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Old 08-26-2006, 05:58 AM   #1
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Registered: Sep 2004
Location: Sweden
Distribution: Ubuntu, Debian
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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.

/* 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);
[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?
Old 08-26-2006, 10:59 PM   #2
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Location: Manalapan, NJ
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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

Last edited by macemoneta; 08-26-2006 at 11:07 PM.
Old 08-27-2006, 05:49 AM   #3
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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).


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