Pseudo-random stack pointer under Linux?

Question

I played with some code when I noticed something strange:

[~] main% cat test.cc #include <stdio.h> void f() { int i; fprintf(stderr, "&i = 0x%08X\n", (long)&i); } int main(int argc, char**argv) { f(); } [~] main% g++ test.cc [~] main% ./a.out &i = 0xBFA27AB4 [~] main% ./a.out &i = 0xBFAD7E24 [~] main% ./a.out &i = 0xBFCA3464 [~] main% ./a.out &i = 0xBF96C064 [~] main% 

The odd thing for me is a change in the address of variable i.

I assume that the kernel supplies different starting addresses of the stack in order to try to break some kind of crack. What is the real reason?

Answer 1

randomization of the location of the address space is used precisely for several operating systems for this very reason. Perhaps your variations in stack pointer addresses are caused by this - most likely, this will be the case in recent versions of Linux and / or BSD. Other IIRC versions also do this.