net: accept UFO datagrams from tuntap and packet

Tuntap and similar devices can inject GSO packets. Accept type
VIRTIO_NET_HDR_GSO_UDP, even though not generating UFO natively.

Processes are expected to use feature negotiation such as TUNSETOFFLOAD
to detect supported offload types and refrain from injecting other
packets. This process breaks down with live migration: guest kernels
do not renegotiate flags, so destination hosts need to expose all
features that the source host does.

Partially revert the UFO removal from 182e0b6b5846~1..d9d30adf5677.
This patch introduces nearly(*) no new code to simplify verification.
It brings back verbatim tuntap UFO negotiation, VIRTIO_NET_HDR_GSO_UDP
insertion and software UFO segmentation.

It does not reinstate protocol stack support, hardware offload
(NETIF_F_UFO), SKB_GSO_UDP tunneling in SKB_GSO_SOFTWARE or reception
of VIRTIO_NET_HDR_GSO_UDP packets in tuntap.

To support SKB_GSO_UDP reappearing in the stack, also reinstate
logic in act_csum and openvswitch. Achieve equivalence with v4.13 HEAD
by squashing in commit 939912216fa8 ("net: skb_needs_check() removes
CHECKSUM_UNNECESSARY check for tx.") and reverting commit 8d63bee643f1
("net: avoid skb_warn_bad_offload false positives on UFO").

(*) To avoid having to bring back skb_shinfo(skb)->ip6_frag_id,
ipv6_proxy_select_ident is changed to return a __be32 and this is
assigned directly to the frag_hdr. Also, SKB_GSO_UDP is inserted
at the end of the enum to minimize code churn.

Tested
  Booted a v4.13 guest kernel with QEMU. On a host kernel before this
  patch `ethtool -k eth0` shows UFO disabled. After the patch, it is
  enabled, same as on a v4.13 host kernel.

  A UFO packet sent from the guest appears on the tap device:
    host:
      nc -l -p -u 8000 &
      tcpdump -n -i tap0

    guest:
      dd if=/dev/zero of=payload.txt bs=1 count=2000
      nc -u 192.16.1.1 8000 < payload.txt

  Direct tap to tap transmission of VIRTIO_NET_HDR_GSO_UDP succeeds,
  packets arriving fragmented:

    ./with_tap_pair.sh ./tap_send_ufo tap0 tap1
    (from https://github.com/wdebruij/kerneltools/tree/master/tests)

Changes
  v1 -> v2
    - simplified set_offload change (review comment)
    - documented test procedure

Link: http://lkml.kernel.org/r/<CAF=yD-LuUeDuL9YWPJD9ykOZ0QCjNeznPDr6whqZ9NGMNF12Mw@mail.gmail.com>
Fixes: fb652fdfe837 ("macvlan/macvtap: Remove NETIF_F_UFO advertisement.")
Reported-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

15 files changed, 209 insertions, 14 deletions

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index b13890953ebb..e9489b88407c 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -1077,7 +1077,7 @@ static long tap_ioctl(struct file *file, unsigned int cmd,
         case TUNSETOFFLOAD:
                 /* let the user check for future flags */
                 if (arg & ~(TUN_F_CSUM | TUN_F_TSO4 | TUN_F_TSO6 |
-                            TUN_F_TSO_ECN))
+                            TUN_F_TSO_ECN | TUN_F_UFO))
                         return -EINVAL;
 
                 rtnl_lock();
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 5a2ea78a008f..6a7bde9bc4b2 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2370,6 +2370,8 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
                                 features |= NETIF_F_TSO6;
                         arg &= ~(TUN_F_TSO4|TUN_F_TSO6);
                 }
+
+                arg &= ~TUN_F_UFO;
         }
 
         /* This gives the user a way to test for new features in future by
diff --git a/include/linux/netdev_features.h b/include/linux/netdev_features.h
index dc8b4896b77b..b1b0ca7ccb2b 100644
--- a/include/linux/netdev_features.h
+++ b/include/linux/netdev_features.h
@@ -54,8 +54,9 @@ enum {
         NETIF_F_GSO_TUNNEL_REMCSUM_BIT, /* ... TUNNEL with TSO & REMCSUM */
         NETIF_F_GSO_SCTP_BIT,           /* ... SCTP fragmentation */
         NETIF_F_GSO_ESP_BIT,            /* ... ESP with TSO */
+        NETIF_F_GSO_UDP_BIT,            /* ... UFO, deprecated except tuntap */
         /**/NETIF_F_GSO_LAST =          /* last bit, see GSO_MASK */
-                NETIF_F_GSO_ESP_BIT,
+                NETIF_F_GSO_UDP_BIT,
 
         NETIF_F_FCOE_CRC_BIT,           /* FCoE CRC32 */
         NETIF_F_SCTP_CRC_BIT,           /* SCTP checksum offload */
@@ -132,6 +133,7 @@ enum {
 #define NETIF_F_GSO_TUNNEL_REMCSUM __NETIF_F(GSO_TUNNEL_REMCSUM)
 #define NETIF_F_GSO_SCTP        __NETIF_F(GSO_SCTP)
 #define NETIF_F_GSO_ESP         __NETIF_F(GSO_ESP)
+#define NETIF_F_GSO_UDP         __NETIF_F(GSO_UDP)
 #define NETIF_F_HW_VLAN_STAG_FILTER __NETIF_F(HW_VLAN_STAG_FILTER)
 #define NETIF_F_HW_VLAN_STAG_RX __NETIF_F(HW_VLAN_STAG_RX)
 #define NETIF_F_HW_VLAN_STAG_TX __NETIF_F(HW_VLAN_STAG_TX)
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 6b274bfe489f..ef789e1d679e 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4140,6 +4140,7 @@ static inline bool net_gso_ok(netdev_features_t features, int gso_type)
         BUILD_BUG_ON(SKB_GSO_TUNNEL_REMCSUM != (NETIF_F_GSO_TUNNEL_REMCSUM >> NETIF_F_GSO_SHIFT));
         BUILD_BUG_ON(SKB_GSO_SCTP    != (NETIF_F_GSO_SCTP >> NETIF_F_GSO_SHIFT));
         BUILD_BUG_ON(SKB_GSO_ESP != (NETIF_F_GSO_ESP >> NETIF_F_GSO_SHIFT));
+        BUILD_BUG_ON(SKB_GSO_UDP != (NETIF_F_GSO_UDP >> NETIF_F_GSO_SHIFT));
 
         return (features & feature) == feature;
 }
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ed06e1c28fc7..bc486ef23f20 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -568,6 +568,8 @@ enum {
         SKB_GSO_SCTP = 1 << 14,
 
         SKB_GSO_ESP = 1 << 15,
+
+        SKB_GSO_UDP = 1 << 16,
 };
 
 #if BITS_PER_LONG > 32
diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 210034c896e3..f144216febc6 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -9,7 +9,7 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
                                         const struct virtio_net_hdr *hdr,
                                         bool little_endian)
 {
-        unsigned short gso_type = 0;
+        unsigned int gso_type = 0;
 
         if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
                 switch (hdr->gso_type & ~VIRTIO_NET_HDR_GSO_ECN) {
@@ -19,6 +19,9 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
                 case VIRTIO_NET_HDR_GSO_TCPV6:
                         gso_type = SKB_GSO_TCPV6;
                         break;
+                case VIRTIO_NET_HDR_GSO_UDP:
+                        gso_type = SKB_GSO_UDP;
+                        break;
                 default:
                         return -EINVAL;
                 }
diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index ec14f0d5a3a1..f73797e2fa60 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -767,6 +767,7 @@ static inline int ipv6_addr_diff(const struct in6_addr *a1, const struct in6_add
 __be32 ipv6_select_ident(struct net *net,
                          const struct in6_addr *daddr,
                          const struct in6_addr *saddr);
+__be32 ipv6_proxy_select_ident(struct net *net, struct sk_buff *skb);
 
 int ip6_dst_hoplimit(struct dst_entry *dst);
 
diff --git a/net/core/dev.c b/net/core/dev.c
index 8ee29f4f5fa9..bbba19112f02 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2746,7 +2746,8 @@ EXPORT_SYMBOL(skb_mac_gso_segment);
 static inline bool skb_needs_check(struct sk_buff *skb, bool tx_path)
 {
         if (tx_path)
-                return skb->ip_summed != CHECKSUM_PARTIAL;
+                return skb->ip_summed != CHECKSUM_PARTIAL &&
+                       skb->ip_summed != CHECKSUM_UNNECESSARY;
 
         return skb->ip_summed == CHECKSUM_NONE;
 }
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index ce4aa827be05..f00499a46927 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1223,9 +1223,10 @@ EXPORT_SYMBOL(inet_sk_rebuild_header);
 struct sk_buff *inet_gso_segment(struct sk_buff *skb,
                                  netdev_features_t features)
 {
-        bool fixedid = false, gso_partial, encap;
+        bool udpfrag = false, fixedid = false, gso_partial, encap;
         struct sk_buff *segs = ERR_PTR(-EINVAL);
         const struct net_offload *ops;
+        unsigned int offset = 0;
         struct iphdr *iph;
         int proto, tot_len;
         int nhoff;
@@ -1260,6 +1261,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
         segs = ERR_PTR(-EPROTONOSUPPORT);
 
         if (!skb->encapsulation || encap) {
+                udpfrag = !!(skb_shinfo(skb)->gso_type & SKB_GSO_UDP);
                 fixedid = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TCP_FIXEDID);
 
                 /* fixed ID is invalid if DF bit is not set */
@@ -1279,7 +1281,13 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
         skb = segs;
         do {
                 iph = (struct iphdr *)(skb_mac_header(skb) + nhoff);
-                if (skb_is_gso(skb)) {
+                if (udpfrag) {
+                        iph->frag_off = htons(offset >> 3);
+                        if (skb->next)
+                                iph->frag_off |= htons(IP_MF);
+                        offset += skb->len - nhoff - ihl;
+                        tot_len = skb->len - nhoff;
+                } else if (skb_is_gso(skb)) {
                         if (!fixedid) {
                                 iph->id = htons(id);
                                 id += skb_shinfo(skb)->gso_segs;
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index e360d55be555..01801b77bd0d 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -187,16 +187,57 @@ out_unlock:
 }
 EXPORT_SYMBOL(skb_udp_tunnel_segment);
 
-static struct sk_buff *udp4_tunnel_segment(struct sk_buff *skb,
-                                           netdev_features_t features)
+static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
+                                         netdev_features_t features)
 {
         struct sk_buff *segs = ERR_PTR(-EINVAL);
+        unsigned int mss;
+        __wsum csum;
+        struct udphdr *uh;
+        struct iphdr *iph;
 
         if (skb->encapsulation &&
             (skb_shinfo(skb)->gso_type &
-             (SKB_GSO_UDP_TUNNEL|SKB_GSO_UDP_TUNNEL_CSUM)))
+             (SKB_GSO_UDP_TUNNEL|SKB_GSO_UDP_TUNNEL_CSUM))) {
                 segs = skb_udp_tunnel_segment(skb, features, false);
+                goto out;
+        }
+
+        if (!pskb_may_pull(skb, sizeof(struct udphdr)))
+                goto out;
+
+        mss = skb_shinfo(skb)->gso_size;
+        if (unlikely(skb->len <= mss))
+                goto out;
+
+        /* Do software UFO. Complete and fill in the UDP checksum as
+         * HW cannot do checksum of UDP packets sent as multiple
+         * IP fragments.
+         */
 
+        uh = udp_hdr(skb);
+        iph = ip_hdr(skb);
+
+        uh->check = 0;
+        csum = skb_checksum(skb, 0, skb->len, 0);
+        uh->check = udp_v4_check(skb->len, iph->saddr, iph->daddr, csum);
+        if (uh->check == 0)
+                uh->check = CSUM_MANGLED_0;
+
+        skb->ip_summed = CHECKSUM_UNNECESSARY;
+
+        /* If there is no outer header we can fake a checksum offload
+         * due to the fact that we have already done the checksum in
+         * software prior to segmenting the frame.
+         */
+        if (!skb->encap_hdr_csum)
+                features |= NETIF_F_HW_CSUM;
+
+        /* Fragment the skb. IP headers of the fragments are updated in
+         * inet_gso_segment()
+         */
+        segs = skb_segment(skb, features);
+out:
         return segs;
 }
 
@@ -330,7 +371,7 @@ static int udp4_gro_complete(struct sk_buff *skb, int nhoff)
 
 static const struct net_offload udpv4_offload = {
         .callbacks = {
-                .gso_segment = udp4_tunnel_segment,
+                .gso_segment = udp4_ufo_fragment,
                 .gro_receive  = udp4_gro_receive,
                 .gro_complete = udp4_gro_complete,
         },
diff --git a/net/ipv6/output_core.c b/net/ipv6/output_core.c
index 4a7e5ffa5108..4fe7c90962dd 100644
--- a/net/ipv6/output_core.c
+++ b/net/ipv6/output_core.c
@@ -31,6 +31,37 @@ static u32 __ipv6_select_ident(struct net *net, u32 hashrnd,
         return id;
 }
 
+/* This function exists only for tap drivers that must support broken
+ * clients requesting UFO without specifying an IPv6 fragment ID.
+ *
+ * This is similar to ipv6_select_ident() but we use an independent hash
+ * seed to limit information leakage.
+ *
+ * The network header must be set before calling this.
+ */
+__be32 ipv6_proxy_select_ident(struct net *net, struct sk_buff *skb)
+{
+        static u32 ip6_proxy_idents_hashrnd __read_mostly;
+        struct in6_addr buf[2];
+        struct in6_addr *addrs;
+        u32 id;
+
+        addrs = skb_header_pointer(skb,
+                                   skb_network_offset(skb) +
+                                   offsetof(struct ipv6hdr, saddr),
+                                   sizeof(buf), buf);
+        if (!addrs)
+                return 0;
+
+        net_get_random_once(&ip6_proxy_idents_hashrnd,
+                            sizeof(ip6_proxy_idents_hashrnd));
+
+        id = __ipv6_select_ident(net, ip6_proxy_idents_hashrnd,
+                                 &addrs[1], &addrs[0]);
+        return htonl(id);
+}
+EXPORT_SYMBOL_GPL(ipv6_proxy_select_ident);
+
 __be32 ipv6_select_ident(struct net *net,
                          const struct in6_addr *daddr,
                          const struct in6_addr *saddr)
diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index 455fd4e39333..a0f89ad76f9d 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c
@@ -17,15 +17,94 @@
 #include <net/ip6_checksum.h>
 #include "ip6_offload.h"
 
-static struct sk_buff *udp6_tunnel_segment(struct sk_buff *skb,
-                                           netdev_features_t features)
+static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
+                                         netdev_features_t features)
 {
         struct sk_buff *segs = ERR_PTR(-EINVAL);
+        unsigned int mss;
+        unsigned int unfrag_ip6hlen, unfrag_len;
+        struct frag_hdr *fptr;
+        u8 *packet_start, *prevhdr;
+        u8 nexthdr;
+        u8 frag_hdr_sz = sizeof(struct frag_hdr);
+        __wsum csum;
+        int tnl_hlen;
+        int err;
+
+        mss = skb_shinfo(skb)->gso_size;
+        if (unlikely(skb->len <= mss))
+                goto out;
 
         if (skb->encapsulation && skb_shinfo(skb)->gso_type &
             (SKB_GSO_UDP_TUNNEL|SKB_GSO_UDP_TUNNEL_CSUM))
                 segs = skb_udp_tunnel_segment(skb, features, true);
+        else {
+                const struct ipv6hdr *ipv6h;
+                struct udphdr *uh;
+
+                if (!pskb_may_pull(skb, sizeof(struct udphdr)))
+                        goto out;
+
+                /* Do software UFO. Complete and fill in the UDP checksum as HW cannot
+                 * do checksum of UDP packets sent as multiple IP fragments.
+                 */
+
+                uh = udp_hdr(skb);
+                ipv6h = ipv6_hdr(skb);
+
+                uh->check = 0;
+                csum = skb_checksum(skb, 0, skb->len, 0);
+                uh->check = udp_v6_check(skb->len, &ipv6h->saddr,
+                                          &ipv6h->daddr, csum);
+                if (uh->check == 0)
+                        uh->check = CSUM_MANGLED_0;
+
+                skb->ip_summed = CHECKSUM_UNNECESSARY;
+
+                /* If there is no outer header we can fake a checksum offload
+                 * due to the fact that we have already done the checksum in
+                 * software prior to segmenting the frame.
+                 */
+                if (!skb->encap_hdr_csum)
+                        features |= NETIF_F_HW_CSUM;
+
+                /* Check if there is enough headroom to insert fragment header. */
+                tnl_hlen = skb_tnl_header_len(skb);
+                if (skb->mac_header < (tnl_hlen + frag_hdr_sz)) {
+                        if (gso_pskb_expand_head(skb, tnl_hlen + frag_hdr_sz))
+                                goto out;
+                }
+
+                /* Find the unfragmentable header and shift it left by frag_hdr_sz
+                 * bytes to insert fragment header.
+                 */
+                err = ip6_find_1stfragopt(skb, &prevhdr);
+                if (err < 0)
+                        return ERR_PTR(err);
+                unfrag_ip6hlen = err;
+                nexthdr = *prevhdr;
+                *prevhdr = NEXTHDR_FRAGMENT;
+                unfrag_len = (skb_network_header(skb) - skb_mac_header(skb)) +
+                             unfrag_ip6hlen + tnl_hlen;
+                packet_start = (u8 *) skb->head + SKB_GSO_CB(skb)->mac_offset;
+                memmove(packet_start-frag_hdr_sz, packet_start, unfrag_len);
+
+                SKB_GSO_CB(skb)->mac_offset -= frag_hdr_sz;
+                skb->mac_header -= frag_hdr_sz;
+                skb->network_header -= frag_hdr_sz;
+
+                fptr = (struct frag_hdr *)(skb_network_header(skb) + unfrag_ip6hlen);
+                fptr->nexthdr = nexthdr;
+                fptr->reserved = 0;
+                fptr->identification = ipv6_proxy_select_ident(dev_net(skb->dev), skb);
+
+                /* Fragment the skb. ipv6 header and the remaining fields of the
+                 * fragment header are updated in ipv6_gso_segment()
+                 */
+                segs = skb_segment(skb, features);
+        }
 
+out:
         return segs;
 }
 
@@ -75,7 +154,7 @@ static int udp6_gro_complete(struct sk_buff *skb, int nhoff)
 
 static const struct net_offload udpv6_offload = {
         .callbacks = {
-                .gso_segment    =       udp6_tunnel_segment,
+                .gso_segment    =       udp6_ufo_fragment,
                 .gro_receive    =       udp6_gro_receive,
                 .gro_complete   =       udp6_gro_complete,
         },
diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index 0dab33fb9844..99cfafc2a139 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -308,6 +308,8 @@ static int queue_gso_packets(struct datapath *dp, struct sk_buff *skb,
                              const struct dp_upcall_info *upcall_info,
                                  uint32_t cutlen)
 {
+        unsigned short gso_type = skb_shinfo(skb)->gso_type;
+        struct sw_flow_key later_key;
         struct sk_buff *segs, *nskb;
         int err;
 
@@ -318,9 +320,21 @@ static int queue_gso_packets(struct datapath *dp, struct sk_buff *skb,
         if (segs == NULL)
                 return -EINVAL;
 
+        if (gso_type & SKB_GSO_UDP) {
+                /* The initial flow key extracted by ovs_flow_key_extract()
+                 * in this case is for a first fragment, so we need to
+                 * properly mark later fragments.
+                 */
+                later_key = *key;
+                later_key.ip.frag = OVS_FRAG_TYPE_LATER;
+        }
+
         /* Queue all of the segments. */
         skb = segs;
         do {
+                if (gso_type & SKB_GSO_UDP && skb != segs)
+                        key = &later_key;
+
                 err = queue_userspace_packet(dp, skb, key, upcall_info, cutlen);
                 if (err)
                         break;
diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c
index 864ddb1e3642..dbe2379329c5 100644
--- a/net/openvswitch/flow.c
+++ b/net/openvswitch/flow.c
@@ -631,7 +631,8 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
                         key->ip.frag = OVS_FRAG_TYPE_LATER;
                         return 0;
                 }
-                if (nh->frag_off & htons(IP_MF))
+                if (nh->frag_off & htons(IP_MF) ||
+                        skb_shinfo(skb)->gso_type & SKB_GSO_UDP)
                         key->ip.frag = OVS_FRAG_TYPE_FIRST;
                 else
                         key->ip.frag = OVS_FRAG_TYPE_NONE;
@@ -747,6 +748,9 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 
                 if (key->ip.frag == OVS_FRAG_TYPE_LATER)
                         return 0;
+                if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP)
+                        key->ip.frag = OVS_FRAG_TYPE_FIRST;
+
                 /* Transport layer. */
                 if (key->ip.proto == NEXTHDR_TCP) {
                         if (tcphdr_ok(skb)) {
diff --git a/net/sched/act_csum.c b/net/sched/act_csum.c
index 1c40caadcff9..d836f998117b 100644
--- a/net/sched/act_csum.c
+++ b/net/sched/act_csum.c
@@ -229,6 +229,9 @@ static int tcf_csum_ipv4_udp(struct sk_buff *skb, unsigned int ihl,
         const struct iphdr *iph;
         u16 ul;
 
+        if (skb_is_gso(skb) && skb_shinfo(skb)->gso_type & SKB_GSO_UDP)
+                return 1;
+
         /*
          * Support both UDP and UDPLITE checksum algorithms, Don't use
          * udph->len to get the real length without any protocol check,
@@ -282,6 +285,9 @@ static int tcf_csum_ipv6_udp(struct sk_buff *skb, unsigned int ihl,
         const struct ipv6hdr *ip6h;
         u16 ul;
 
+        if (skb_is_gso(skb) && skb_shinfo(skb)->gso_type & SKB_GSO_UDP)
+                return 1;
+
         /*
          * Support both UDP and UDPLITE checksum algorithms, Don't use
          * udph->len to get the real length without any protocol check,