Getting the address of a function in C?

Question

I'm getting a segfault when running this code as root in userspace. I don't understand why. I believe I have a rootkit and I want to check if the addresses are the same as the ones as in /boot/System.map-3.2.0-4-amd64

unsigned long hex;
unsigned long **sys_call_table;

for(hex = 0xffffffff810f8989; hex < 0xffffffff8160e370; hex += sizeof(void *))
{
    sys_call_table = (unsigned long **)hex;

    if(sys_call_table[3] == (unsigned long *)0xffffffff810f8989)
    {
        puts("sys_close's address has not been replaced by the rootkit");
    }
}

cat /boot/System.map-3.2.0-4-amd64 | grep "string you want"

ffffffff81401200 R sys_call_table
ffffffff810f9f9e T sys_read         // sys_call_table[0]
ffffffff810fa009 T sys_write        // sys_call_table[1]
ffffffff810f950d T sys_open         // sys_call_table[2]
ffffffff810f8989 T sys_close        // sys_call_table[3]
ffffffff8160e370 D loops_per_jiffy

Answer 1

Running from root is not enough - the problem is that you run it in user space - run it in the kernel space, as a kernel module, for example. Although having root privileges is enough for invoking system calls you cannot access the table - in user space you can only access allocated memory to you.