CODEWITHSUNDEEP
cpointerslinux-kernel
Tom Riddel
Tom Riddel

Reputation: 87

Change address of kernel function pointer

I'm written a module to try and change the address of the exported symbol 'do_fork' to point to my function first before calling the original do_fork address. So far I can't seem to change the address as it gives me the error 'lvalue required as left operand of assignment.'

I'm not sure how to change the pointer to do_fork() to my function fake_fork();

#include <linux/fs.h>
#include <linux/init.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/sched.h>
#include <linux/module.h>

int c=0;

long fake_fork(unsigned long a, unsigned long b, unsigned long c, int __user *d, int __user *e)
{
    ++c;

    return do_fork(a, b, c, d, e);
}

EXPORT_SYMBOL(fake_fork);

static int fork_proc_show(struct seq_file *m, void *v)
{
    seq_printf(m, "System Call fork called: %d times.\n", c);
    return 0;
}

static int fork_proc_open(struct inode *inode, struct file *file)
{
    return single_open(file, fork_proc_show, NULL);
}

static const struct file_operations fork_proc_fops = {
    .open       = fork_proc_open,
    .read       = seq_read,
    .llseek     = seq_lseek,
    .release    = single_release,
};

static int __init proc_fork_init(void)
{
    do_fork = fake_fork;  // <-- Not working

    printk("init proc forkcounter\n");
    proc_create("forkcounter", 0, NULL, &fork_proc_fops);
    return 0;
}

static void __exit cleanup_fork_module(void)
{
    remove_proc_entry("forkcounter",NULL);

    printk("cleanup proc forkcounter\n");
}

module_init(proc_fork_init);
module_exit(cleanup_fork_module);

Upvotes: 0

Views: 840

Answers (1)

Andrej Adamenko
Andrej Adamenko

Reputation: 1668

You cannot change do_fork, it is a constant at run time. It is the address of do_fork() function.

You cannot assign anything to a function. It is because name of a function is not a variable. It is a constant pointer.

It is the same as 

5 = 2 + 2;

You would get the same error message.

I assume you want your function to be called every time do_fork() is called. It it going to be more complicated to implement. I would start for example with this link.

Upvotes: 3

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