Limiting a Lua script's memory usage? - lua

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.