// SPDX-License-Identifier: GPL-2.0-or-later
/*
 *      GRE over IPv4 demultiplexer driver
 *
 *      Authors: Dmitry Kozlov (xeb@mail.ru)
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/module.h>
#include <linux/if.h>
#include <linux/icmp.h>
#include <linux/kernel.h>
#include <linux/kmod.h>
#include <linux/skbuff.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/netdevice.h>
#include <linux/if_tunnel.h>
#include <linux/spinlock.h>
#include <net/protocol.h>
#include <net/gre.h>
#include <net/erspan.h>

#include <net/icmp.h>
#include <net/route.h>
#include <net/xfrm.h>

static const struct gre_protocol __rcu *gre_proto[GREPROTO_MAX] __read_mostly;

int gre_add_protocol(const struct gre_protocol *proto, u8 version)
{
        if (version >= GREPROTO_MAX)
                return -EINVAL;

        return (cmpxchg((const struct gre_protocol **)&gre_proto[version], NULL, proto) == NULL) ?
                0 : -EBUSY;
}
EXPORT_SYMBOL_GPL(gre_add_protocol);

int gre_del_protocol(const struct gre_protocol *proto, u8 version)
{
        int ret;

        if (version >= GREPROTO_MAX)
                return -EINVAL;

        ret = (cmpxchg((const struct gre_protocol **)&gre_proto[version], proto, NULL) == proto) ?
                0 : -EBUSY;

        if (ret)
                return ret;

        synchronize_rcu();
        return 0;
}
EXPORT_SYMBOL_GPL(gre_del_protocol);

/* Fills in tpi and returns header length to be pulled.
 * Note that caller must use pskb_may_pull() before pulling GRE header.
 */
int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
                     bool *csum_err, __be16 proto, int nhs)
{
        const struct gre_base_hdr *greh;
        __be32 *options;
        int hdr_len;

        if (unlikely(!pskb_may_pull(skb, nhs + sizeof(struct gre_base_hdr))))
                return -EINVAL;

        greh = (struct gre_base_hdr *)(skb->data + nhs);
        if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING)))
                return -EINVAL;

        gre_flags_to_tnl_flags(tpi->flags, greh->flags);
        hdr_len = gre_calc_hlen(tpi->flags);

        if (!pskb_may_pull(skb, nhs + hdr_len))
                return -EINVAL;

        greh = (struct gre_base_hdr *)(skb->data + nhs);
        tpi->proto = greh->protocol;

        options = (__be32 *)(greh + 1);
        if (greh->flags & GRE_CSUM) {
                if (!skb_checksum_simple_validate(skb)) {
                        skb_checksum_try_convert(skb, IPPROTO_GRE,
                                                 null_compute_pseudo);
                } else if (csum_err) {
                        *csum_err = true;
                        return -EINVAL;
                }

                options++;
        }

        if (greh->flags & GRE_KEY) {
                tpi->key = *options;
                options++;
        } else {
                tpi->key = 0;
        }
        if (unlikely(greh->flags & GRE_SEQ)) {
                tpi->seq = *options;
                options++;
        } else {
                tpi->seq = 0;
        }
        /* WCCP version 1 and 2 protocol decoding.
         * - Change protocol to IPv4/IPv6
         * - When dealing with WCCPv2, Skip extra 4 bytes in GRE header
         */
        if (greh->flags == 0 && tpi->proto == htons(ETH_P_WCCP)) {
                u8 _val, *val;

                val = skb_header_pointer(skb, nhs + hdr_len,
                                         sizeof(_val), &_val);
                if (!val)
                        return -EINVAL;
                tpi->proto = proto;
                if ((*val & 0xF0) != 0x40)
                        hdr_len += 4;
        }
        tpi->hdr_len = hdr_len;

        /* ERSPAN ver 1 and 2 protocol sets GRE key field
         * to 0 and sets the configured key in the
         * inner erspan header field
         */
        if ((greh->protocol == htons(ETH_P_ERSPAN) && hdr_len != 4) ||
            greh->protocol == htons(ETH_P_ERSPAN2)) {
                struct erspan_base_hdr *ershdr;

                if (!pskb_may_pull(skb, nhs + hdr_len + sizeof(*ershdr)))
                        return -EINVAL;

                ershdr = (struct erspan_base_hdr *)(skb->data + nhs + hdr_len);
                tpi->key = cpu_to_be32(get_session_id(ershdr));
        }

        return hdr_len;
}
EXPORT_SYMBOL(gre_parse_header);

static int gre_rcv(struct sk_buff *skb)
{
        const struct gre_protocol *proto;
        u8 ver;
        int ret;

        if (!pskb_may_pull(skb, 12))
                goto drop;

        ver = skb->data[1]&0x7f;
        if (ver >= GREPROTO_MAX)
                goto drop;

        rcu_read_lock();
        proto = rcu_dereference(gre_proto[ver]);
        if (!proto || !proto->handler)
                goto drop_unlock;
        ret = proto->handler(skb);
        rcu_read_unlock();
        return ret;

drop_unlock:
        rcu_read_unlock();
drop:
        kfree_skb(skb);
        return NET_RX_DROP;
}

static int gre_err(struct sk_buff *skb, u32 info)
{
        const struct gre_protocol *proto;
        const struct iphdr *iph = (const struct iphdr *)skb->data;
        u8 ver = skb->data[(iph->ihl<<2) + 1]&0x7f;
        int err = 0;

        if (ver >= GREPROTO_MAX)
                return -EINVAL;

        rcu_read_lock();
        proto = rcu_dereference(gre_proto[ver]);
        if (proto && proto->err_handler)
                proto->err_handler(skb, info);
        else
                err = -EPROTONOSUPPORT;
        rcu_read_unlock();

        return err;
}

static const struct net_protocol net_gre_protocol = {
        .handler     = gre_rcv,
        .err_handler = gre_err,
};

static int __init gre_init(void)
{
        pr_info("GRE over IPv4 demultiplexer driver\n");

        if (inet_add_protocol(&net_gre_protocol, IPPROTO_GRE) < 0) {
                pr_err("can't add protocol\n");
                return -EAGAIN;
        }
        return 0;
}

static void __exit gre_exit(void)
{
        inet_del_protocol(&net_gre_protocol, IPPROTO_GRE);
}

module_init(gre_init);
module_exit(gre_exit);

MODULE_DESCRIPTION("GRE over IPv4 demultiplexer driver");
MODULE_AUTHOR("D. Kozlov <xeb@mail.ru>");
MODULE_LICENSE("GPL");