checksum.c

/*
 * Copyright (c) 2007-2011 Nicira, Inc.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of version 2 of the GNU General Public
 * License as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/in.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/udp.h>

#include "checksum.h"
#include "datapath.h"

#ifdef NEED_CSUM_NORMALIZE

#if defined(CONFIG_XEN) && defined(HAVE_PROTO_DATA_VALID)
/* This code is based on skb_checksum_setup() from Xen's net/dev/core.c.  We
 * can't call this function directly because it isn't exported in all
 * versions. */
static int vswitch_skb_checksum_setup(struct sk_buff *skb)
{
	struct iphdr *iph;
	unsigned char *th;
	int err = -EPROTO;
	__u16 csum_start, csum_offset;

	if (!skb->proto_csum_blank)
		return 0;

	if (skb->protocol != htons(ETH_P_IP))
		goto out;

	if (!pskb_may_pull(skb, skb_network_header(skb) + sizeof(struct iphdr) - skb->data))
		goto out;

	iph = ip_hdr(skb);
	th = skb_network_header(skb) + 4 * iph->ihl;

	csum_start = th - skb->head;
	switch (iph->protocol) {
	case IPPROTO_TCP:
		csum_offset = offsetof(struct tcphdr, check);
		break;
	case IPPROTO_UDP:
		csum_offset = offsetof(struct udphdr, check);
		break;
	default:
		if (net_ratelimit())
			pr_err("Attempting to checksum a non-TCP/UDP packet, "
			       "dropping a protocol %d packet",
			       iph->protocol);
		goto out;
	}

	if (!pskb_may_pull(skb, th + csum_offset + 2 - skb->data))
		goto out;

	skb->proto_csum_blank = 0;
	set_ip_summed(skb, OVS_CSUM_PARTIAL);
	set_skb_csum_pointers(skb, csum_start, csum_offset);

	err = 0;

out:
	return err;
}
#else
static int vswitch_skb_checksum_setup(struct sk_buff *skb)
{
	return 0;
}
#endif /* not Xen old style checksums */

/*
 *	compute_ip_summed - map external checksum state onto OVS representation
 *
 * @skb: Packet to manipulate.
 * @xmit: Whether we were on transmit path of network stack.  For example,
 *	  this is true for the internal dev vport because it receives skbs
 *	  that passed through dev_queue_xmit() but false for the netdev vport
 *	  because its packets come from netif_receive_skb().
 *
 * Older kernels (and various versions of Xen) were not explicit enough about
 * checksum offload parameters and rely on a combination of context and
 * non standard fields.  This deals with all those variations so that we
 * can internally manipulate checksum offloads without worrying about kernel
 * version.
 *
 * Types of checksums that we can receive (these all refer to L4 checksums):
 * 1. CHECKSUM_NONE: Device that did not compute checksum, contains full
 *	(though not verified) checksum in packet but not in skb->csum.  Packets
 *	from the bridge local port will also have this type.
 * 2. CHECKSUM_COMPLETE (CHECKSUM_HW): Good device that computes checksums,
 *	also the GRE module.  This is the same as CHECKSUM_NONE, except it has
 *	a valid skb->csum.  Importantly, both contain a full checksum (not
 *	verified) in the packet itself.  The only difference is that if the
 *	packet gets to L4 processing on this machine (not in DomU) we won't
 *	have to recompute the checksum to verify.  Most hardware devices do not
 *	produce packets with this type, even if they support receive checksum
 *	offloading (they produce type #5).
 * 3. CHECKSUM_PARTIAL (CHECKSUM_HW): Packet without full checksum and needs to
 *	be computed if it is sent off box.  Unfortunately on earlier kernels,
 *	this case is impossible to distinguish from #2, despite having opposite
 *	meanings.  Xen adds an extra field on earlier kernels (see #4) in order
 *	to distinguish the different states.
 * 4. CHECKSUM_UNNECESSARY (with proto_csum_blank true): This packet was
 *	generated locally by a Xen DomU and has a partial checksum.  If it is
 *	handled on this machine (Dom0 or DomU), then the checksum will not be
 *	computed.  If it goes off box, the checksum in the packet needs to be
 *	completed.  Calling skb_checksum_setup converts this to CHECKSUM_HW
 *	(CHECKSUM_PARTIAL) so that the checksum can be completed.  In later
 *	kernels, this combination is replaced with CHECKSUM_PARTIAL.
 * 5. CHECKSUM_UNNECESSARY (with proto_csum_blank false): Packet with a correct
 *	full checksum or using a protocol without a checksum.  skb->csum is
 *	undefined.  This is common from devices with receive checksum
 *	offloading.  This is somewhat similar to CHECKSUM_NONE, except that
 *	nobody will try to verify the checksum with CHECKSUM_UNNECESSARY.
 *
 * Note that on earlier kernels, CHECKSUM_COMPLETE and CHECKSUM_PARTIAL are
 * both defined as CHECKSUM_HW.  Normally the meaning of CHECKSUM_HW is clear
 * based on whether it is on the transmit or receive path.  After the datapath
 * it will be intepreted as CHECKSUM_PARTIAL.  If the packet already has a
 * checksum, we will panic.  Since we can receive packets with checksums, we
 * assume that all CHECKSUM_HW packets have checksums and map them to
 * CHECKSUM_NONE, which has a similar meaning (the it is only different if the
 * packet is processed by the local IP stack, in which case it will need to
 * be reverified).  If we receive a packet with CHECKSUM_HW that really means
 * CHECKSUM_PARTIAL, it will be sent with the wrong checksum.  However, there
 * shouldn't be any devices that do this with bridging.
 */
int compute_ip_summed(struct sk_buff *skb, bool xmit)
{
	/* For our convenience these defines change repeatedly between kernel
	 * versions, so we can't just copy them over...
	 */
	switch (skb->ip_summed) {
	case CHECKSUM_NONE:
		set_ip_summed(skb, OVS_CSUM_NONE);
		break;
	case CHECKSUM_UNNECESSARY:
		set_ip_summed(skb, OVS_CSUM_UNNECESSARY);
		break;
#ifdef CHECKSUM_HW
	/* In theory this could be either CHECKSUM_PARTIAL or CHECKSUM_COMPLETE.
	 * However, on the receive side we should only get CHECKSUM_PARTIAL
	 * packets from Xen, which uses some special fields to represent this
	 * (see vswitch_skb_checksum_setup()).  Since we can only make one type
	 * work, pick the one that actually happens in practice.
	 *
	 * On the transmit side (basically after skb_checksum_setup()
	 * has been run or on internal dev transmit), packets with
	 * CHECKSUM_COMPLETE aren't generated, so assume CHECKSUM_PARTIAL.
	 */
	case CHECKSUM_HW:
		if (!xmit)
			set_ip_summed(skb, OVS_CSUM_COMPLETE);
		else
			set_ip_summed(skb, OVS_CSUM_PARTIAL);
		break;
#else
	case CHECKSUM_COMPLETE:
		set_ip_summed(skb, OVS_CSUM_COMPLETE);
		break;
	case CHECKSUM_PARTIAL:
		set_ip_summed(skb, OVS_CSUM_PARTIAL);
		break;
#endif
	}

	OVS_CB(skb)->csum_start = skb_headroom(skb) + skb_transport_offset(skb);

	return vswitch_skb_checksum_setup(skb);
}

/*
 *     forward_ip_summed - map internal checksum state back onto native
 *			   kernel fields.
 *
 * @skb: Packet to manipulate.
 * @xmit: Whether we are about send on the transmit path the network stack.
 *	  This follows the same logic as the @xmit field in compute_ip_summed().
 *	  Generally, a given vport will have opposite values for @xmit passed to
 *	  these two functions.
 *
 * When a packet is about to egress from OVS take our internal fields (including
 * any modifications we have made) and recreate the correct representation for
 * this kernel.  This may do things like change the transport header offset.
 */
void forward_ip_summed(struct sk_buff *skb, bool xmit)
{
	switch (get_ip_summed(skb)) {
	case OVS_CSUM_NONE:
		skb->ip_summed = CHECKSUM_NONE;
		break;
	case OVS_CSUM_UNNECESSARY:
		skb->ip_summed = CHECKSUM_UNNECESSARY;
#if defined(CONFIG_XEN) && defined(HAVE_PROTO_DATA_VALID)
		skb->proto_data_valid = 1;
#endif
		break;
#ifdef CHECKSUM_HW
	case OVS_CSUM_COMPLETE:
		if (!xmit)
			skb->ip_summed = CHECKSUM_HW;
		else
			skb->ip_summed = CHECKSUM_NONE;
		break;
	case OVS_CSUM_PARTIAL:
		if (!xmit) {
			skb->ip_summed = CHECKSUM_UNNECESSARY;
#if defined(CONFIG_XEN) && defined(HAVE_PROTO_DATA_VALID)
			skb->proto_csum_blank = 1;
#endif
		} else {
			skb->ip_summed = CHECKSUM_HW;
		}
		break;
#else
	case OVS_CSUM_COMPLETE:
		skb->ip_summed = CHECKSUM_COMPLETE;
		break;
	case OVS_CSUM_PARTIAL:
		skb->ip_summed = CHECKSUM_PARTIAL;
		break;
#endif
	}

	if (get_ip_summed(skb) == OVS_CSUM_PARTIAL)
		skb_set_transport_header(skb, OVS_CB(skb)->csum_start -
					      skb_headroom(skb));
}

u8 get_ip_summed(struct sk_buff *skb)
{
	return OVS_CB(skb)->ip_summed;
}

void set_ip_summed(struct sk_buff *skb, u8 ip_summed)
{
	OVS_CB(skb)->ip_summed = ip_summed;
}

void get_skb_csum_pointers(const struct sk_buff *skb, u16 *csum_start,
			   u16 *csum_offset)
{
	*csum_start = OVS_CB(skb)->csum_start;
	*csum_offset = skb->csum;
}

void set_skb_csum_pointers(struct sk_buff *skb, u16 csum_start,
			   u16 csum_offset)
{
	OVS_CB(skb)->csum_start = csum_start;
	skb->csum = csum_offset;
}
#endif /* NEED_CSUM_NORMALIZE */