Re: Bug in lua 5.2.1?

[Roberto Ierusalimschy <roberto@...>]

> I think that I have found a bug in lua_insert(), in Lua 5.2.1.
> 
> lua_insert() does the following loop:
>   for (q = L->top; q>p; q--) setobjs2s(L, q, q-1);
> (first iteration writes value into L->top)
> 
> But according to index2addr():
>   if (o >= L->top) return NONVALIDVALUE;
> (= means that L->top should not be accessed).
> 
> The problem is actually that my application crashes sometimes inside
> lua_insert(). I think it happens when lua_insert() is called on a "full"
> stack (i.e. when it tries to access L->top which points outside of the
> memory block). If I add lua_checkstack() before lua_insert(), the problem
> disappears.

I am afraid your stack is more than full: it is already overflowed by
a few entries before you call lua_insert. If you grep for EXTRA_STACK in
the Lua source code, you will see that Lua always keeps some extra space
after the "end" of the stack, for some internal uses. For instance,
lua_insert uses L->top as a temporary, and it is shure that there is
such a slot (because of EXTRA_STACK).

-- Roberto