Re: [Tinycc-devel] Zeroing stack variables CValue

[Michael Matz]

Hi,

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

> Ok now I understand your point, here is the minimal program that I was using
> to trace it:
> ----
> int main() {
> int x;
> static void *label_return = &&lbl_return;
> //printf("label_return = %p\n", label_return);
> goto *label_return; //<<<<< here segfault on linux X86_64 without the memset
> on vset 
> //printf("unreachable\n");
> lbl_return:
> return 0;
> }
> ----

Thanks.  While it doesn't crash for me on x86-64 (with rev aa561d70, i.e. 
before your memset patch) I do see the wrong vset flowing into init_putv; 
it's unary(), case TOK_LAND, which does

        vset(&s->type, VT_CONST | VT_SYM, 0);
        vtop->sym = s;
        next();
        break;

Where s->type will be VT_PTR.  Indeed vset as it is right now cannot be 
used to initialize values of such type.  One could extend vset (together 
with vpush64 the only routine that accepts an arbitrary type but sets a 
specific CValue member) to check for the type and initialize the correct 
member.  Possibly it's better to try to get rid of as many explicit vset 
calls as possible (many of the current ones actually don't need the 
immediate value, as it's always zero, and those others that actually pass 
some offset or location seem mostly dubious in that they might incorrectly 
truncate the value while passing it to vset).  Needs some pondering ...


Ciao,
Michael.