LinuxQuestions.org - Variable at random location in memory?

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Variable at random location in memory?

IIRC global variables like PATH used to exist at a certain memory address. But now it seems you never know where it is. I made a program before that printed out the address of a variable, and when I use it now it keeps showing up seemingly random addresses.

Code:

/* getenvaddr.c */

#include <stdlib.h>

#include <stdio.h>



int main(int argc, char *argv[]) {

        char *addr;

        addr = getenv(argv[1]);

        printf("Addr: %p\n", addr);

}

Code:

[root@garanka laboratory] # ./getenvaddr PATH

Addr: 0xaf9dfe9b

[root@garanka laboratory] # ./getenvaddr PATH

Addr: 0xafe83e9b

[root@garanka laboratory] # ./getenvaddr PATH

Addr: 0xaf88be9b

What has happened and why?

This may be part of the Execshield implementation:

cat /proc/sys/kernel/exec-shield-randomize

0 — Randomized VM mapping is disabled
1 — Randomized VM mapping is enabled

Update: on later kernels, the value may be in:

cat /proc/sys/kernel/randomize_va_space

Yeah I've read a little bit and it seems that we now have ASLR in Linux. On an interesting note this will also come to Vista (in a lesser secure way it seems, though).