net/netfilter/nf_sockopt.c
 static struct nf_sockopt_ops *nf_sockopt_find(struct sock *sk, int pf,
         int val, int get)
 {
     struct nf_sockopt_ops *ops;
  
     if (sock_net(sk) != &init_net)
         return ERR_PTR(-ENOPROTOOPT);
  
     if (mutex_lock_interruptible(&nf_sockopt_mutex) != 0)
         return ERR_PTR(-EINTR);
  
     list_for_each_entry(ops, &nf_sockopts, list) {
         if (ops->pf == pf) {
             if (!try_module_get(ops->owner))
                 goto out_nosup;
  
             if (get) {
                 if (val >= ops->get_optmin &&
                         val < ops->get_optmax)
                     goto out;
             } else {
                 if (val >= ops->set_optmin &&
                         val < ops->set_optmax)
                     goto out;
             }
             module_put(ops->owner);
         }
     }
 out_nosup:
     ops = ERR_PTR(-ENOPROTOOPT);
 out:
     mutex_unlock(&nf_sockopt_mutex);
     return ops;
 }