nsh: add GSO support

Add a new nsh/ directory. It currently holds only GSO functions but more
will come: in particular, code shared by openvswitch and tc to manipulate
NSH headers.

For now, assume there's no hardware support for NSH segmentation. We can
always introduce netdev->nsh_features later.
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/Kconfig

@@ -235,6 +235,7 @@ source "net/openvswitch/Kconfig"
 source "net/vmw_vsock/Kconfig"
 source "net/netlink/Kconfig"
 source "net/mpls/Kconfig"
+source "net/nsh/Kconfig"
 source "net/hsr/Kconfig"
 source "net/switchdev/Kconfig"
 source "net/l3mdev/Kconfig"

net/Makefile

@@ -75,6 +75,7 @@ obj-$(CONFIG_NET_IFE)		+= ife/
 obj-$(CONFIG_OPENVSWITCH)	+= openvswitch/
 obj-$(CONFIG_VSOCKETS)	+= vmw_vsock/
 obj-$(CONFIG_MPLS)		+= mpls/
+obj-$(CONFIG_NET_NSH)		+= nsh/
 obj-$(CONFIG_HSR)		+= hsr/
 ifneq ($(CONFIG_NET_SWITCHDEV),)
 obj-y				+= switchdev/

net/nsh/Kconfig

+menuconfig NET_NSH
+	tristate "Network Service Header (NSH) protocol"
+	default n
+	---help---
+	  Network Service Header is an implementation of Service Function
+	  Chaining (RFC 7665). The current implementation in Linux supports
+	  only MD type 1 and only with the openvswitch module.
+
+	  If unsure, say N.

net/nsh/Makefile

+obj-$(CONFIG_NET_NSH) += nsh.o

net/nsh/nsh.c

+/*
+ * Network Service Header
+ *
+ * Copyright (c) 2017 Red Hat, Inc. -- Jiri Benc <jbenc@redhat.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/module.h>
+#include <linux/netdevice.h>
+#include <linux/skbuff.h>
+#include <net/nsh.h>
+#include <net/tun_proto.h>
+
+static struct sk_buff *nsh_gso_segment(struct sk_buff *skb,
+				       netdev_features_t features)
+{
+	struct sk_buff *segs = ERR_PTR(-EINVAL);
+	unsigned int nsh_len, mac_len;
+	__be16 proto;
+	int nhoff;
+
+	skb_reset_network_header(skb);
+
+	nhoff = skb->network_header - skb->mac_header;
+	mac_len = skb->mac_len;
+
+	if (unlikely(!pskb_may_pull(skb, NSH_BASE_HDR_LEN)))
+		goto out;
+	nsh_len = nsh_hdr_len(nsh_hdr(skb));
+	if (unlikely(!pskb_may_pull(skb, nsh_len)))
+		goto out;
+
+	proto = tun_p_to_eth_p(nsh_hdr(skb)->np);
+	if (!proto)
+		goto out;
+
+	__skb_pull(skb, nsh_len);
+
+	skb_reset_mac_header(skb);
+	skb_reset_mac_len(skb);
+	skb->protocol = proto;
+
+	features &= NETIF_F_SG;
+	segs = skb_mac_gso_segment(skb, features);
+	if (IS_ERR_OR_NULL(segs)) {
+		skb_gso_error_unwind(skb, htons(ETH_P_NSH), nsh_len,
+				     skb->network_header - nhoff,
+				     mac_len);
+		goto out;
+	}
+
+	for (skb = segs; skb; skb = skb->next) {
+		skb->protocol = htons(ETH_P_NSH);
+		__skb_push(skb, nsh_len);
+		skb_set_mac_header(skb, -nhoff);
+		skb->network_header = skb->mac_header + mac_len;
+		skb->mac_len = mac_len;
+	}
+
+out:
+	return segs;
+}
+
+static struct packet_offload nsh_packet_offload __read_mostly = {
+	.type = htons(ETH_P_NSH),
+	.priority = 15,
+	.callbacks = {
+		.gso_segment = nsh_gso_segment,
+	},
+};
+
+static int __init nsh_init_module(void)
+{
+	dev_add_offload(&nsh_packet_offload);
+	return 0;
+}
+
+static void __exit nsh_cleanup_module(void)
+{
+	dev_remove_offload(&nsh_packet_offload);
+}
+
+module_init(nsh_init_module);
+module_exit(nsh_cleanup_module);
+
+MODULE_AUTHOR("Jiri Benc <jbenc@redhat.com>");
+MODULE_DESCRIPTION("NSH protocol");
+MODULE_LICENSE("GPL v2");