/* IPv4 specific functions of netfilter core */
#include <linux/kernel.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/skbuff.h>
#include <linux/gfp.h>
#include <linux/export.h>
#include <net/route.h>
#include <net/xfrm.h>
#include <net/ip.h>
#include <net/netfilter/nf_queue.h>

/* route_me_harder function, used by iptable_nat, iptable_mangle + ip_queue */
int ip_route_me_harder(struct sk_buff *skb, unsigned int addr_type)
{
        struct net *net = dev_net(skb_dst(skb)->dev);
        const struct iphdr *iph = ip_hdr(skb);
        struct rtable *rt;
        struct flowi4 fl4 = {};
        __be32 saddr = iph->saddr;
        __u8 flags = skb->sk ? inet_sk_flowi_flags(skb->sk) : 0;
        unsigned int hh_len;

        if (addr_type == RTN_UNSPEC)
                addr_type = inet_addr_type(net, saddr);
        if (addr_type == RTN_LOCAL || addr_type == RTN_UNICAST)
                flags |= FLOWI_FLAG_ANYSRC;
        else
                saddr = 0;

        /* some non-standard hacks like ipt_REJECT.c:send_reset() can cause
         * packets with foreign saddr to appear on the NF_INET_LOCAL_OUT hook.
         */
        fl4.daddr = iph->daddr;
        fl4.saddr = saddr;
        fl4.flowi4_tos = RT_TOS(iph->tos);
        fl4.flowi4_oif = skb->sk ? skb->sk->sk_bound_dev_if : 0;
        fl4.flowi4_mark = skb->mark;
        fl4.flowi4_flags = flags;
        rt = ip_route_output_key(net, &fl4);
        if (IS_ERR(rt))
                return -1;

        /* Drop old route. */
        skb_dst_drop(skb);
        skb_dst_set(skb, &rt->dst);

        if (skb_dst(skb)->error)
                return -1;

#ifdef CONFIG_XFRM
        if (!(IPCB(skb)->flags & IPSKB_XFRM_TRANSFORMED) &&
            xfrm_decode_session(skb, flowi4_to_flowi(&fl4), AF_INET) == 0) {
                struct dst_entry *dst = skb_dst(skb);
                skb_dst_set(skb, NULL);
                dst = xfrm_lookup(net, dst, flowi4_to_flowi(&fl4), skb->sk, 0);
                if (IS_ERR(dst))
                        return -1;
                skb_dst_set(skb, dst);
        }
#endif

        /* Change in oif may mean change in hh_len. */
        hh_len = skb_dst(skb)->dev->hard_header_len;
        if (skb_headroom(skb) < hh_len &&
            pskb_expand_head(skb, HH_DATA_ALIGN(hh_len - skb_headroom(skb)),
                                0, GFP_ATOMIC))
                return -1;

        return 0;
}
EXPORT_SYMBOL(ip_route_me_harder);

#ifdef CONFIG_XFRM
int ip_xfrm_me_harder(struct sk_buff *skb)
{
        struct flowi fl;
        unsigned int hh_len;
        struct dst_entry *dst;

        if (IPCB(skb)->flags & IPSKB_XFRM_TRANSFORMED)
                return 0;
        if (xfrm_decode_session(skb, &fl, AF_INET) < 0)
                return -1;

        dst = skb_dst(skb);
        if (dst->xfrm)
                dst = ((struct xfrm_dst *)dst)->route;
        dst_hold(dst);

        dst = xfrm_lookup(dev_net(dst->dev), dst, &fl, skb->sk, 0);
        if (IS_ERR(dst))
                return -1;

        skb_dst_drop(skb);
        skb_dst_set(skb, dst);

        /* Change in oif may mean change in hh_len. */
        hh_len = skb_dst(skb)->dev->hard_header_len;
        if (skb_headroom(skb) < hh_len &&
            pskb_expand_head(skb, hh_len - skb_headroom(skb), 0, GFP_ATOMIC))
                return -1;
        return 0;
}
EXPORT_SYMBOL(ip_xfrm_me_harder);
#endif

void (*ip_nat_decode_session)(struct sk_buff *, struct flowi *);
EXPORT_SYMBOL(ip_nat_decode_session);

/*
 * Extra routing may needed on local out, as the QUEUE target never
 * returns control to the table.
 */

struct ip_rt_info {
        __be32 daddr;
        __be32 saddr;
        u_int8_t tos;
        u_int32_t mark;
};

static void nf_ip_saveroute(const struct sk_buff *skb,
                            struct nf_queue_entry *entry)
{
        struct ip_rt_info *rt_info = nf_queue_entry_reroute(entry);

        if (entry->hook == NF_INET_LOCAL_OUT) {
                const struct iphdr *iph = ip_hdr(skb);

                rt_info->tos = iph->tos;
                rt_info->daddr = iph->daddr;
                rt_info->saddr = iph->saddr;
                rt_info->mark = skb->mark;
        }
}

static int nf_ip_reroute(struct sk_buff *skb,
                         const struct nf_queue_entry *entry)
{
        const struct ip_rt_info *rt_info = nf_queue_entry_reroute(entry);

        if (entry->hook == NF_INET_LOCAL_OUT) {
                const struct iphdr *iph = ip_hdr(skb);

                if (!(iph->tos == rt_info->tos &&
                      skb->mark == rt_info->mark &&
                      iph->daddr == rt_info->daddr &&
                      iph->saddr == rt_info->saddr))
                        return ip_route_me_harder(skb, RTN_UNSPEC);
        }
        return 0;
}

__sum16 nf_ip_checksum(struct sk_buff *skb, unsigned int hook,
                            unsigned int dataoff, u_int8_t protocol)
{
        const struct iphdr *iph = ip_hdr(skb);
        __sum16 csum = 0;

        switch (skb->ip_summed) {
        case CHECKSUM_COMPLETE:
                if (hook != NF_INET_PRE_ROUTING && hook != NF_INET_LOCAL_IN)
                        break;
                if ((protocol == 0 && !csum_fold(skb->csum)) ||
                    !csum_tcpudp_magic(iph->saddr, iph->daddr,
                                       skb->len - dataoff, protocol,
                                       skb->csum)) {
                        skb->ip_summed = CHECKSUM_UNNECESSARY;
                        break;
                }
                /* fall through */
        case CHECKSUM_NONE:
                if (protocol == 0)
                        skb->csum = 0;
                else
                        skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr,
                                                       skb->len - dataoff,
                                                       protocol, 0);
                csum = __skb_checksum_complete(skb);
        }
        return csum;
}
EXPORT_SYMBOL(nf_ip_checksum);

static __sum16 nf_ip_checksum_partial(struct sk_buff *skb, unsigned int hook,
                                      unsigned int dataoff, unsigned int len,
                                      u_int8_t protocol)
{
        const struct iphdr *iph = ip_hdr(skb);
        __sum16 csum = 0;

        switch (skb->ip_summed) {
        case CHECKSUM_COMPLETE:
                if (len == skb->len - dataoff)
                        return nf_ip_checksum(skb, hook, dataoff, protocol);
                /* fall through */
        case CHECKSUM_NONE:
                skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr, protocol,
                                               skb->len - dataoff, 0);
                skb->ip_summed = CHECKSUM_NONE;
                return __skb_checksum_complete_head(skb, dataoff + len);
        }
        return csum;
}

static int nf_ip_route(struct net *net, struct dst_entry **dst,
                       struct flowi *fl, bool strict __always_unused)
{
        struct rtable *rt = ip_route_output_key(net, &fl->u.ip4);
        if (IS_ERR(rt))
                return PTR_ERR(rt);
        *dst = &rt->dst;
        return 0;
}

static const struct nf_afinfo nf_ip_afinfo = {
        .family                 = AF_INET,
        .checksum               = nf_ip_checksum,
        .checksum_partial       = nf_ip_checksum_partial,
        .route                  = nf_ip_route,
        .saveroute              = nf_ip_saveroute,
        .reroute                = nf_ip_reroute,
        .route_key_size         = sizeof(struct ip_rt_info),
};

static int ipv4_netfilter_init(void)
{
        return nf_register_afinfo(&nf_ip_afinfo);
}

static void ipv4_netfilter_fini(void)
{
        nf_unregister_afinfo(&nf_ip_afinfo);
}

module_init(ipv4_netfilter_init);
module_exit(ipv4_netfilter_fini);