How to disable address randomization ?

[Chan Kim]

Hi,

To print kernel virtual address, you should use %px instead of %p in the printk.

Probably that’s why you couldn’t see the pointer values correctly.

Chan

 

From: admin LI <admin at hifiphile.com> 
Sent: Friday, January 14, 2022 6:02 AM
To: kernelnewbies at kernelnewbies.org
Subject: How to disable address randomization ?

 

Hi,

I'm developing a kernel module for an ARM machine, while debugging I found addresses 
printed are all randomized and useless for debugging.

To prove I was not crazy I wrote this small program:

---------------------------------
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Somebody");
MODULE_DESCRIPTION("A simple example Linux module.");
MODULE_VERSION("0.01");

static int __init example_init(void) {
    uint32_t a;
    uint32_t b;
    uint32_t c;
    uint8_t d[10];
    uint8_t *e;

    printk(KERN_INFO "Hello, World!\n");
    printk(KERN_INFO "&a %p\n",&a);
    printk(KERN_INFO "&b %p\n",&b);
    printk(KERN_INFO "&c %p\n",&c);
    printk(KERN_INFO "&d %p\n",d);
    printk(KERN_INFO "&d[0] %p\n",&d[0]);
    printk(KERN_INFO "&d[1] %p\n",&d[1]);

    e = kmalloc(10, GFP_KERNEL);
    printk(KERN_INFO "&e[0] %p\n",&e[0]);
    printk(KERN_INFO "&e[1] %p\n",&e[1]);

    kfree(e);

 return 0;
}

static void __exit example_exit(void) {
 printk(KERN_INFO "Goodbye, World!\n");
}

module_init(example_init);
module_exit(example_exit);
---------------------------------
And it gave me this output:

Hello, World!
&a b3f9fa31
&b 27e1c68a
&c da50d287
&d 9f9aec2b
&d[0] 9f9aec2b
&d[1] cc627580
&e[0] 98b8c9eb
&e[1] 45f248f8

Then I tested on my debian host machine which gave me the same kind of randomized addresses.

When I search randomization the only thing I found is KASLR which I don't think is the same thing.

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