In this article, I will demonstrate how to edit a global variable of a user land process using GDB. A simple use case of this method that comes to mind is editing the value representing the number of licenses of a product.

\#include<stdio.h>  
\#include<stdlib.h>  
int y = 1000;  
f1(int c)  
{  
        int x;  
    x = 2\*c\*y;  
        printf("f1: double the value = %d\n", x);  
}  

main()  
{  
        int r;  
        while(1){  
            r = rand() % 10;  
            printf("main: rand value = %d\n",r);  
            sleep(1);  
            f1(r);  
        }  
}  

compile the code above using -g option of gcc

$ gdb -g testcode.c  

run the a.out

$ ./a.out  
main: rand value = 3  
f1: double the value = 6000  
main: rand value = 6  
f1: double the value = 12000  
main: rand value = 7  
f1: double the value = 14000  

We will now change the value of global variable y to 100. Once that value is changed, doubled values will be multiplied by 100

First, find out the pid of a.out
then call gdb with that executable and the pid

\# gdb a.out <pid>  

root@ubuntu-vbox-mc:/home/nishanth# gdb a.out 1966  
GNU gdb (GDB) 7.2-ubuntu  
Copyright (C) 2010 Free Software Foundation, Inc.  
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>  
This is free software: you are free to change and redistribute it.  
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"  
and "show warranty" for details.  
This GDB was configured as "i686-linux-gnu".  
For bug reporting instructions, please see:  
<http://www.gnu.org/software/gdb/bugs/>...  
Reading symbols from /home/bhojas/a.out...done.  
Attaching to program: /home/bhojas/a.out, process 1966  
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.  
Loaded symbols for /lib/libc.so.6  
Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.  
Loaded symbols for /lib/ld-linux.so.2  
0x004cd416 in __kernel_vsyscall ()  
(gdb) print y  
$1 = 1000  
(gdb) set var y=100  
(gdb)c  

The output immediately changes like so

main: rand value = 5  
f1: double the value = 1000  
main: rand value = 6  
f1: double the value = 1200  
main: rand value = 7  
f1: double the value = 1400