kostyl.c

static struct sock *cookie_v4_check_wrapper(struct sock *sk, struct sk_buff *skb) {
        struct sock* (*old_func)(struct sock *sk, struct sk_buff *skb) = (void*)((unsigned long)cookie_v4_check_ptr + MCOUNT_INSN_SIZE);

        tcp_synq_overflow(sk);
        return old_func(sk, skb);
}

static void notrace
tcpsecrets_ftrace_handler(unsigned long ip, unsigned long parent_ip,
                      struct ftrace_ops *fops, struct pt_regs *regs)
{
        regs->ip = (unsigned long)cookie_v4_check_wrapper;
}

static struct ftrace_ops tcpsecrets_ftrace_ops __read_mostly = {
        .func = tcpsecrets_ftrace_handler,
        .flags = FTRACE_OPS_FL_SAVE_REGS,
};

static void fix_cookie_v4_check(void) {
        int ret;

        ret = ftrace_set_filter_ip(&tcpsecrets_ftrace_ops, (unsigned long)cookie_v4_check_ptr, 0, 0);
        if (ret) {
                printk("cant set ftrace filter\n");
        }
        ret = register_ftrace_function(&tcpsecrets_ftrace_ops);
        if (ret) {
                printk("cant set ftrace function\n");
        }
}