// SPDX-License-Identifier: GPL-2.0-or-later
/*
 *      SNAP data link layer. Derived from 802.2
 *
 *              Alan Cox <alan@lxorguk.ukuu.org.uk>,
 *              from the 802.2 layer by Greg Page.
 *              Merged in additions from Greg Page's psnap.c.
 */

#include <linux/module.h>
#include <linux/netdevice.h>
#include <linux/skbuff.h>
#include <linux/slab.h>
#include <net/datalink.h>
#include <net/llc.h>
#include <net/psnap.h>
#include <linux/mm.h>
#include <linux/in.h>
#include <linux/init.h>
#include <linux/rculist.h>

static LIST_HEAD(snap_list);
static DEFINE_SPINLOCK(snap_lock);
static struct llc_sap *snap_sap;

/*
 *      Find a snap client by matching the 5 bytes.
 */
static struct datalink_proto *find_snap_client(const unsigned char *desc)
{
        struct datalink_proto *proto = NULL, *p;

        list_for_each_entry_rcu(p, &snap_list, node, lockdep_is_held(&snap_lock)) {
                if (!memcmp(p->type, desc, 5)) {
                        proto = p;
                        break;
                }
        }
        return proto;
}

/*
 *      A SNAP packet has arrived
 */
static int snap_rcv(struct sk_buff *skb, struct net_device *dev,
                    struct packet_type *pt, struct net_device *orig_dev)
{
        int rc = 1;
        struct datalink_proto *proto;
        static struct packet_type snap_packet_type = {
                .type = cpu_to_be16(ETH_P_SNAP),
        };

        if (unlikely(!pskb_may_pull(skb, 5)))
                goto drop;

        rcu_read_lock();
        proto = find_snap_client(skb->data);
        if (proto) {
                /* Pass the frame on. */
                skb_pull_rcsum(skb, 5);
                skb_reset_transport_header(skb);
                rc = proto->rcvfunc(skb, dev, &snap_packet_type, orig_dev);
        }
        rcu_read_unlock();

        if (unlikely(!proto))
                goto drop;

out:
        return rc;

drop:
        kfree_skb(skb);
        goto out;
}

/*
 *      Put a SNAP header on a frame and pass to 802.2
 */
static int snap_request(struct datalink_proto *dl,
                        struct sk_buff *skb, const u8 *dest)
{
        memcpy(skb_push(skb, 5), dl->type, 5);
        llc_build_and_send_ui_pkt(snap_sap, skb, dest, snap_sap->laddr.lsap);
        return 0;
}

/*
 *      Set up the SNAP layer
 */
EXPORT_SYMBOL(register_snap_client);
EXPORT_SYMBOL(unregister_snap_client);

static const char snap_err_msg[] __initconst =
        KERN_CRIT "SNAP - unable to register with 802.2\n";

static int __init snap_init(void)
{
        snap_sap = llc_sap_open(0xAA, snap_rcv);
        if (!snap_sap) {
                printk(snap_err_msg);
                return -EBUSY;
        }

        return 0;
}

module_init(snap_init);

static void __exit snap_exit(void)
{
        llc_sap_put(snap_sap);
}

module_exit(snap_exit);


/*
 *      Register SNAP clients. We don't yet use this for IP.
 */
struct datalink_proto *register_snap_client(const unsigned char *desc,
                                            int (*rcvfunc)(struct sk_buff *,
                                                           struct net_device *,
                                                           struct packet_type *,
                                                           struct net_device *))
{
        struct datalink_proto *proto = NULL;

        spin_lock_bh(&snap_lock);

        if (find_snap_client(desc))
                goto out;

        proto = kmalloc_obj(*proto, GFP_ATOMIC);
        if (proto) {
                memcpy(proto->type, desc, 5);
                proto->rcvfunc          = rcvfunc;
                proto->header_length    = 5 + 3; /* snap + 802.2 */
                proto->request          = snap_request;
                list_add_rcu(&proto->node, &snap_list);
        }
out:
        spin_unlock_bh(&snap_lock);

        return proto;
}

/*
 *      Unregister SNAP clients. Protocols no longer want to play with us ...
 */
void unregister_snap_client(struct datalink_proto *proto)
{
        spin_lock_bh(&snap_lock);
        list_del_rcu(&proto->node);
        spin_unlock_bh(&snap_lock);

        synchronize_net();

        kfree(proto);
}

MODULE_DESCRIPTION("SNAP data link layer. Derived from 802.2");
MODULE_LICENSE("GPL");