text: // 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;
  
  	tpi->flags = gre_flags_to_tnl_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) ||
  	    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,
  	.netns_ok    = 1,
  };
  
  static int __init gre_init(void)
  {
  	pr_info("GRE over IPv4 demultiplexor 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");
  