Obtain interface netmask in Linux kernel module

I write Linux kernel module with netfilter hook. I want to block any packet that is not from my subnet.

Is there any simple method to get netmask of the interface in kernel-mode? I found only the way to get it using ioctl() in user-mode.

There is a pretty easy way to get it.
Network device is described by struct net_device.
<linux/netdevice.h>:

struct net_device {
    ...
    struct in_device __rcu  *ip_ptr;
    ...

net_device has a pointer to "inet" device (in_device).

<linux/inetdevice.h>:

struct in_device {
    ...
    struct in_ifaddr    *ifa_list;  /* IP ifaddr chain      */
    ...

which finnaly points to chain of in_ifaddr that contains all the interface info:

struct in_ifaddr {
    struct hlist_node   hash;
    struct in_ifaddr    *ifa_next;
    struct in_device    *ifa_dev;
    struct rcu_head     rcu_head;
    __be32              ifa_local;
    __be32              ifa_address;
    __be32              ifa_mask;
    __u32               ifa_rt_priority;
    __be32              ifa_broadcast;
    unsigned char       ifa_scope;
    unsigned char       ifa_prefixlen;
    __u32               ifa_flags;
    char                ifa_label[IFNAMSIZ];

    /* In seconds, relative to tstamp. Expiry is at tstamp + HZ * lft. */
    __u32               ifa_valid_lft;
    __u32               ifa_preferred_lft;
    unsigned long       ifa_cstamp; /* created timestamp */
    unsigned long       ifa_tstamp; /* updated timestamp */
};

---

To make my answer more versatile, here is an abstract example (without binding to netfilter and skb devices logic):

struct in_ifaddr *ifa;
struct net_device *dev = dev_get_by_name(&init_net, "wlp7s0");
if(!dev) {
    printk(KERN_ERR "Can't obtain device\n");
    return;
}

// roughly
rcu_read_lock();
for(ifa = rcu_dereference(dev->ip_ptr->ifa_list);
          ifa;
          ifa = rcu_dereference(ifa->ifa_next))
    printk("address: %pI4, mask: %pI4\n", &ifa->ifa_address, &ifa->ifa_mask);
rcu_read_unlock();

From example you can see that you can handle the whole chain(that @larsks mentioned in comment) depending on some specific logic.

P.S. don't forget to include <linux/netdevice.h> and <linux/inetdevice.h>.