L,"_G"); // pushes _G, which will be the __index metatable entry S: 654321
 lua_setfield(L,-2,"__index"); // metatable on top S: 54321
 lua_pushvalue(L,-1); // copy the metatable S: 554321
 lua_setmetatable(L,-3); // set the last copy for env2 S: 54321
 lua_setmetatable(L,-3); // set the original for env1 S: 4321
 // here we end up having 2 tables on the stack for 2 environments
 lua_setupvalue(L,1,1); // first upvalue == _ENV so set it. S: 321
 lua_setupvalue(L,2,1); // set _ENV for file S: 21
 // Remaining on the stack: 2 chunks with env set.
 lua_pcall(L,0,LUA_MULTRET,0);
 lua_pcall(L,0,LUA_MULTRET,0);
 lua_close(L);
 return 0;
}

And for the 2 Lua files:

-- file1.lua
function init()
 A="foo"
 print("Hello from file1")
 print(A)
end
init()
-- file2.lua
-- this shows that stuff defined in file1 will not polute the environment for file2
print("init function is",tostring(init))
function init()
 A="bar"
 print("Hello from file2")
 print(A)
end
init()

• Steve: thanks but i know these resources - however im unable to reproduce this in C/C++
• Rook: Indeed, the problem is that the C equivalents of load and loadfile do not have quite such a straightfoward interface or documentation. The lua-users wiki page also appears to be a little out of data, and contains features that never made it into the 5.2 spec.
• Steve: jpjacobs, we met in lua-irc today and as i said i got the solution but thanks again for laying out an alternative example!

Answer 2

both define an init function and other functions with similar names.

First of all, why are those functions global? They should be local to the script. If you're going to require them in other files, they should create and return a table containing the functions that they wish to expose.

The modern idiom when requiring these files is to do something like this:

local Library = require 'library'
Library.Func1(...)

Thus, you do not pollute the global Lua namespace. You use local variables.

However, if you insist on using globals like this, you can do exactly what the documentation said: change the first upvalue of the compiled chunk.

Basically if i replace setenv with setuservalue - i get an access violation.

Of course you do. That's not what lua_setuservalue does. It's for setting values associated with userdata. What you want is appropriately called lua_setupvalue.

Using the sample code you cite, the correct answer would be:

lua_setupvalue(L, -2, 1);

• Steve: i accidentally figured it out myself - lua_setupvalue(L, -2, 0); crashes! - i got away with using lua_setupvalue(L, -2, 1); - my knowledge is limited and i do exactly know what 1 is in this context. but it seems to work - now is it also possible to expose my object only in this "namespace" and not in the global table? im using luabind to expose my classes and currently im using luabind::globals(myLuaState)["myObj"] = myObj
• Steve: as for "why" are the functions global - i like to create an event system with scripts for multiple entities which use the same function signature e.g. init, since the scripts are generally not created by developers i want to make it as painless as possible and not bother anyone with libaries or modules - therefor i want to make sure each script is executed in its own environment - its not sourced anyway!
• Nicol Bolas: @Steve: The documentation isn't clear if the lua_setupvalue function takes zero-based or one-based indices. It looks like they're one-based.
• Steve: so basically - im not even sure what that parameter means - is it always 1 ? - and furthermore to you happen to know how to expose my c++ obj only for this environment?
• Nicol Bolas: @Steve: Functions of any type can have 0 or more upvalues. Lua functions (compiled Lua code) will have at least one upvalue. The first upvalue, index 1, is the environment for that function (which is why every Lua function has at least one). As to the latter, that's up to you. Putting it into the environment table is one way to do it.