Question

I’ve seen it said multiple times that there is no way to limit a Lua script’s memory usage, including people jumping through hoops to prevent Lua scripts from creating functions and tables. But given that lua_newstate allows you to pass a custom allocator, couldn’t one just use that to limit memory consumption? At worst, one could use an arena-based allocator and put a hard limit even on the amount of memory that could be used by fragmentation.

Am I missing something here?

Answer 1

 static void *l_alloc_restricted (void *ud, void *ptr, size_t osize, size_t nsize)
 {
   const int MAX_SIZE = 1024; /* set limit here */
   int *used = (int *)ud;

   if(ptr == NULL) {
     /*
      * <http://www.lua.org/manual/5.2/manual.html#lua_Alloc>:
      * When ptr is NULL, osize encodes the kind of object that Lua is
      * allocating.
      *
      * Since we don’t care about that, just mark it as 0.
      */
     osize = 0;
   }

   if (nsize == 0)
   {
     free(ptr);
     *used -= osize; /* substract old size from used memory */
     return NULL;
   }
   else
   {
     if (*used + (nsize - osize) > MAX_SIZE) /* too much memory in use */
       return NULL;
     ptr = realloc(ptr, nsize);
     if (ptr) /* reallocation successful? */
       *used += (nsize - osize);
     return ptr;
   }
 }

To make Lua use your allocator, you can use

 int *ud = malloc(sizeof(int)); *ud = 0;
 lua_State *L = lua_State *lua_newstate (l_alloc_restricted, ud);

Note: I haven’t tested the source, but it should work.