Re: [Tinycc-devel] Zeroing stack variables CValue

[Michael Matz]

Hi,

On Sat, 29 Mar 2014, Domingo Alvarez Duarte wrote:

> How do you propose to solve this specific problem ?
>
> ----
> ST_FUNC void vset(TCCState* tcc_state, CType *type, int r, int v)
> {
>     CValue cval;
>     memset(&cval, 0, sizeof(CValue));
>
>     cval.i = v; //,<<<<<<<<<<< here is the main bug that mix with garbage
>     vsetc(tcc_state, type, r, &cval);
> }
> ----
>
> ----
> /* store a value or an expression directly in global data or in local array
>  */
> static void init_putv(TCCState* tcc_state, CType *type, Section *sec, unsig
> ned long c,
>                       int v, int expr_type)
> {
> ...
>         case VT_PTR:
>             if (tcc_state->tccgen_vtop->r & VT_SYM) {
>                 greloc(tcc_state, sec, tcc_state->tccgen_vtop->sym, c, R_DA
> TA_PTR);
>             }
>
> //<<< on the next line is where we try to get the assigned value to cvalue.
> i as cvalue.ull
>             *(addr_t *)ptr |= (tcc_state->tccgen_vtop->c.ull & bit_mask) <<
>  bit_pos;
>             break;
> ----

There is no specific problem with the above snippets.  It's simply two 
unrelated functions, one setting cval1.i and another accessing cval2.ull. 
There is only a problem when the cval set by vset is used in init_putv in 
the VT_PTR case.  _That_ would be a problem, but from the above two 
snippets it's not clear that this happens.  That's why I asked for a 
testcase, we need to know _which_ vset call (only setting .i) leaks into 
init_putv.  E.g. such code would be okay:

vset (type, 0, 42);
vtop->c.ull = 42;
...
init_putv called accessing above vtop.


Ciao,
Michael.