gro: Allow tunnel stacking in the case of FOU/GUE

[linux/fpc-iii.git] / net / ipv4 / ip_output.c

/*
 * INET         An implementation of the TCP/IP protocol suite for the LINUX
 *              operating system.  INET is implemented using the  BSD Socket
 *              interface as the means of communication with the user level.
 *
 *              The Internet Protocol (IP) output module.
 *
 * Authors:     Ross Biro
 *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *              Donald Becker, <becker@super.org>
 *              Alan Cox, <Alan.Cox@linux.org>
 *              Richard Underwood
 *              Stefan Becker, <stefanb@yello.ping.de>
 *              Jorge Cwik, <jorge@laser.satlink.net>
 *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *              Hirokazu Takahashi, <taka@valinux.co.jp>
 *
 *      See ip_input.c for original log
 *
 *      Fixes:
 *              Alan Cox        :       Missing nonblock feature in ip_build_xmit.
 *              Mike Kilburn    :       htons() missing in ip_build_xmit.
 *              Bradford Johnson:       Fix faulty handling of some frames when
 *                                      no route is found.
 *              Alexander Demenshin:    Missing sk/skb free in ip_queue_xmit
 *                                      (in case if packet not accepted by
 *                                      output firewall rules)
 *              Mike McLagan    :       Routing by source
 *              Alexey Kuznetsov:       use new route cache
 *              Andi Kleen:             Fix broken PMTU recovery and remove
 *                                      some redundant tests.
 *      Vitaly E. Lavrov        :       Transparent proxy revived after year coma.
 *              Andi Kleen      :       Replace ip_reply with ip_send_reply.
 *              Andi Kleen      :       Split fast and slow ip_build_xmit path
 *                                      for decreased register pressure on x86
 *                                      and more readibility.
 *              Marc Boucher    :       When call_out_firewall returns FW_QUEUE,
 *                                      silently drop skb instead of failing with -EPERM.
 *              Detlev Wengorz  :       Copy protocol for fragments.
 *              Hirokazu Takahashi:     HW checksumming for outgoing UDP
 *                                      datagrams.
 *              Hirokazu Takahashi:     sendfile() on UDP works now.
 */

#include <asm/uaccess.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/errno.h>
#include <linux/highmem.h>
#include <linux/slab.h>

#include <linux/socket.h>
#include <linux/sockios.h>
#include <linux/in.h>
#include <linux/inet.h>
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/proc_fs.h>
#include <linux/stat.h>
#include <linux/init.h>

#include <net/snmp.h>
#include <net/ip.h>
#include <net/protocol.h>
#include <net/route.h>
#include <net/xfrm.h>
#include <linux/skbuff.h>
#include <net/sock.h>
#include <net/arp.h>
#include <net/icmp.h>
#include <net/checksum.h>
#include <net/inetpeer.h>
#include <linux/igmp.h>
#include <linux/netfilter_ipv4.h>
#include <linux/netfilter_bridge.h>
#include <linux/mroute.h>
#include <linux/netlink.h>
#include <linux/tcp.h>

int sysctl_ip_default_ttl __read_mostly = IPDEFTTL;
EXPORT_SYMBOL(sysctl_ip_default_ttl);

/* Generate a checksum for an outgoing IP datagram. */
void ip_send_check(struct iphdr *iph)
{
        iph->check = 0;
        iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
}
EXPORT_SYMBOL(ip_send_check);

int __ip_local_out_sk(struct sock *sk, struct sk_buff *skb)
{
        struct iphdr *iph = ip_hdr(skb);

        iph->tot_len = htons(skb->len);
        ip_send_check(iph);
        return nf_hook(NFPROTO_IPV4, NF_INET_LOCAL_OUT, sk, skb, NULL,
                       skb_dst(skb)->dev, dst_output_sk);
}

int __ip_local_out(struct sk_buff *skb)
{
        return __ip_local_out_sk(skb->sk, skb);
}

int ip_local_out_sk(struct sock *sk, struct sk_buff *skb)
{
        int err;

        err = __ip_local_out(skb);
        if (likely(err == 1))
                err = dst_output_sk(sk, skb);

        return err;
}
EXPORT_SYMBOL_GPL(ip_local_out_sk);

static inline int ip_select_ttl(struct inet_sock *inet, struct dst_entry *dst)
{
        int ttl = inet->uc_ttl;

        if (ttl < 0)
                ttl = ip4_dst_hoplimit(dst);
        return ttl;
}

/*
 *              Add an ip header to a skbuff and send it out.
 *
 */
int ip_build_and_send_pkt(struct sk_buff *skb, struct sock *sk,
                          __be32 saddr, __be32 daddr, struct ip_options_rcu *opt)
{
        struct inet_sock *inet = inet_sk(sk);
        struct rtable *rt = skb_rtable(skb);
        struct iphdr *iph;

        /* Build the IP header. */
        skb_push(skb, sizeof(struct iphdr) + (opt ? opt->opt.optlen : 0));
        skb_reset_network_header(skb);
        iph = ip_hdr(skb);
        iph->version  = 4;
        iph->ihl      = 5;
        iph->tos      = inet->tos;
        if (ip_dont_fragment(sk, &rt->dst))
                iph->frag_off = htons(IP_DF);
        else
                iph->frag_off = 0;
        iph->ttl      = ip_select_ttl(inet, &rt->dst);
        iph->daddr    = (opt && opt->opt.srr ? opt->opt.faddr : daddr);
        iph->saddr    = saddr;
        iph->protocol = sk->sk_protocol;
        ip_select_ident(sock_net(sk), skb, sk);

        if (opt && opt->opt.optlen) {
                iph->ihl += opt->opt.optlen>>2;
                ip_options_build(skb, &opt->opt, daddr, rt, 0);
        }

        skb->priority = sk->sk_priority;
        skb->mark = sk->sk_mark;

        /* Send it out. */
        return ip_local_out(skb);
}
EXPORT_SYMBOL_GPL(ip_build_and_send_pkt);

static inline int ip_finish_output2(struct sock *sk, struct sk_buff *skb)
{
        struct dst_entry *dst = skb_dst(skb);
        struct rtable *rt = (struct rtable *)dst;
        struct net_device *dev = dst->dev;
        unsigned int hh_len = LL_RESERVED_SPACE(dev);
        struct neighbour *neigh;
        u32 nexthop;

        if (rt->rt_type == RTN_MULTICAST) {
                IP_UPD_PO_STATS(dev_net(dev), IPSTATS_MIB_OUTMCAST, skb->len);
        } else if (rt->rt_type == RTN_BROADCAST)
                IP_UPD_PO_STATS(dev_net(dev), IPSTATS_MIB_OUTBCAST, skb->len);

        /* Be paranoid, rather than too clever. */
        if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
                struct sk_buff *skb2;

                skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev));
                if (!skb2) {
                        kfree_skb(skb);
                        return -ENOMEM;
                }
                if (skb->sk)
                        skb_set_owner_w(skb2, skb->sk);
                consume_skb(skb);
                skb = skb2;
        }

        rcu_read_lock_bh();
        nexthop = (__force u32) rt_nexthop(rt, ip_hdr(skb)->daddr);
        neigh = __ipv4_neigh_lookup_noref(dev, nexthop);
        if (unlikely(!neigh))
                neigh = __neigh_create(&arp_tbl, &nexthop, dev, false);
        if (!IS_ERR(neigh)) {
                int res = dst_neigh_output(dst, neigh, skb);

                rcu_read_unlock_bh();
                return res;
        }
        rcu_read_unlock_bh();

        net_dbg_ratelimited("%s: No header cache and no neighbour!\n",
                            __func__);
        kfree_skb(skb);
        return -EINVAL;
}

static int ip_finish_output_gso(struct sock *sk, struct sk_buff *skb)
{
        netdev_features_t features;
        struct sk_buff *segs;
        int ret = 0;

        /* common case: locally created skb or seglen is <= mtu */
        if (((IPCB(skb)->flags & IPSKB_FORWARDED) == 0) ||
              skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb))
                return ip_finish_output2(sk, skb);

        /* Slowpath -  GSO segment length is exceeding the dst MTU.
         *
         * This can happen in two cases:
         * 1) TCP GRO packet, DF bit not set
         * 2) skb arrived via virtio-net, we thus get TSO/GSO skbs directly
         * from host network stack.
         */
        features = netif_skb_features(skb);
        BUILD_BUG_ON(sizeof(*IPCB(skb)) > SKB_SGO_CB_OFFSET);
        segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
        if (IS_ERR_OR_NULL(segs)) {
                kfree_skb(skb);
                return -ENOMEM;
        }

        consume_skb(skb);

        do {
                struct sk_buff *nskb = segs->next;
                int err;

                segs->next = NULL;
                err = ip_fragment(sk, segs, ip_finish_output2);

                if (err && ret == 0)
                        ret = err;
                segs = nskb;
        } while (segs);

        return ret;
}

static int ip_finish_output(struct sock *sk, struct sk_buff *skb)
{
#if defined(CONFIG_NETFILTER) && defined(CONFIG_XFRM)
        /* Policy lookup after SNAT yielded a new policy */
        if (skb_dst(skb)->xfrm) {
                IPCB(skb)->flags |= IPSKB_REROUTED;
                return dst_output_sk(sk, skb);
        }
#endif
        if (skb_is_gso(skb))
                return ip_finish_output_gso(sk, skb);

        if (skb->len > ip_skb_dst_mtu(skb))
                return ip_fragment(sk, skb, ip_finish_output2);

        return ip_finish_output2(sk, skb);
}

int ip_mc_output(struct sock *sk, struct sk_buff *skb)
{
        struct rtable *rt = skb_rtable(skb);
        struct net_device *dev = rt->dst.dev;

        /*
         *      If the indicated interface is up and running, send the packet.
         */
        IP_UPD_PO_STATS(dev_net(dev), IPSTATS_MIB_OUT, skb->len);

        skb->dev = dev;
        skb->protocol = htons(ETH_P_IP);

        /*
         *      Multicasts are looped back for other local users
         */

        if (rt->rt_flags&RTCF_MULTICAST) {
                if (sk_mc_loop(sk)
#ifdef CONFIG_IP_MROUTE
                /* Small optimization: do not loopback not local frames,
                   which returned after forwarding; they will be  dropped
                   by ip_mr_input in any case.
                   Note, that local frames are looped back to be delivered
                   to local recipients.

                   This check is duplicated in ip_mr_input at the moment.
                 */
                    &&
                    ((rt->rt_flags & RTCF_LOCAL) ||
                     !(IPCB(skb)->flags & IPSKB_FORWARDED))
#endif
                   ) {
                        struct sk_buff *newskb = skb_clone(skb, GFP_ATOMIC);
                        if (newskb)
                                NF_HOOK(NFPROTO_IPV4, NF_INET_POST_ROUTING,
                                        sk, newskb, NULL, newskb->dev,
                                        dev_loopback_xmit);
                }

                /* Multicasts with ttl 0 must not go beyond the host */

                if (ip_hdr(skb)->ttl == 0) {
                        kfree_skb(skb);
                        return 0;
                }
        }

        if (rt->rt_flags&RTCF_BROADCAST) {
                struct sk_buff *newskb = skb_clone(skb, GFP_ATOMIC);
                if (newskb)
                        NF_HOOK(NFPROTO_IPV4, NF_INET_POST_ROUTING, sk, newskb,
                                NULL, newskb->dev, dev_loopback_xmit);
        }

        return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING, sk, skb, NULL,
                            skb->dev, ip_finish_output,
                            !(IPCB(skb)->flags & IPSKB_REROUTED));
}

int ip_output(struct sock *sk, struct sk_buff *skb)
{
        struct net_device *dev = skb_dst(skb)->dev;

        IP_UPD_PO_STATS(dev_net(dev), IPSTATS_MIB_OUT, skb->len);

        skb->dev = dev;
        skb->protocol = htons(ETH_P_IP);

        return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING, sk, skb,
                            NULL, dev,
                            ip_finish_output,
                            !(IPCB(skb)->flags & IPSKB_REROUTED));
}

/*
 * copy saddr and daddr, possibly using 64bit load/stores
 * Equivalent to :
 *   iph->saddr = fl4->saddr;
 *   iph->daddr = fl4->daddr;
 */
static void ip_copy_addrs(struct iphdr *iph, const struct flowi4 *fl4)
{
        BUILD_BUG_ON(offsetof(typeof(*fl4), daddr) !=
                     offsetof(typeof(*fl4), saddr) + sizeof(fl4->saddr));
        memcpy(&iph->saddr, &fl4->saddr,
               sizeof(fl4->saddr) + sizeof(fl4->daddr));
}

/* Note: skb->sk can be different from sk, in case of tunnels */
int ip_queue_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl)
{
        struct inet_sock *inet = inet_sk(sk);
        struct ip_options_rcu *inet_opt;
        struct flowi4 *fl4;
        struct rtable *rt;
        struct iphdr *iph;
        int res;

        /* Skip all of this if the packet is already routed,
         * f.e. by something like SCTP.
         */
        rcu_read_lock();
        inet_opt = rcu_dereference(inet->inet_opt);
        fl4 = &fl->u.ip4;
        rt = skb_rtable(skb);
        if (rt)
                goto packet_routed;

        /* Make sure we can route this packet. */
        rt = (struct rtable *)__sk_dst_check(sk, 0);
        if (!rt) {
                __be32 daddr;

                /* Use correct destination address if we have options. */
                daddr = inet->inet_daddr;
                if (inet_opt && inet_opt->opt.srr)
                        daddr = inet_opt->opt.faddr;

                /* If this fails, retransmit mechanism of transport layer will
                 * keep trying until route appears or the connection times
                 * itself out.
                 */
                rt = ip_route_output_ports(sock_net(sk), fl4, sk,
                                           daddr, inet->inet_saddr,
                                           inet->inet_dport,
                                           inet->inet_sport,
                                           sk->sk_protocol,
                                           RT_CONN_FLAGS(sk),
                                           sk->sk_bound_dev_if);
                if (IS_ERR(rt))
                        goto no_route;
                sk_setup_caps(sk, &rt->dst);
        }
        skb_dst_set_noref(skb, &rt->dst);

packet_routed:
        if (inet_opt && inet_opt->opt.is_strictroute && rt->rt_uses_gateway)
                goto no_route;

        /* OK, we know where to send it, allocate and build IP header. */
        skb_push(skb, sizeof(struct iphdr) + (inet_opt ? inet_opt->opt.optlen : 0));
        skb_reset_network_header(skb);
        iph = ip_hdr(skb);
        *((__be16 *)iph) = htons((4 << 12) | (5 << 8) | (inet->tos & 0xff));
        if (ip_dont_fragment(sk, &rt->dst) && !skb->ignore_df)
                iph->frag_off = htons(IP_DF);
        else
                iph->frag_off = 0;
        iph->ttl      = ip_select_ttl(inet, &rt->dst);
        iph->protocol = sk->sk_protocol;
        ip_copy_addrs(iph, fl4);

        /* Transport layer set skb->h.foo itself. */

        if (inet_opt && inet_opt->opt.optlen) {
                iph->ihl += inet_opt->opt.optlen >> 2;
                ip_options_build(skb, &inet_opt->opt, inet->inet_daddr, rt, 0);
        }

        ip_select_ident_segs(sock_net(sk), skb, sk,
                             skb_shinfo(skb)->gso_segs ?: 1);

        /* TODO : should we use skb->sk here instead of sk ? */
        skb->priority = sk->sk_priority;
        skb->mark = sk->sk_mark;

        res = ip_local_out(skb);
        rcu_read_unlock();
        return res;

no_route:
        rcu_read_unlock();
        IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
        kfree_skb(skb);
        return -EHOSTUNREACH;
}
EXPORT_SYMBOL(ip_queue_xmit);

static void ip_copy_metadata(struct sk_buff *to, struct sk_buff *from)
{
        to->pkt_type = from->pkt_type;
        to->priority = from->priority;
        to->protocol = from->protocol;
        skb_dst_drop(to);
        skb_dst_copy(to, from);
        to->dev = from->dev;
        to->mark = from->mark;

        /* Copy the flags to each fragment. */
        IPCB(to)->flags = IPCB(from)->flags;

#ifdef CONFIG_NET_SCHED
        to->tc_index = from->tc_index;
#endif
        nf_copy(to, from);
#if defined(CONFIG_IP_VS) || defined(CONFIG_IP_VS_MODULE)
        to->ipvs_property = from->ipvs_property;
#endif
        skb_copy_secmark(to, from);
}

/*
 *      This IP datagram is too large to be sent in one piece.  Break it up into
 *      smaller pieces (each of size equal to IP header plus
 *      a block of the data of the original IP data part) that will yet fit in a
 *      single device frame, and queue such a frame for sending.
 */

int ip_fragment(struct sock *sk, struct sk_buff *skb,
                int (*output)(struct sock *, struct sk_buff *))
{
        struct iphdr *iph;
        int ptr;
        struct net_device *dev;
        struct sk_buff *skb2;
        unsigned int mtu, hlen, left, len, ll_rs;
        int offset;
        __be16 not_last_frag;
        struct rtable *rt = skb_rtable(skb);
        int err = 0;

        dev = rt->dst.dev;

        /*
         *      Point into the IP datagram header.
         */

        iph = ip_hdr(skb);

        mtu = ip_skb_dst_mtu(skb);
        if (unlikely(((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) ||
                     (IPCB(skb)->frag_max_size &&
                      IPCB(skb)->frag_max_size > mtu))) {
                IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
                icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
                          htonl(mtu));
                kfree_skb(skb);
                return -EMSGSIZE;
        }

        /*
         *      Setup starting values.
         */

        hlen = iph->ihl * 4;
        mtu = mtu - hlen;       /* Size of data space */
#if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
        if (skb->nf_bridge)
                mtu -= nf_bridge_mtu_reduction(skb);
#endif
        IPCB(skb)->flags |= IPSKB_FRAG_COMPLETE;

        /* When frag_list is given, use it. First, check its validity:
         * some transformers could create wrong frag_list or break existing
         * one, it is not prohibited. In this case fall back to copying.
         *
         * LATER: this step can be merged to real generation of fragments,
         * we can switch to copy when see the first bad fragment.
         */
        if (skb_has_frag_list(skb)) {
                struct sk_buff *frag, *frag2;
                int first_len = skb_pagelen(skb);

                if (first_len - hlen > mtu ||
                    ((first_len - hlen) & 7) ||
                    ip_is_fragment(iph) ||
                    skb_cloned(skb))
                        goto slow_path;

                skb_walk_frags(skb, frag) {
                        /* Correct geometry. */
                        if (frag->len > mtu ||
                            ((frag->len & 7) && frag->next) ||
                            skb_headroom(frag) < hlen)
                                goto slow_path_clean;

                        /* Partially cloned skb? */
                        if (skb_shared(frag))
                                goto slow_path_clean;

                        BUG_ON(frag->sk);
                        if (skb->sk) {
                                frag->sk = skb->sk;
                                frag->destructor = sock_wfree;
                        }
                        skb->truesize -= frag->truesize;
                }

                /* Everything is OK. Generate! */

                err = 0;
                offset = 0;
                frag = skb_shinfo(skb)->frag_list;
                skb_frag_list_init(skb);
                skb->data_len = first_len - skb_headlen(skb);
                skb->len = first_len;
                iph->tot_len = htons(first_len);
                iph->frag_off = htons(IP_MF);
                ip_send_check(iph);

                for (;;) {
                        /* Prepare header of the next frame,
                         * before previous one went down. */
                        if (frag) {
                                frag->ip_summed = CHECKSUM_NONE;
                                skb_reset_transport_header(frag);
                                __skb_push(frag, hlen);
                                skb_reset_network_header(frag);
                                memcpy(skb_network_header(frag), iph, hlen);
                                iph = ip_hdr(frag);
                                iph->tot_len = htons(frag->len);
                                ip_copy_metadata(frag, skb);
                                if (offset == 0)
                                        ip_options_fragment(frag);
                                offset += skb->len - hlen;
                                iph->frag_off = htons(offset>>3);
                                if (frag->next)
                                        iph->frag_off |= htons(IP_MF);
                                /* Ready, complete checksum */
                                ip_send_check(iph);
                        }

                        err = output(sk, skb);

                        if (!err)
                                IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGCREATES);
                        if (err || !frag)
                                break;

                        skb = frag;
                        frag = skb->next;
                        skb->next = NULL;
                }

                if (err == 0) {
                        IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGOKS);
                        return 0;
                }

                while (frag) {
                        skb = frag->next;
                        kfree_skb(frag);
                        frag = skb;
                }
                IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
                return err;

slow_path_clean:
                skb_walk_frags(skb, frag2) {
                        if (frag2 == frag)
                                break;
                        frag2->sk = NULL;
                        frag2->destructor = NULL;
                        skb->truesize += frag2->truesize;
                }
        }

slow_path:
        /* for offloaded checksums cleanup checksum before fragmentation */
        if ((skb->ip_summed == CHECKSUM_PARTIAL) && skb_checksum_help(skb))
                goto fail;
        iph = ip_hdr(skb);

        left = skb->len - hlen;         /* Space per frame */
        ptr = hlen;             /* Where to start from */

        ll_rs = LL_RESERVED_SPACE(rt->dst.dev);

        /*
         *      Fragment the datagram.
         */

        offset = (ntohs(iph->frag_off) & IP_OFFSET) << 3;
        not_last_frag = iph->frag_off & htons(IP_MF);

        /*
         *      Keep copying data until we run out.
         */

        while (left > 0) {
                len = left;
                /* IF: it doesn't fit, use 'mtu' - the data space left */
                if (len > mtu)
                        len = mtu;
                /* IF: we are not sending up to and including the packet end
                   then align the next start on an eight byte boundary */
                if (len < left) {
                        len &= ~7;
                }

                /* Allocate buffer */
                skb2 = alloc_skb(len + hlen + ll_rs, GFP_ATOMIC);
                if (!skb2) {
                        err = -ENOMEM;
                        goto fail;
                }

                /*
                 *      Set up data on packet
                 */

                ip_copy_metadata(skb2, skb);
                skb_reserve(skb2, ll_rs);
                skb_put(skb2, len + hlen);
                skb_reset_network_header(skb2);
                skb2->transport_header = skb2->network_header + hlen;

                /*
                 *      Charge the memory for the fragment to any owner
                 *      it might possess
                 */

                if (skb->sk)
                        skb_set_owner_w(skb2, skb->sk);

                /*
                 *      Copy the packet header into the new buffer.
                 */

                skb_copy_from_linear_data(skb, skb_network_header(skb2), hlen);

                /*
                 *      Copy a block of the IP datagram.
                 */
                if (skb_copy_bits(skb, ptr, skb_transport_header(skb2), len))
                        BUG();
                left -= len;

                /*
                 *      Fill in the new header fields.
                 */
                iph = ip_hdr(skb2);
                iph->frag_off = htons((offset >> 3));

                /* ANK: dirty, but effective trick. Upgrade options only if
                 * the segment to be fragmented was THE FIRST (otherwise,
                 * options are already fixed) and make it ONCE
                 * on the initial skb, so that all the following fragments
                 * will inherit fixed options.
                 */
                if (offset == 0)
                        ip_options_fragment(skb);

                /*
                 *      Added AC : If we are fragmenting a fragment that's not the
                 *                 last fragment then keep MF on each bit
                 */
                if (left > 0 || not_last_frag)
                        iph->frag_off |= htons(IP_MF);
                ptr += len;
                offset += len;

                /*
                 *      Put this fragment into the sending queue.
                 */
                iph->tot_len = htons(len + hlen);

                ip_send_check(iph);

                err = output(sk, skb2);
                if (err)
                        goto fail;

                IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGCREATES);
        }
        consume_skb(skb);
        IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGOKS);
        return err;

fail:
        kfree_skb(skb);
        IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
        return err;
}
EXPORT_SYMBOL(ip_fragment);

int
ip_generic_getfrag(void *from, char *to, int offset, int len, int odd, struct sk_buff *skb)
{
        struct msghdr *msg = from;

        if (skb->ip_summed == CHECKSUM_PARTIAL) {
                if (copy_from_iter(to, len, &msg->msg_iter) != len)
                        return -EFAULT;
        } else {
                __wsum csum = 0;
                if (csum_and_copy_from_iter(to, len, &csum, &msg->msg_iter) != len)
                        return -EFAULT;
                skb->csum = csum_block_add(skb->csum, csum, odd);
        }
        return 0;
}
EXPORT_SYMBOL(ip_generic_getfrag);

static inline __wsum
csum_page(struct page *page, int offset, int copy)
{
        char *kaddr;
        __wsum csum;
        kaddr = kmap(page);
        csum = csum_partial(kaddr + offset, copy, 0);
        kunmap(page);
        return csum;
}

static inline int ip_ufo_append_data(struct sock *sk,
                        struct sk_buff_head *queue,
                        int getfrag(void *from, char *to, int offset, int len,
                               int odd, struct sk_buff *skb),
                        void *from, int length, int hh_len, int fragheaderlen,
                        int transhdrlen, int maxfraglen, unsigned int flags)
{
        struct sk_buff *skb;
        int err;

        /* There is support for UDP fragmentation offload by network
         * device, so create one single skb packet containing complete
         * udp datagram
         */
        skb = skb_peek_tail(queue);
        if (!skb) {
                skb = sock_alloc_send_skb(sk,
                        hh_len + fragheaderlen + transhdrlen + 20,
                        (flags & MSG_DONTWAIT), &err);

                if (!skb)
                        return err;

                /* reserve space for Hardware header */
                skb_reserve(skb, hh_len);

                /* create space for UDP/IP header */
                skb_put(skb, fragheaderlen + transhdrlen);

                /* initialize network header pointer */
                skb_reset_network_header(skb);

                /* initialize protocol header pointer */
                skb->transport_header = skb->network_header + fragheaderlen;

                skb->csum = 0;

                __skb_queue_tail(queue, skb);
        } else if (skb_is_gso(skb)) {
                goto append;
        }

        skb->ip_summed = CHECKSUM_PARTIAL;
        /* specify the length of each IP datagram fragment */
        skb_shinfo(skb)->gso_size = maxfraglen - fragheaderlen;
        skb_shinfo(skb)->gso_type = SKB_GSO_UDP;

append:
        return skb_append_datato_frags(sk, skb, getfrag, from,
                                       (length - transhdrlen));
}

static int __ip_append_data(struct sock *sk,
                            struct flowi4 *fl4,
                            struct sk_buff_head *queue,
                            struct inet_cork *cork,
                            struct page_frag *pfrag,
                            int getfrag(void *from, char *to, int offset,
                                        int len, int odd, struct sk_buff *skb),
                            void *from, int length, int transhdrlen,
                            unsigned int flags)
{
        struct inet_sock *inet = inet_sk(sk);
        struct sk_buff *skb;

        struct ip_options *opt = cork->opt;
        int hh_len;
        int exthdrlen;
        int mtu;
        int copy;
        int err;
        int offset = 0;
        unsigned int maxfraglen, fragheaderlen, maxnonfragsize;
        int csummode = CHECKSUM_NONE;
        struct rtable *rt = (struct rtable *)cork->dst;
        u32 tskey = 0;

        skb = skb_peek_tail(queue);

        exthdrlen = !skb ? rt->dst.header_len : 0;
        mtu = cork->fragsize;
        if (cork->tx_flags & SKBTX_ANY_SW_TSTAMP &&
            sk->sk_tsflags & SOF_TIMESTAMPING_OPT_ID)
                tskey = sk->sk_tskey++;

        hh_len = LL_RESERVED_SPACE(rt->dst.dev);

        fragheaderlen = sizeof(struct iphdr) + (opt ? opt->optlen : 0);
        maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen;
        maxnonfragsize = ip_sk_ignore_df(sk) ? 0xFFFF : mtu;

        if (cork->length + length > maxnonfragsize - fragheaderlen) {
                ip_local_error(sk, EMSGSIZE, fl4->daddr, inet->inet_dport,
                               mtu - (opt ? opt->optlen : 0));
                return -EMSGSIZE;
        }

        /*
         * transhdrlen > 0 means that this is the first fragment and we wish
         * it won't be fragmented in the future.
         */
        if (transhdrlen &&
            length + fragheaderlen <= mtu &&
            rt->dst.dev->features & NETIF_F_V4_CSUM &&
            !exthdrlen)
                csummode = CHECKSUM_PARTIAL;

        cork->length += length;
        if (((length > mtu) || (skb && skb_is_gso(skb))) &&
            (sk->sk_protocol == IPPROTO_UDP) &&
            (rt->dst.dev->features & NETIF_F_UFO) && !rt->dst.header_len &&
            (sk->sk_type == SOCK_DGRAM) && !sk->sk_no_check_tx) {
                err = ip_ufo_append_data(sk, queue, getfrag, from, length,
                                         hh_len, fragheaderlen, transhdrlen,
                                         maxfraglen, flags);
                if (err)
                        goto error;
                return 0;
        }

        /* So, what's going on in the loop below?
         *
         * We use calculated fragment length to generate chained skb,
         * each of segments is IP fragment ready for sending to network after
         * adding appropriate IP header.
         */

        if (!skb)
                goto alloc_new_skb;

        while (length > 0) {
                /* Check if the remaining data fits into current packet. */
                copy = mtu - skb->len;
                if (copy < length)
                        copy = maxfraglen - skb->len;
                if (copy <= 0) {
                        char *data;
                        unsigned int datalen;
                        unsigned int fraglen;
                        unsigned int fraggap;
                        unsigned int alloclen;
                        struct sk_buff *skb_prev;
alloc_new_skb:
                        skb_prev = skb;
                        if (skb_prev)
                                fraggap = skb_prev->len - maxfraglen;
                        else
                                fraggap = 0;

                        /*
                         * If remaining data exceeds the mtu,
                         * we know we need more fragment(s).
                         */
                        datalen = length + fraggap;
                        if (datalen > mtu - fragheaderlen)
                                datalen = maxfraglen - fragheaderlen;
                        fraglen = datalen + fragheaderlen;

                        if ((flags & MSG_MORE) &&
                            !(rt->dst.dev->features&NETIF_F_SG))
                                alloclen = mtu;
                        else
                                alloclen = fraglen;

                        alloclen += exthdrlen;

                        /* The last fragment gets additional space at tail.
                         * Note, with MSG_MORE we overallocate on fragments,
                         * because we have no idea what fragment will be
                         * the last.
                         */
                        if (datalen == length + fraggap)
                                alloclen += rt->dst.trailer_len;

                        if (transhdrlen) {
                                skb = sock_alloc_send_skb(sk,
                                                alloclen + hh_len + 15,
                                                (flags & MSG_DONTWAIT), &err);
                        } else {
                                skb = NULL;
                                if (atomic_read(&sk->sk_wmem_alloc) <=
                                    2 * sk->sk_sndbuf)
                                        skb = sock_wmalloc(sk,
                                                           alloclen + hh_len + 15, 1,
                                                           sk->sk_allocation);
                                if (unlikely(!skb))
                                        err = -ENOBUFS;
                        }
                        if (!skb)
                                goto error;

                        /*
                         *      Fill in the control structures
                         */
                        skb->ip_summed = csummode;
                        skb->csum = 0;
                        skb_reserve(skb, hh_len);

                        /* only the initial fragment is time stamped */
                        skb_shinfo(skb)->tx_flags = cork->tx_flags;
                        cork->tx_flags = 0;
                        skb_shinfo(skb)->tskey = tskey;
                        tskey = 0;

                        /*
                         *      Find where to start putting bytes.
                         */
                        data = skb_put(skb, fraglen + exthdrlen);
                        skb_set_network_header(skb, exthdrlen);
                        skb->transport_header = (skb->network_header +
                                                 fragheaderlen);
                        data += fragheaderlen + exthdrlen;

                        if (fraggap) {
                                skb->csum = skb_copy_and_csum_bits(
                                        skb_prev, maxfraglen,
                                        data + transhdrlen, fraggap, 0);
                                skb_prev->csum = csum_sub(skb_prev->csum,
                                                          skb->csum);
                                data += fraggap;
                                pskb_trim_unique(skb_prev, maxfraglen);
                        }

                        copy = datalen - transhdrlen - fraggap;
                        if (copy > 0 && getfrag(from, data + transhdrlen, offset, copy, fraggap, skb) < 0) {
                                err = -EFAULT;
                                kfree_skb(skb);
                                goto error;
                        }

                        offset += copy;
                        length -= datalen - fraggap;
                        transhdrlen = 0;
                        exthdrlen = 0;
                        csummode = CHECKSUM_NONE;

                        /*
                         * Put the packet on the pending queue.
                         */
                        __skb_queue_tail(queue, skb);
                        continue;
                }

                if (copy > length)
                        copy = length;

                if (!(rt->dst.dev->features&NETIF_F_SG)) {
                        unsigned int off;

                        off = skb->len;
                        if (getfrag(from, skb_put(skb, copy),
                                        offset, copy, off, skb) < 0) {
                                __skb_trim(skb, off);
                                err = -EFAULT;
                                goto error;
                        }
                } else {
                        int i = skb_shinfo(skb)->nr_frags;

                        err = -ENOMEM;
                        if (!sk_page_frag_refill(sk, pfrag))
                                goto error;

                        if (!skb_can_coalesce(skb, i, pfrag->page,
                                              pfrag->offset)) {
                                err = -EMSGSIZE;
                                if (i == MAX_SKB_FRAGS)
                                        goto error;

                                __skb_fill_page_desc(skb, i, pfrag->page,
                                                     pfrag->offset, 0);
                                skb_shinfo(skb)->nr_frags = ++i;
                                get_page(pfrag->page);
                        }
                        copy = min_t(int, copy, pfrag->size - pfrag->offset);
                        if (getfrag(from,
                                    page_address(pfrag->page) + pfrag->offset,
                                    offset, copy, skb->len, skb) < 0)
                                goto error_efault;

                        pfrag->offset += copy;
                        skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
                        skb->len += copy;
                        skb->data_len += copy;
                        skb->truesize += copy;
                        atomic_add(copy, &sk->sk_wmem_alloc);
                }
                offset += copy;
                length -= copy;
        }

        return 0;

error_efault:
        err = -EFAULT;
error:
        cork->length -= length;
        IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTDISCARDS);
        return err;
}

static int ip_setup_cork(struct sock *sk, struct inet_cork *cork,
                         struct ipcm_cookie *ipc, struct rtable **rtp)
{
        struct ip_options_rcu *opt;
        struct rtable *rt;

        /*
         * setup for corking.
         */
        opt = ipc->opt;
        if (opt) {
                if (!cork->opt) {
                        cork->opt = kmalloc(sizeof(struct ip_options) + 40,
                                            sk->sk_allocation);
                        if (unlikely(!cork->opt))
                                return -ENOBUFS;
                }
                memcpy(cork->opt, &opt->opt, sizeof(struct ip_options) + opt->opt.optlen);
                cork->flags |= IPCORK_OPT;
                cork->addr = ipc->addr;
        }
        rt = *rtp;
        if (unlikely(!rt))
                return -EFAULT;
        /*
         * We steal reference to this route, caller should not release it
         */
        *rtp = NULL;
        cork->fragsize = ip_sk_use_pmtu(sk) ?
                         dst_mtu(&rt->dst) : rt->dst.dev->mtu;
        cork->dst = &rt->dst;
        cork->length = 0;
        cork->ttl = ipc->ttl;
        cork->tos = ipc->tos;
        cork->priority = ipc->priority;
        cork->tx_flags = ipc->tx_flags;

        return 0;
}

/*
 *      ip_append_data() and ip_append_page() can make one large IP datagram
 *      from many pieces of data. Each pieces will be holded on the socket
 *      until ip_push_pending_frames() is called. Each piece can be a page
 *      or non-page data.
 *
 *      Not only UDP, other transport protocols - e.g. raw sockets - can use
 *      this interface potentially.
 *
 *      LATER: length must be adjusted by pad at tail, when it is required.
 */
int ip_append_data(struct sock *sk, struct flowi4 *fl4,
                   int getfrag(void *from, char *to, int offset, int len,
                               int odd, struct sk_buff *skb),
                   void *from, int length, int transhdrlen,
                   struct ipcm_cookie *ipc, struct rtable **rtp,
                   unsigned int flags)
{
        struct inet_sock *inet = inet_sk(sk);
        int err;

        if (flags&MSG_PROBE)
                return 0;

        if (skb_queue_empty(&sk->sk_write_queue)) {
                err = ip_setup_cork(sk, &inet->cork.base, ipc, rtp);
                if (err)
                        return err;
        } else {
                transhdrlen = 0;
        }

        return __ip_append_data(sk, fl4, &sk->sk_write_queue, &inet->cork.base,
                                sk_page_frag(sk), getfrag,
                                from, length, transhdrlen, flags);
}

ssize_t ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,
                       int offset, size_t size, int flags)
{
        struct inet_sock *inet = inet_sk(sk);
        struct sk_buff *skb;
        struct rtable *rt;
        struct ip_options *opt = NULL;
        struct inet_cork *cork;
        int hh_len;
        int mtu;
        int len;
        int err;
        unsigned int maxfraglen, fragheaderlen, fraggap, maxnonfragsize;

        if (inet->hdrincl)
                return -EPERM;

        if (flags&MSG_PROBE)
                return 0;

        if (skb_queue_empty(&sk->sk_write_queue))
                return -EINVAL;

        cork = &inet->cork.base;
        rt = (struct rtable *)cork->dst;
        if (cork->flags & IPCORK_OPT)
                opt = cork->opt;

        if (!(rt->dst.dev->features&NETIF_F_SG))
                return -EOPNOTSUPP;

        hh_len = LL_RESERVED_SPACE(rt->dst.dev);
        mtu = cork->fragsize;

        fragheaderlen = sizeof(struct iphdr) + (opt ? opt->optlen : 0);
        maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen;
        maxnonfragsize = ip_sk_ignore_df(sk) ? 0xFFFF : mtu;

        if (cork->length + size > maxnonfragsize - fragheaderlen) {
                ip_local_error(sk, EMSGSIZE, fl4->daddr, inet->inet_dport,
                               mtu - (opt ? opt->optlen : 0));
                return -EMSGSIZE;
        }

        skb = skb_peek_tail(&sk->sk_write_queue);
        if (!skb)
                return -EINVAL;

        cork->length += size;
        if ((size + skb->len > mtu) &&
            (sk->sk_protocol == IPPROTO_UDP) &&
            (rt->dst.dev->features & NETIF_F_UFO)) {
                skb_shinfo(skb)->gso_size = mtu - fragheaderlen;
                skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
        }

        while (size > 0) {
                int i;

                if (skb_is_gso(skb))
                        len = size;
                else {

                        /* Check if the remaining data fits into current packet. */
                        len = mtu - skb->len;
                        if (len < size)
                                len = maxfraglen - skb->len;
                }
                if (len <= 0) {
                        struct sk_buff *skb_prev;
                        int alloclen;

                        skb_prev = skb;
                        fraggap = skb_prev->len - maxfraglen;

                        alloclen = fragheaderlen + hh_len + fraggap + 15;
                        skb = sock_wmalloc(sk, alloclen, 1, sk->sk_allocation);
                        if (unlikely(!skb)) {
                                err = -ENOBUFS;
                                goto error;
                        }

                        /*
                         *      Fill in the control structures
                         */
                        skb->ip_summed = CHECKSUM_NONE;
                        skb->csum = 0;
                        skb_reserve(skb, hh_len);

                        /*
                         *      Find where to start putting bytes.
                         */
                        skb_put(skb, fragheaderlen + fraggap);
                        skb_reset_network_header(skb);
                        skb->transport_header = (skb->network_header +
                                                 fragheaderlen);
                        if (fraggap) {
                                skb->csum = skb_copy_and_csum_bits(skb_prev,
                                                                   maxfraglen,
                                                    skb_transport_header(skb),
                                                                   fraggap, 0);
                                skb_prev->csum = csum_sub(skb_prev->csum,
                                                          skb->csum);
                                pskb_trim_unique(skb_prev, maxfraglen);
                        }

                        /*
                         * Put the packet on the pending queue.
                         */
                        __skb_queue_tail(&sk->sk_write_queue, skb);
                        continue;
                }

                i = skb_shinfo(skb)->nr_frags;
                if (len > size)
                        len = size;
                if (skb_can_coalesce(skb, i, page, offset)) {
                        skb_frag_size_add(&skb_shinfo(skb)->frags[i-1], len);
                } else if (i < MAX_SKB_FRAGS) {
                        get_page(page);
                        skb_fill_page_desc(skb, i, page, offset, len);
                } else {
                        err = -EMSGSIZE;
                        goto error;
                }

                if (skb->ip_summed == CHECKSUM_NONE) {
                        __wsum csum;
                        csum = csum_page(page, offset, len);
                        skb->csum = csum_block_add(skb->csum, csum, skb->len);
                }

                skb->len += len;
                skb->data_len += len;
                skb->truesize += len;
                atomic_add(len, &sk->sk_wmem_alloc);
                offset += len;
                size -= len;
        }
        return 0;

error:
        cork->length -= size;
        IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTDISCARDS);
        return err;
}

static void ip_cork_release(struct inet_cork *cork)
{
        cork->flags &= ~IPCORK_OPT;
        kfree(cork->opt);
        cork->opt = NULL;
        dst_release(cork->dst);
        cork->dst = NULL;
}

/*
 *      Combined all pending IP fragments on the socket as one IP datagram
 *      and push them out.
 */
struct sk_buff *__ip_make_skb(struct sock *sk,
                              struct flowi4 *fl4,
                              struct sk_buff_head *queue,
                              struct inet_cork *cork)
{
        struct sk_buff *skb, *tmp_skb;
        struct sk_buff **tail_skb;
        struct inet_sock *inet = inet_sk(sk);
        struct net *net = sock_net(sk);
        struct ip_options *opt = NULL;
        struct rtable *rt = (struct rtable *)cork->dst;
        struct iphdr *iph;
        __be16 df = 0;
        __u8 ttl;

        skb = __skb_dequeue(queue);
        if (!skb)
                goto out;
        tail_skb = &(skb_shinfo(skb)->frag_list);

        /* move skb->data to ip header from ext header */
        if (skb->data < skb_network_header(skb))
                __skb_pull(skb, skb_network_offset(skb));
        while ((tmp_skb = __skb_dequeue(queue)) != NULL) {
                __skb_pull(tmp_skb, skb_network_header_len(skb));
                *tail_skb = tmp_skb;
                tail_skb = &(tmp_skb->next);
                skb->len += tmp_skb->len;
                skb->data_len += tmp_skb->len;
                skb->truesize += tmp_skb->truesize;
                tmp_skb->destructor = NULL;
                tmp_skb->sk = NULL;
        }

        /* Unless user demanded real pmtu discovery (IP_PMTUDISC_DO), we allow
         * to fragment the frame generated here. No matter, what transforms
         * how transforms change size of the packet, it will come out.
         */
        skb->ignore_df = ip_sk_ignore_df(sk);

        /* DF bit is set when we want to see DF on outgoing frames.
         * If ignore_df is set too, we still allow to fragment this frame
         * locally. */
        if (inet->pmtudisc == IP_PMTUDISC_DO ||
            inet->pmtudisc == IP_PMTUDISC_PROBE ||
            (skb->len <= dst_mtu(&rt->dst) &&
             ip_dont_fragment(sk, &rt->dst)))
                df = htons(IP_DF);

        if (cork->flags & IPCORK_OPT)
                opt = cork->opt;

        if (cork->ttl != 0)
                ttl = cork->ttl;
        else if (rt->rt_type == RTN_MULTICAST)
                ttl = inet->mc_ttl;
        else
                ttl = ip_select_ttl(inet, &rt->dst);

        iph = ip_hdr(skb);
        iph->version = 4;
        iph->ihl = 5;
        iph->tos = (cork->tos != -1) ? cork->tos : inet->tos;
        iph->frag_off = df;
        iph->ttl = ttl;
        iph->protocol = sk->sk_protocol;
        ip_copy_addrs(iph, fl4);
        ip_select_ident(net, skb, sk);

        if (opt) {
                iph->ihl += opt->optlen>>2;
                ip_options_build(skb, opt, cork->addr, rt, 0);
        }

        skb->priority = (cork->tos != -1) ? cork->priority: sk->sk_priority;
        skb->mark = sk->sk_mark;
        /*
         * Steal rt from cork.dst to avoid a pair of atomic_inc/atomic_dec
         * on dst refcount
         */
        cork->dst = NULL;
        skb_dst_set(skb, &rt->dst);

        if (iph->protocol == IPPROTO_ICMP)
                icmp_out_count(net, ((struct icmphdr *)
                        skb_transport_header(skb))->type);

        ip_cork_release(cork);
out:
        return skb;
}

int ip_send_skb(struct net *net, struct sk_buff *skb)
{
        int err;

        err = ip_local_out(skb);
        if (err) {
                if (err > 0)
                        err = net_xmit_errno(err);
                if (err)
                        IP_INC_STATS(net, IPSTATS_MIB_OUTDISCARDS);
        }

        return err;
}

int ip_push_pending_frames(struct sock *sk, struct flowi4 *fl4)
{
        struct sk_buff *skb;

        skb = ip_finish_skb(sk, fl4);
        if (!skb)
                return 0;

        /* Netfilter gets whole the not fragmented skb. */
        return ip_send_skb(sock_net(sk), skb);
}

/*
 *      Throw away all pending data on the socket.
 */
static void __ip_flush_pending_frames(struct sock *sk,
                                      struct sk_buff_head *queue,
                                      struct inet_cork *cork)
{
        struct sk_buff *skb;

        while ((skb = __skb_dequeue_tail(queue)) != NULL)
                kfree_skb(skb);

        ip_cork_release(cork);
}

void ip_flush_pending_frames(struct sock *sk)
{
        __ip_flush_pending_frames(sk, &sk->sk_write_queue, &inet_sk(sk)->cork.base);
}

struct sk_buff *ip_make_skb(struct sock *sk,
                            struct flowi4 *fl4,
                            int getfrag(void *from, char *to, int offset,
                                        int len, int odd, struct sk_buff *skb),
                            void *from, int length, int transhdrlen,
                            struct ipcm_cookie *ipc, struct rtable **rtp,
                            unsigned int flags)
{
        struct inet_cork cork;
        struct sk_buff_head queue;
        int err;

        if (flags & MSG_PROBE)
                return NULL;

        __skb_queue_head_init(&queue);

        cork.flags = 0;
        cork.addr = 0;
        cork.opt = NULL;
        err = ip_setup_cork(sk, &cork, ipc, rtp);
        if (err)
                return ERR_PTR(err);

        err = __ip_append_data(sk, fl4, &queue, &cork,
                               &current->task_frag, getfrag,
                               from, length, transhdrlen, flags);
        if (err) {
                __ip_flush_pending_frames(sk, &queue, &cork);
                return ERR_PTR(err);
        }

        return __ip_make_skb(sk, fl4, &queue, &cork);
}

/*
 *      Fetch data from kernel space and fill in checksum if needed.
 */
static int ip_reply_glue_bits(void *dptr, char *to, int offset,
                              int len, int odd, struct sk_buff *skb)
{
        __wsum csum;

        csum = csum_partial_copy_nocheck(dptr+offset, to, len, 0);
        skb->csum = csum_block_add(skb->csum, csum, odd);
        return 0;
}

/*
 *      Generic function to send a packet as reply to another packet.
 *      Used to send some TCP resets/acks so far.
 */
void ip_send_unicast_reply(struct sock *sk, struct sk_buff *skb,
                           const struct ip_options *sopt,
                           __be32 daddr, __be32 saddr,
                           const struct ip_reply_arg *arg,
                           unsigned int len)
{
        struct ip_options_data replyopts;
        struct ipcm_cookie ipc;
        struct flowi4 fl4;
        struct rtable *rt = skb_rtable(skb);
        struct net *net = sock_net(sk);
        struct sk_buff *nskb;
        int err;

        if (__ip_options_echo(&replyopts.opt.opt, skb, sopt))
                return;

        ipc.addr = daddr;
        ipc.opt = NULL;
        ipc.tx_flags = 0;
        ipc.ttl = 0;
        ipc.tos = -1;

        if (replyopts.opt.opt.optlen) {
                ipc.opt = &replyopts.opt;

                if (replyopts.opt.opt.srr)
                        daddr = replyopts.opt.opt.faddr;
        }

        flowi4_init_output(&fl4, arg->bound_dev_if,
                           IP4_REPLY_MARK(net, skb->mark),
                           RT_TOS(arg->tos),
                           RT_SCOPE_UNIVERSE, ip_hdr(skb)->protocol,
                           ip_reply_arg_flowi_flags(arg),
                           daddr, saddr,
                           tcp_hdr(skb)->source, tcp_hdr(skb)->dest);
        security_skb_classify_flow(skb, flowi4_to_flowi(&fl4));
        rt = ip_route_output_key(net, &fl4);
        if (IS_ERR(rt))
                return;

        inet_sk(sk)->tos = arg->tos;

        sk->sk_priority = skb->priority;
        sk->sk_protocol = ip_hdr(skb)->protocol;
        sk->sk_bound_dev_if = arg->bound_dev_if;
        sk->sk_sndbuf = sysctl_wmem_default;
        err = ip_append_data(sk, &fl4, ip_reply_glue_bits, arg->iov->iov_base,
                             len, 0, &ipc, &rt, MSG_DONTWAIT);
        if (unlikely(err)) {
                ip_flush_pending_frames(sk);
                goto out;
        }

        nskb = skb_peek(&sk->sk_write_queue);
        if (nskb) {
                if (arg->csumoffset >= 0)
                        *((__sum16 *)skb_transport_header(nskb) +
                          arg->csumoffset) = csum_fold(csum_add(nskb->csum,
                                                                arg->csum));
                nskb->ip_summed = CHECKSUM_NONE;
                skb_set_queue_mapping(nskb, skb_get_queue_mapping(skb));
                ip_push_pending_frames(sk, &fl4);
        }
out:
        ip_rt_put(rt);
}

void __init ip_init(void)
{
        ip_rt_init();
        inet_initpeers();

#if defined(CONFIG_IP_MULTICAST)
        igmp_mc_init();
#endif
}