Re: Non-static variables and nested function pointers [bug #28392]

["Grégoire Sutre" <gregoire.sutre@labri.fr>]

Seth Goldberg wrote:

>  Exactly -- the presence of the execstack attribute in the segment is 
> merely a request -- the kernel is free to discard it, and many OSes do, 
> as you've found :).

The problem is more complex: I tried a simple example with a pointer to 
a nested function, and it runs without any segfault on NetBSD/i386.
This would suggest that, by default, the stack is executable -- at least 
if trampolines are used?

The same example segfaults on Debian/amd64 if compiled with 
-Wl,-z,noexecstack (and does not segfault otherwise).

After some digging, I found threads in the archives of the mailing list 
on the problem of executable stack on NetBSD. This led to a patch that 
is now part of trunk if I'm not mistaken.

http://lists.gnu.org/archive/html/grub-devel/2008-02/msg00095.html

I will try to test on NetBSD/amd64 and report here (in a few days), as 
NetBSD's support for non-executable stack and heap seems better on amd64.

Grégoire


#include <stdio.h>

int apply(void (*hook) (int *))
{
    int a = 0;
    hook(&a);
    hook(&a);
    return a;
}

int main (int argc, char *argv[])
{
    int j = 5;
    int res;

    void hook(int *n)
    {
       *n = *n + j;
       j--;
    }

    res = apply(hook);
    printf("result: %d, j=%d\n", res, j);
    return 0;
}