net/ipv4/gre.c - LeafOS-Devices/android_kernel_samsung_gta4xl - Gitiles

 /*
  *	GRE over IPv4 demultiplexer driver
  *
  *	Authors: Dmitry Kozlov (xeb@mail.ru)
  *
  *	This program is free software; you can redistribute it and/or
  *	modify it under the terms of the GNU General Public License
  *	as published by the Free Software Foundation; either version
  *	2 of the License, or (at your option) any later version.
  *
  */

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

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

 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);

 static __sum16 check_checksum(struct sk_buff *skb)
 {
 	__sum16 csum = 0;

 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE:
 		csum = csum_fold(skb->csum);

 		if (!csum)
 			break;
 		/* Fall through. */

 	case CHECKSUM_NONE:
 		skb->csum = 0;
 		csum = __skb_checksum_complete(skb);
 		skb->ip_summed = CHECKSUM_COMPLETE;
 		break;
 	}

 	return csum;
 }

 static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 			    bool *csum_err)
 {
 	unsigned int ip_hlen = ip_hdrlen(skb);
 	const struct gre_base_hdr *greh;
 	__be32 *options;
 	int hdr_len;

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

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

 	tpi->flags = gre_flags_to_tnl_flags(greh->flags);
 	hdr_len = ip_gre_calc_hlen(tpi->flags);

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

 	greh = (struct gre_base_hdr *)(skb_network_header(skb) + ip_hlen);
 	tpi->proto = greh->protocol;

 	options = (__be32 *)(greh + 1);
 	if (greh->flags & GRE_CSUM) {
 		if (check_checksum(skb)) {
 			*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 IP
 	 * - When dealing with WCCPv2, Skip extra 4 bytes in GRE header
 	 */
 	if (greh->flags == 0 && tpi->proto == htons(ETH_P_WCCP)) {
 		tpi->proto = htons(ETH_P_IP);
 		if ((*(u8 *)options & 0xF0) != 0x40) {
 			hdr_len += 4;
 			if (!pskb_may_pull(skb, hdr_len))
 				return -EINVAL;
 		}
 	}
 	return 0;
 }

 static int gre_cisco_rcv(struct sk_buff *skb)
 {
 	struct tnl_ptk_info tpi;
 	int i;
 	bool csum_err = false;

 	if (parse_gre_header(skb, &tpi, &csum_err) < 0)
 		goto drop;

 	rcu_read_lock();
 	for (i = 0; i < GRE_IP_PROTO_MAX; i++) {
 		struct gre_cisco_protocol *proto;
 		int ret;

 		proto = rcu_dereference(gre_cisco_proto_list[i]);
 		if (!proto)
 			continue;
 		ret = proto->handler(skb, &tpi);
 		if (ret == PACKET_RCVD) {
 			rcu_read_unlock();
 			return 0;
 		}
 	}
 	rcu_read_unlock();

 	icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0);
 drop:
 	kfree_skb(skb);
 	return 0;
 }

 static void gre_cisco_err(struct sk_buff *skb, u32 info)
 {
 	/* All the routers (except for Linux) return only
 	 * 8 bytes of packet payload. It means, that precise relaying of
 	 * ICMP in the real Internet is absolutely infeasible.
 	 *
 	 * Moreover, Cisco "wise men" put GRE key to the third word
 	 * in GRE header. It makes impossible maintaining even soft
 	 * state for keyed
 	 * GRE tunnels with enabled checksum. Tell them "thank you".
 	 *
 	 * Well, I wonder, rfc1812 was written by Cisco employee,
 	 * what the hell these idiots break standards established
 	 * by themselves???
 	 */

 	const int type = icmp_hdr(skb)->type;
 	const int code = icmp_hdr(skb)->code;
 	struct tnl_ptk_info tpi;
 	bool csum_err = false;
 	int i;

 	if (parse_gre_header(skb, &tpi, &csum_err)) {
 		if (!csum_err)		/* ignore csum errors. */
 			return;
 	}

 	if (type == ICMP_DEST_UNREACH && code == ICMP_FRAG_NEEDED) {
 		ipv4_update_pmtu(skb, dev_net(skb->dev), info,
 				skb->dev->ifindex, 0, IPPROTO_GRE, 0);
 		return;
 	}
 	if (type == ICMP_REDIRECT) {
 		ipv4_redirect(skb, dev_net(skb->dev), skb->dev->ifindex, 0,
 				IPPROTO_GRE, 0);
 		return;
 	}

 	rcu_read_lock();
 	for (i = 0; i < GRE_IP_PROTO_MAX; i++) {
 		struct gre_cisco_protocol *proto;

 		proto = rcu_dereference(gre_cisco_proto_list[i]);
 		if (!proto)
 			continue;

 		if (proto->err_handler(skb, info, &tpi) == PACKET_RCVD)
 			goto out;

 	}
 out:
 	rcu_read_unlock();
 }

 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 void 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;

 	if (ver >= GREPROTO_MAX)
 		return;

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

 static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 				       netdev_features_t features)
 {
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
 	netdev_features_t enc_features;
 	int ghl = GRE_HEADER_SECTION;
 	struct gre_base_hdr *greh;
 	int mac_len = skb->mac_len;
 	__be16 protocol = skb->protocol;
 	int tnl_hlen;
 	bool csum;

 	if (unlikely(skb_shinfo(skb)->gso_type &
 				~(SKB_GSO_TCPV4 |
 				  SKB_GSO_TCPV6 |
 				  SKB_GSO_UDP |
 				  SKB_GSO_DODGY |
 				  SKB_GSO_TCP_ECN |
 				  SKB_GSO_GRE)))
 		goto out;

 	if (unlikely(!pskb_may_pull(skb, sizeof(*greh))))
 		goto out;

 	greh = (struct gre_base_hdr *)skb_transport_header(skb);

 	if (greh->flags & GRE_KEY)
 		ghl += GRE_HEADER_SECTION;
 	if (greh->flags & GRE_SEQ)
 		ghl += GRE_HEADER_SECTION;
 	if (greh->flags & GRE_CSUM) {
 		ghl += GRE_HEADER_SECTION;
 		csum = true;
 	} else
 		csum = false;

 	/* setup inner skb. */
 	skb->protocol = greh->protocol;
 	skb->encapsulation = 0;

 	if (unlikely(!pskb_may_pull(skb, ghl)))
 		goto out;
 	__skb_pull(skb, ghl);
 	skb_reset_mac_header(skb);
 	skb_set_network_header(skb, skb_inner_network_offset(skb));
 	skb->mac_len = skb_inner_network_offset(skb);

 	/* segment inner packet. */
 	enc_features = skb->dev->hw_enc_features & netif_skb_features(skb);
 	segs = skb_mac_gso_segment(skb, enc_features);
 	if (!segs || IS_ERR(segs))
 		goto out;

 	skb = segs;
 	tnl_hlen = skb_tnl_header_len(skb);
 	do {
 		__skb_push(skb, ghl);
 		if (csum) {
 			__be32 *pcsum;

 			if (skb_has_shared_frag(skb)) {
 				int err;

 				err = __skb_linearize(skb);
 				if (err) {
 					kfree_skb(segs);
 					segs = ERR_PTR(err);
 					goto out;
 				}
 			}

 			greh = (struct gre_base_hdr *)(skb->data);
 			pcsum = (__be32 *)(greh + 1);
 			*pcsum = 0;
 			*(__sum16 *)pcsum = csum_fold(skb_checksum(skb, 0, skb->len, 0));
 		}
 		__skb_push(skb, tnl_hlen - ghl);

 		skb_reset_mac_header(skb);
 		skb_set_network_header(skb, mac_len);
 		skb->mac_len = mac_len;
 		skb->protocol = protocol;
 	} while ((skb = skb->next));
 out:
 	return segs;
 }

 static int gre_gso_send_check(struct sk_buff *skb)
 {
 	if (!skb->encapsulation)
 		return -EINVAL;
 	return 0;
 }

 static const struct net_protocol net_gre_protocol = {
 	.handler     = gre_rcv,
 	.err_handler = gre_err,
 	.netns_ok    = 1,
 };

 static const struct net_offload gre_offload = {
 	.callbacks = {
 		.gso_send_check =	gre_gso_send_check,
 		.gso_segment    =	gre_gso_segment,
 	},
 };

 static const struct gre_protocol ipgre_protocol = {
 	.handler     = gre_cisco_rcv,
 	.err_handler = gre_cisco_err,
 };

 int gre_cisco_register(struct gre_cisco_protocol *newp)
 {
 	struct gre_cisco_protocol **proto = (struct gre_cisco_protocol **)
 					    &gre_cisco_proto_list[newp->priority];

 	return (cmpxchg(proto, NULL, newp) == NULL) ? 0 : -EBUSY;
 }
 EXPORT_SYMBOL_GPL(gre_cisco_register);

 int gre_cisco_unregister(struct gre_cisco_protocol *del_proto)
 {
 	struct gre_cisco_protocol **proto = (struct gre_cisco_protocol **)
 					    &gre_cisco_proto_list[del_proto->priority];
 	int ret;

 	ret = (cmpxchg(proto, del_proto, NULL) == del_proto) ? 0 : -EINVAL;

 	if (ret)
 		return ret;

 	synchronize_net();
 	return 0;
 }
 EXPORT_SYMBOL_GPL(gre_cisco_unregister);

 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");
 		goto err;
 	}

 	if (gre_add_protocol(&ipgre_protocol, GREPROTO_CISCO) < 0) {
 		pr_info("%s: can't add ipgre handler\n", __func__);
 		goto err_gre;
 	}

 	if (inet_add_offload(&gre_offload, IPPROTO_GRE)) {
 		pr_err("can't add protocol offload\n");
 		goto err_gso;
 	}

 	return 0;
 err_gso:
 	gre_del_protocol(&ipgre_protocol, GREPROTO_CISCO);
 err_gre:
 	inet_del_protocol(&net_gre_protocol, IPPROTO_GRE);
 err:
 	return -EAGAIN;
 }

 static void __exit gre_exit(void)
 {
 	inet_del_offload(&gre_offload, IPPROTO_GRE);
 	gre_del_protocol(&ipgre_protocol, GREPROTO_CISCO);
 	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");
	/*
	* GRE over IPv4 demultiplexer driver
	*
	* Authors: Dmitry Kozlov (xeb@mail.ru)
	*
	* This program is free software; you can redistribute it and/or
	* modify it under the terms of the GNU General Public License
	* as published by the Free Software Foundation; either version
	* 2 of the License, or (at your option) any later version.
	*
	*/

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

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

	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);

	static __sum16 check_checksum(struct sk_buff *skb)
	{
	__sum16 csum = 0;

	switch (skb->ip_summed) {
	case CHECKSUM_COMPLETE:
	csum = csum_fold(skb->csum);

	if (!csum)
	break;
	/* Fall through. */

	case CHECKSUM_NONE:
	skb->csum = 0;
	csum = __skb_checksum_complete(skb);
	skb->ip_summed = CHECKSUM_COMPLETE;
	break;
	}

	return csum;
	}

	static int parse_gre_header(struct sk_buff skb, struct tnl_ptk_info tpi,
	bool *csum_err)
	{
	unsigned int ip_hlen = ip_hdrlen(skb);
	const struct gre_base_hdr *greh;
	__be32 *options;
	int hdr_len;

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

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

	tpi->flags = gre_flags_to_tnl_flags(greh->flags);
	hdr_len = ip_gre_calc_hlen(tpi->flags);

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

	greh = (struct gre_base_hdr *)(skb_network_header(skb) + ip_hlen);
	tpi->proto = greh->protocol;

	options = (__be32 *)(greh + 1);
	if (greh->flags & GRE_CSUM) {
	if (check_checksum(skb)) {
	*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 IP
	* - When dealing with WCCPv2, Skip extra 4 bytes in GRE header
	*/
	if (greh->flags == 0 && tpi->proto == htons(ETH_P_WCCP)) {
	tpi->proto = htons(ETH_P_IP);
	if (((u8 )options & 0xF0) != 0x40) {
	hdr_len += 4;
	if (!pskb_may_pull(skb, hdr_len))
	return -EINVAL;
	}
	}
	return 0;
	}

	static int gre_cisco_rcv(struct sk_buff *skb)
	{
	struct tnl_ptk_info tpi;
	int i;
	bool csum_err = false;

	if (parse_gre_header(skb, &tpi, &csum_err) < 0)
	goto drop;

	rcu_read_lock();
	for (i = 0; i < GRE_IP_PROTO_MAX; i++) {
	struct gre_cisco_protocol *proto;
	int ret;

	proto = rcu_dereference(gre_cisco_proto_list[i]);
	if (!proto)
	continue;
	ret = proto->handler(skb, &tpi);
	if (ret == PACKET_RCVD) {
	rcu_read_unlock();
	return 0;
	}
	}
	rcu_read_unlock();

	icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0);
	drop:
	kfree_skb(skb);
	return 0;
	}

	static void gre_cisco_err(struct sk_buff *skb, u32 info)
	{
	/* All the routers (except for Linux) return only
	* 8 bytes of packet payload. It means, that precise relaying of
	* ICMP in the real Internet is absolutely infeasible.
	*
	* Moreover, Cisco "wise men" put GRE key to the third word
	* in GRE header. It makes impossible maintaining even soft
	* state for keyed
	* GRE tunnels with enabled checksum. Tell them "thank you".
	*
	* Well, I wonder, rfc1812 was written by Cisco employee,
	* what the hell these idiots break standards established
	* by themselves???
	*/

	const int type = icmp_hdr(skb)->type;
	const int code = icmp_hdr(skb)->code;
	struct tnl_ptk_info tpi;
	bool csum_err = false;
	int i;

	if (parse_gre_header(skb, &tpi, &csum_err)) {
	if (!csum_err) /* ignore csum errors. */
	return;
	}

	if (type == ICMP_DEST_UNREACH && code == ICMP_FRAG_NEEDED) {
	ipv4_update_pmtu(skb, dev_net(skb->dev), info,
	skb->dev->ifindex, 0, IPPROTO_GRE, 0);
	return;
	}
	if (type == ICMP_REDIRECT) {
	ipv4_redirect(skb, dev_net(skb->dev), skb->dev->ifindex, 0,
	IPPROTO_GRE, 0);
	return;
	}

	rcu_read_lock();
	for (i = 0; i < GRE_IP_PROTO_MAX; i++) {
	struct gre_cisco_protocol *proto;

	proto = rcu_dereference(gre_cisco_proto_list[i]);
	if (!proto)
	continue;

	if (proto->err_handler(skb, info, &tpi) == PACKET_RCVD)
	goto out;

	}
	out:
	rcu_read_unlock();
	}

	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 void 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;

	if (ver >= GREPROTO_MAX)
	return;

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

	static struct sk_buff gre_gso_segment(struct sk_buff skb,
	netdev_features_t features)
	{
	struct sk_buff *segs = ERR_PTR(-EINVAL);
	netdev_features_t enc_features;
	int ghl = GRE_HEADER_SECTION;
	struct gre_base_hdr *greh;
	int mac_len = skb->mac_len;
	__be16 protocol = skb->protocol;
	int tnl_hlen;
	bool csum;

	if (unlikely(skb_shinfo(skb)->gso_type &
	~(SKB_GSO_TCPV4 \|
	SKB_GSO_TCPV6 \|
	SKB_GSO_UDP \|
	SKB_GSO_DODGY \|
	SKB_GSO_TCP_ECN \|
	SKB_GSO_GRE)))
	goto out;

	if (unlikely(!pskb_may_pull(skb, sizeof(*greh))))
	goto out;

	greh = (struct gre_base_hdr *)skb_transport_header(skb);

	if (greh->flags & GRE_KEY)
	ghl += GRE_HEADER_SECTION;
	if (greh->flags & GRE_SEQ)
	ghl += GRE_HEADER_SECTION;
	if (greh->flags & GRE_CSUM) {
	ghl += GRE_HEADER_SECTION;
	csum = true;
	} else
	csum = false;

	/* setup inner skb. */
	skb->protocol = greh->protocol;
	skb->encapsulation = 0;

	if (unlikely(!pskb_may_pull(skb, ghl)))
	goto out;
	__skb_pull(skb, ghl);
	skb_reset_mac_header(skb);
	skb_set_network_header(skb, skb_inner_network_offset(skb));
	skb->mac_len = skb_inner_network_offset(skb);

	/* segment inner packet. */
	enc_features = skb->dev->hw_enc_features & netif_skb_features(skb);
	segs = skb_mac_gso_segment(skb, enc_features);
	if (!segs \|\| IS_ERR(segs))
	goto out;

	skb = segs;
	tnl_hlen = skb_tnl_header_len(skb);
	do {
	__skb_push(skb, ghl);
	if (csum) {
	__be32 *pcsum;

	if (skb_has_shared_frag(skb)) {
	int err;

	err = __skb_linearize(skb);
	if (err) {
	kfree_skb(segs);
	segs = ERR_PTR(err);
	goto out;
	}
	}

	greh = (struct gre_base_hdr *)(skb->data);
	pcsum = (__be32 *)(greh + 1);
	*pcsum = 0;
	(__sum16 )pcsum = csum_fold(skb_checksum(skb, 0, skb->len, 0));
	}
	__skb_push(skb, tnl_hlen - ghl);

	skb_reset_mac_header(skb);
	skb_set_network_header(skb, mac_len);
	skb->mac_len = mac_len;
	skb->protocol = protocol;
	} while ((skb = skb->next));
	out:
	return segs;
	}

	static int gre_gso_send_check(struct sk_buff *skb)
	{
	if (!skb->encapsulation)
	return -EINVAL;
	return 0;
	}

	static const struct net_protocol net_gre_protocol = {
	.handler = gre_rcv,
	.err_handler = gre_err,
	.netns_ok = 1,
	};

	static const struct net_offload gre_offload = {
	.callbacks = {
	.gso_send_check = gre_gso_send_check,
	.gso_segment = gre_gso_segment,
	},
	};

	static const struct gre_protocol ipgre_protocol = {
	.handler = gre_cisco_rcv,
	.err_handler = gre_cisco_err,
	};

	int gre_cisco_register(struct gre_cisco_protocol *newp)
	{
	struct gre_cisco_protocol proto = (struct gre_cisco_protocol )
	&gre_cisco_proto_list[newp->priority];

	return (cmpxchg(proto, NULL, newp) == NULL) ? 0 : -EBUSY;
	}
	EXPORT_SYMBOL_GPL(gre_cisco_register);

	int gre_cisco_unregister(struct gre_cisco_protocol *del_proto)
	{
	struct gre_cisco_protocol proto = (struct gre_cisco_protocol )
	&gre_cisco_proto_list[del_proto->priority];
	int ret;

	ret = (cmpxchg(proto, del_proto, NULL) == del_proto) ? 0 : -EINVAL;

	if (ret)
	return ret;

	synchronize_net();
	return 0;
	}
	EXPORT_SYMBOL_GPL(gre_cisco_unregister);

	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");
	goto err;
	}

	if (gre_add_protocol(&ipgre_protocol, GREPROTO_CISCO) < 0) {
	pr_info("%s: can't add ipgre handler\n", __func__);
	goto err_gre;
	}

	if (inet_add_offload(&gre_offload, IPPROTO_GRE)) {
	pr_err("can't add protocol offload\n");
	goto err_gso;
	}

	return 0;
	err_gso:
	gre_del_protocol(&ipgre_protocol, GREPROTO_CISCO);
	err_gre:
	inet_del_protocol(&net_gre_protocol, IPPROTO_GRE);
	err:
	return -EAGAIN;
	}

	static void __exit gre_exit(void)
	{
	inet_del_offload(&gre_offload, IPPROTO_GRE);
	gre_del_protocol(&ipgre_protocol, GREPROTO_CISCO);
	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");