Is this a bug?

[xiayong <xiayong@...>]

Hello everyone!

         I use lua some years, thank you all for developing lua language!

        Some days ago, our program crash. I found the crash in lua code. So I check lua code, found the stack overflow.

Please look this code In function luaD_precall:

 

1  if (!cl->isC) {  /* Lua function? prepare its call */

   2       CallInfo *ci;

   3       StkId st, base;

   4       Proto *p = cl->p;

   5       luaD_checkstack(L, p->maxstacksize);

   6       func = restorestack(L, funcr);

   7       if (!p->is_vararg) {  /* no varargs? */

   8              base = func + 1;

   9              if (L->top > base + p->numparams)

   10             L->top = base + p->numparams;

   11      }

   12      else {  /* vararg function */

   13             int nargs = cast_int(L->top - func) - 1;

   14             base = adjust_varargs(L, p, nargs);

   15             func = restorestack(L, funcr);  /* previous call may change the stack */

   16      }

   17      ci = inc_ci(L);  /* now `enter' new function */

   18      ci->func = func;

   19      L->base = ci->base = base;

   20      ci->top = L->base + p->maxstacksize;

   21      lua_assert(ci->top <= L->stack_last);

   22      L->savedpc = p->code;  /* starting point */

   23      ci->tailcalls = 0;

   24      ci->nresults = nresults;

   25      for (st = L->top; st < ci->top; st++)

   26             setnilvalue(st);

   27      L->top = ci->top;

   

In my program, the p->maxstacksize is 79 before line 5, the current stacksize is 51, after call  luaD_checkstack,the stacksize grow to 130.

The lua function use vararg, so will run to line 15. Function restorestack will be called.

 

static StkId adjust_varargs (lua_State *L, Proto *p, int actual) {

        int i;

        int nfixargs = p->numparams;

       Table *htab = NULL;

       StkId base, fixed;

       for (; actual < nfixargs; ++actual)

           setnilvalue(L->top++);

#if defined(LUA_COMPAT_VARARG)

       if (p->is_vararg & VARARG_NEEDSARG) { /* compat. with old-style vararg? */

           int nvar = actual - nfixargs;  /* number of extra arguments */

           lua_assert(p->is_vararg & VARARG_HASARG);

           luaC_checkGC(L);

       htab = luaH_new(L, nvar, 1);  /* create `arg' table */

        

In function adjust_varargs(), the lua function use “arg”, So luaC_checkGC will be called.

In luaC_checkGC, the current lua stack size will be reduce to 65!

The call stack like this:

    luaC_step()-à singlestep()-àpropagatemark()àtraversestack()àcheckstacksizes()àluaD_reallocstack()

 

But the p->maxstacksize is 79, the stacksize is not enough…

When the program run to line 27,the L->top is bigger than L->stack_last, in the next operation, will cause crash!

 

Thank you!

 

                                                                                                                                                                                                                                                  Xiayong

                                                                                                                                                                                                                                                  2010/5/7