// SPDX-License-Identifier: GPL-2.0
/*
 * 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 IP forwarding functionality.
 *
 * Authors:     see ip.c
 *
 * Fixes:
 *              Many            :       Split from ip.c , see ip_input.c for
 *                                      history.
 *              Dave Gregorich  :       NULL ip_rt_put fix for multicast
 *                                      routing.
 *              Jos Vos         :       Add call_out_firewall before sending,
 *                                      use output device for accounting.
 *              Jos Vos         :       Call forward firewall after routing
 *                                      (always use output device).
 *              Mike McLagan    :       Routing by source
 */

#include <linux/types.h>
#include <linux/mm.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/icmp.h>
#include <linux/netdevice.h>
#include <linux/slab.h>
#include <net/sock.h>
#include <net/ip.h>
#include <net/tcp.h>
#include <net/udp.h>
#include <net/icmp.h>
#include <linux/tcp.h>
#include <linux/udp.h>
#include <linux/netfilter_ipv4.h>
#include <net/checksum.h>
#include <linux/route.h>
#include <net/route.h>
#include <net/xfrm.h>

static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
{
        if (skb->len <= mtu)
                return false;

        if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0))
                return false;

        /* original fragment exceeds mtu and DF is set */
        if (unlikely(IPCB(skb)->frag_max_size > mtu))
                return true;

        if (skb->ignore_df)
                return false;

        if (skb_is_gso(skb) && skb_gso_validate_network_len(skb, mtu))
                return false;

        return true;
}


static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
{
        struct ip_options *opt  = &(IPCB(skb)->opt);

#ifdef CONFIG_NET_SWITCHDEV
        if (skb->offload_l3_fwd_mark) {
                consume_skb(skb);
                return 0;
        }
#endif

        if (unlikely(opt->optlen))
                ip_forward_options(skb);

        skb_clear_tstamp(skb);
        return dst_output(net, sk, skb);
}

int ip_forward(struct sk_buff *skb)
{
        u32 mtu;
        struct iphdr *iph;      /* Our header */
        struct rtable *rt;      /* Route we use */
        struct ip_options *opt  = &(IPCB(skb)->opt);
        struct net *net;
        SKB_DR(reason);

        /* that should never happen */
        if (skb->pkt_type != PACKET_HOST)
                goto drop;

        if (unlikely(skb->sk))
                goto drop;

        if (skb_warn_if_lro(skb))
                goto drop;

        if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)) {
                SKB_DR_SET(reason, XFRM_POLICY);
                goto drop;
        }

        if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
                return NET_RX_SUCCESS;

        skb_forward_csum(skb);
        net = dev_net(skb->dev);

        /*
         *      According to the RFC, we must first decrease the TTL field. If
         *      that reaches zero, we must reply an ICMP control message telling
         *      that the packet's lifetime expired.
         */
        if (ip_hdr(skb)->ttl <= 1)
                goto too_many_hops;

        if (!xfrm4_route_forward(skb)) {
                SKB_DR_SET(reason, XFRM_POLICY);
                goto drop;
        }

        rt = skb_rtable(skb);

        if (opt->is_strictroute && rt->rt_uses_gateway)
                goto sr_failed;

        __IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);

        IPCB(skb)->flags |= IPSKB_FORWARDED;
        mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
        if (ip_exceeds_mtu(skb, mtu)) {
                IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
                icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
                          htonl(mtu));
                SKB_DR_SET(reason, PKT_TOO_BIG);
                goto drop;
        }

        /* We are about to mangle packet. Copy it! */
        if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
                goto drop;
        iph = ip_hdr(skb);

        /* Decrease ttl after skb cow done */
        ip_decrease_ttl(iph);

        /*
         *      We now generate an ICMP HOST REDIRECT giving the route
         *      we calculated.
         */
        if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
            !skb_sec_path(skb))
                ip_rt_send_redirect(skb);

        if (READ_ONCE(net->ipv4.sysctl_ip_fwd_update_priority))
                skb->priority = rt_tos2priority(iph->tos);

        return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
                       net, NULL, skb, skb->dev, rt->dst.dev,
                       ip_forward_finish);

sr_failed:
        /*
         *      Strict routing permits no gatewaying
         */
         icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
         goto drop;

too_many_hops:
        /* Tell the sender its packet died... */
        __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
        icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
        SKB_DR_SET(reason, IP_INHDR);
drop:
        kfree_skb_reason(skb, reason);
        return NET_RX_DROP;
}