Randomization of environment variable addresses

Question

I am using bash. I have switched off ASLR in Ubuntu 11.04 using

#sysctl -w kernel.randomize_va_space=0

And I have exported a variable from the shell using

$ export MYSHELL=/bin/sh

I wrote a C program to get the address of the MYSHELL :

void main(){
char* shell = getenv("MYSHELL");
if (shell)
    printf("0x%x\n", (unsigned int)shell);
}

It spat out 0xbffffe82 . When I used it as a part of my attack for ret-to-libc, the address changes (although by a very small offset). Why does this happen?

Also when I change the filename of the binary and use the previously successful address, it won't work, and it has been relocated to a different address. Why? In other words, What is the relation of binary names and environment variable addresses? Is this a protection feature by bash? How do I switch this off?

Note: this is not homework.

Answer 1

Stack layout at program startup is documented here . It should be obvious why changing the name of the program (length really) changes the layout.