// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
// Copyright (c) 2019, 2020 Cloudflare

#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>

#include <linux/bpf.h>
#include <linux/icmp.h>
#include <linux/icmpv6.h>
#include <linux/if_ether.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/pkt_cls.h>
#include <linux/tcp.h>
#include <linux/udp.h>

#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>

#include "test_cls_redirect.h"

#ifdef SUBPROGS
#define INLINING __noinline
#else
#define INLINING __always_inline
#endif

#define offsetofend(TYPE, MEMBER) \
        (offsetof(TYPE, MEMBER) + sizeof((((TYPE *)0)->MEMBER)))

#define IP_OFFSET_MASK (0x1FFF)
#define IP_MF (0x2000)

char _license[] SEC("license") = "Dual BSD/GPL";

/**
 * Destination port and IP used for UDP encapsulation.
 */
volatile const __be16 ENCAPSULATION_PORT;
volatile const __be32 ENCAPSULATION_IP;

typedef struct {
        uint64_t processed_packets_total;
        uint64_t l3_protocol_packets_total_ipv4;
        uint64_t l3_protocol_packets_total_ipv6;
        uint64_t l4_protocol_packets_total_tcp;
        uint64_t l4_protocol_packets_total_udp;
        uint64_t accepted_packets_total_syn;
        uint64_t accepted_packets_total_syn_cookies;
        uint64_t accepted_packets_total_last_hop;
        uint64_t accepted_packets_total_icmp_echo_request;
        uint64_t accepted_packets_total_established;
        uint64_t forwarded_packets_total_gue;
        uint64_t forwarded_packets_total_gre;

        uint64_t errors_total_unknown_l3_proto;
        uint64_t errors_total_unknown_l4_proto;
        uint64_t errors_total_malformed_ip;
        uint64_t errors_total_fragmented_ip;
        uint64_t errors_total_malformed_icmp;
        uint64_t errors_total_unwanted_icmp;
        uint64_t errors_total_malformed_icmp_pkt_too_big;
        uint64_t errors_total_malformed_tcp;
        uint64_t errors_total_malformed_udp;
        uint64_t errors_total_icmp_echo_replies;
        uint64_t errors_total_malformed_encapsulation;
        uint64_t errors_total_encap_adjust_failed;
        uint64_t errors_total_encap_buffer_too_small;
        uint64_t errors_total_redirect_loop;
        uint64_t errors_total_encap_mtu_violate;
} metrics_t;

typedef enum {
        INVALID = 0,
        UNKNOWN,
        ECHO_REQUEST,
        SYN,
        SYN_COOKIE,
        ESTABLISHED,
} verdict_t;

typedef struct {
        uint16_t src, dst;
} flow_ports_t;

_Static_assert(
        sizeof(flow_ports_t) !=
                offsetofend(struct bpf_sock_tuple, ipv4.dport) -
                        offsetof(struct bpf_sock_tuple, ipv4.sport) - 1,
        "flow_ports_t must match sport and dport in struct bpf_sock_tuple");
_Static_assert(
        sizeof(flow_ports_t) !=
                offsetofend(struct bpf_sock_tuple, ipv6.dport) -
                        offsetof(struct bpf_sock_tuple, ipv6.sport) - 1,
        "flow_ports_t must match sport and dport in struct bpf_sock_tuple");

typedef int ret_t;

/* This is a bit of a hack. We need a return value which allows us to
 * indicate that the regular flow of the program should continue,
 * while allowing functions to use XDP_PASS and XDP_DROP, etc.
 */
static const ret_t CONTINUE_PROCESSING = -1;

/* Convenience macro to call functions which return ret_t.
 */
#define MAYBE_RETURN(x)                           \
        do {                                      \
                ret_t __ret = x;                  \
                if (__ret != CONTINUE_PROCESSING) \
                        return __ret;             \
        } while (0)

/* Linux packet pointers are either aligned to NET_IP_ALIGN (aka 2 bytes),
 * or not aligned if the arch supports efficient unaligned access.
 *
 * Since the verifier ensures that eBPF packet accesses follow these rules,
 * we can tell LLVM to emit code as if we always had a larger alignment.
 * It will yell at us if we end up on a platform where this is not valid.
 */
typedef uint8_t *net_ptr __attribute__((align_value(8)));

typedef struct buf {
        struct __sk_buff *skb;
        net_ptr head;
        /* NB: tail musn't have alignment other than 1, otherwise
        * LLVM will go and eliminate code, e.g. when checking packet lengths.
        */
        uint8_t *const tail;
} buf_t;

static __always_inline size_t buf_off(const buf_t *buf)
{
        /* Clang seems to optimize constructs like
         *    a - b + c
         * if c is known:
         *    r? = c
         *    r? -= b
         *    r? += a
         *
         * This is a problem if a and b are packet pointers,
         * since the verifier allows subtracting two pointers to
         * get a scalar, but not a scalar and a pointer.
         *
         * Use inline asm to break this optimization.
         */
        size_t off = (size_t)buf->head;
        asm("%0 -= %1" : "+r"(off) : "r"(buf->skb->data));
        return off;
}

static __always_inline bool buf_copy(buf_t *buf, void *dst, size_t len)
{
        if (bpf_skb_load_bytes(buf->skb, buf_off(buf), dst, len)) {
                return false;
        }

        buf->head += len;
        return true;
}

static __always_inline bool buf_skip(buf_t *buf, const size_t len)
{
        /* Check whether off + len is valid in the non-linear part. */
        if (buf_off(buf) + len > buf->skb->len) {
                return false;
        }

        buf->head += len;
        return true;
}

/* Returns a pointer to the start of buf, or NULL if len is
 * larger than the remaining data. Consumes len bytes on a successful
 * call.
 *
 * If scratch is not NULL, the function will attempt to load non-linear
 * data via bpf_skb_load_bytes. On success, scratch is returned.
 */
static __always_inline void *buf_assign(buf_t *buf, const size_t len, void *scratch)
{
        if (buf->head + len > buf->tail) {
                if (scratch == NULL) {
                        return NULL;
                }

                return buf_copy(buf, scratch, len) ? scratch : NULL;
        }

        void *ptr = buf->head;
        buf->head += len;
        return ptr;
}

static INLINING bool pkt_skip_ipv4_options(buf_t *buf, const struct iphdr *ipv4)
{
        if (ipv4->ihl <= 5) {
                return true;
        }

        return buf_skip(buf, (ipv4->ihl - 5) * 4);
}

static INLINING bool ipv4_is_fragment(const struct iphdr *ip)
{
        uint16_t frag_off = ip->frag_off & bpf_htons(IP_OFFSET_MASK);
        return (ip->frag_off & bpf_htons(IP_MF)) != 0 || frag_off > 0;
}

static __always_inline struct iphdr *pkt_parse_ipv4(buf_t *pkt, struct iphdr *scratch)
{
        struct iphdr *ipv4 = buf_assign(pkt, sizeof(*ipv4), scratch);
        if (ipv4 == NULL) {
                return NULL;
        }

        if (ipv4->ihl < 5) {
                return NULL;
        }

        if (!pkt_skip_ipv4_options(pkt, ipv4)) {
                return NULL;
        }

        return ipv4;
}

/* Parse the L4 ports from a packet, assuming a layout like TCP or UDP. */
static INLINING bool pkt_parse_icmp_l4_ports(buf_t *pkt, flow_ports_t *ports)
{
        if (!buf_copy(pkt, ports, sizeof(*ports))) {
                return false;
        }

        /* Ports in the L4 headers are reversed, since we are parsing an ICMP
         * payload which is going towards the eyeball.
         */
        uint16_t dst = ports->src;
        ports->src = ports->dst;
        ports->dst = dst;
        return true;
}

static INLINING uint16_t pkt_checksum_fold(uint32_t csum)
{
        /* The highest reasonable value for an IPv4 header
         * checksum requires two folds, so we just do that always.
         */
        csum = (csum & 0xffff) + (csum >> 16);
        csum = (csum & 0xffff) + (csum >> 16);
        return (uint16_t)~csum;
}

static INLINING void pkt_ipv4_checksum(struct iphdr *iph)
{
        iph->check = 0;

        /* An IP header without options is 20 bytes. Two of those
         * are the checksum, which we always set to zero. Hence,
         * the maximum accumulated value is 18 / 2 * 0xffff = 0x8fff7,
         * which fits in 32 bit.
         */
        _Static_assert(sizeof(struct iphdr) == 20, "iphdr must be 20 bytes");
        uint32_t acc = 0;
        uint16_t *ipw = (uint16_t *)iph;

#pragma clang loop unroll(full)
        for (size_t i = 0; i < sizeof(struct iphdr) / 2; i++) {
                acc += ipw[i];
        }

        iph->check = pkt_checksum_fold(acc);
}

static INLINING
bool pkt_skip_ipv6_extension_headers(buf_t *pkt,
                                     const struct ipv6hdr *ipv6,
                                     uint8_t *upper_proto,
                                     bool *is_fragment)
{
        /* We understand five extension headers.
         * https://tools.ietf.org/html/rfc8200#section-4.1 states that all
         * headers should occur once, except Destination Options, which may
         * occur twice. Hence we give up after 6 headers.
         */
        struct {
                uint8_t next;
                uint8_t len;
        } exthdr = {
                .next = ipv6->nexthdr,
        };
        *is_fragment = false;

#pragma clang loop unroll(full)
        for (int i = 0; i < 6; i++) {
                switch (exthdr.next) {
                case IPPROTO_FRAGMENT:
                        *is_fragment = true;
                        /* NB: We don't check that hdrlen == 0 as per spec. */
                        /* fallthrough; */

                case IPPROTO_HOPOPTS:
                case IPPROTO_ROUTING:
                case IPPROTO_DSTOPTS:
                case IPPROTO_MH:
                        if (!buf_copy(pkt, &exthdr, sizeof(exthdr))) {
                                return false;
                        }

                        /* hdrlen is in 8-octet units, and excludes the first 8 octets. */
                        if (!buf_skip(pkt,
                                      (exthdr.len + 1) * 8 - sizeof(exthdr))) {
                                return false;
                        }

                        /* Decode next header */
                        break;

                default:
                        /* The next header is not one of the known extension
                         * headers, treat it as the upper layer header.
                         *
                         * This handles IPPROTO_NONE.
                         *
                         * Encapsulating Security Payload (50) and Authentication
                         * Header (51) also end up here (and will trigger an
                         * unknown proto error later). They have a custom header
                         * format and seem too esoteric to care about.
                         */
                        *upper_proto = exthdr.next;
                        return true;
                }
        }

        /* We never found an upper layer header. */
        return false;
}

/* This function has to be inlined, because the verifier otherwise rejects it
 * due to returning a pointer to the stack. This is technically correct, since
 * scratch is allocated on the stack. However, this usage should be safe since
 * it's the callers stack after all.
 */
static __always_inline struct ipv6hdr *
pkt_parse_ipv6(buf_t *pkt, struct ipv6hdr *scratch, uint8_t *proto,
               bool *is_fragment)
{
        struct ipv6hdr *ipv6 = buf_assign(pkt, sizeof(*ipv6), scratch);
        if (ipv6 == NULL) {
                return NULL;
        }

        if (!pkt_skip_ipv6_extension_headers(pkt, ipv6, proto, is_fragment)) {
                return NULL;
        }

        return ipv6;
}

/* Global metrics, per CPU
 */
struct {
        __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
        __uint(max_entries, 1);
        __type(key, unsigned int);
        __type(value, metrics_t);
} metrics_map SEC(".maps");

static INLINING metrics_t *get_global_metrics(void)
{
        uint64_t key = 0;
        return bpf_map_lookup_elem(&metrics_map, &key);
}

static INLINING ret_t accept_locally(struct __sk_buff *skb, encap_headers_t *encap)
{
        const int payload_off =
                sizeof(*encap) +
                sizeof(struct in_addr) * encap->unigue.hop_count;
        int32_t encap_overhead = payload_off - sizeof(struct ethhdr);

        // Changing the ethertype if the encapsulated packet is ipv6
        if (encap->gue.proto_ctype == IPPROTO_IPV6) {
                encap->eth.h_proto = bpf_htons(ETH_P_IPV6);
        }

        if (bpf_skb_adjust_room(skb, -encap_overhead, BPF_ADJ_ROOM_MAC,
                                BPF_F_ADJ_ROOM_FIXED_GSO |
                                BPF_F_ADJ_ROOM_NO_CSUM_RESET) ||
            bpf_csum_level(skb, BPF_CSUM_LEVEL_DEC))
                return TC_ACT_SHOT;

        return bpf_redirect(skb->ifindex, BPF_F_INGRESS);
}

static INLINING ret_t forward_with_gre(struct __sk_buff *skb, encap_headers_t *encap,
                                       struct in_addr *next_hop, metrics_t *metrics)
{
        metrics->forwarded_packets_total_gre++;

        const int payload_off =
                sizeof(*encap) +
                sizeof(struct in_addr) * encap->unigue.hop_count;
        int32_t encap_overhead =
                payload_off - sizeof(struct ethhdr) - sizeof(struct iphdr);
        int32_t delta = sizeof(struct gre_base_hdr) - encap_overhead;
        uint16_t proto = ETH_P_IP;
        uint32_t mtu_len = 0;

        /* Loop protection: the inner packet's TTL is decremented as a safeguard
         * against any forwarding loop. As the only interesting field is the TTL
         * hop limit for IPv6, it is easier to use bpf_skb_load_bytes/bpf_skb_store_bytes
         * as they handle the split packets if needed (no need for the data to be
         * in the linear section).
         */
        if (encap->gue.proto_ctype == IPPROTO_IPV6) {
                proto = ETH_P_IPV6;
                uint8_t ttl;
                int rc;

                rc = bpf_skb_load_bytes(
                        skb, payload_off + offsetof(struct ipv6hdr, hop_limit),
                        &ttl, 1);
                if (rc != 0) {
                        metrics->errors_total_malformed_encapsulation++;
                        return TC_ACT_SHOT;
                }

                if (ttl == 0) {
                        metrics->errors_total_redirect_loop++;
                        return TC_ACT_SHOT;
                }

                ttl--;
                rc = bpf_skb_store_bytes(
                        skb, payload_off + offsetof(struct ipv6hdr, hop_limit),
                        &ttl, 1, 0);
                if (rc != 0) {
                        metrics->errors_total_malformed_encapsulation++;
                        return TC_ACT_SHOT;
                }
        } else {
                uint8_t ttl;
                int rc;

                rc = bpf_skb_load_bytes(
                        skb, payload_off + offsetof(struct iphdr, ttl), &ttl,
                        1);
                if (rc != 0) {
                        metrics->errors_total_malformed_encapsulation++;
                        return TC_ACT_SHOT;
                }

                if (ttl == 0) {
                        metrics->errors_total_redirect_loop++;
                        return TC_ACT_SHOT;
                }

                /* IPv4 also has a checksum to patch. While the TTL is only one byte,
                 * this function only works for 2 and 4 bytes arguments (the result is
                 * the same).
                 */
                rc = bpf_l3_csum_replace(
                        skb, payload_off + offsetof(struct iphdr, check), ttl,
                        ttl - 1, 2);
                if (rc != 0) {
                        metrics->errors_total_malformed_encapsulation++;
                        return TC_ACT_SHOT;
                }

                ttl--;
                rc = bpf_skb_store_bytes(
                        skb, payload_off + offsetof(struct iphdr, ttl), &ttl, 1,
                        0);
                if (rc != 0) {
                        metrics->errors_total_malformed_encapsulation++;
                        return TC_ACT_SHOT;
                }
        }

        if (bpf_check_mtu(skb, skb->ifindex, &mtu_len, delta, 0)) {
                metrics->errors_total_encap_mtu_violate++;
                return TC_ACT_SHOT;
        }

        if (bpf_skb_adjust_room(skb, delta, BPF_ADJ_ROOM_NET,
                                BPF_F_ADJ_ROOM_FIXED_GSO |
                                BPF_F_ADJ_ROOM_NO_CSUM_RESET) ||
            bpf_csum_level(skb, BPF_CSUM_LEVEL_INC)) {
                metrics->errors_total_encap_adjust_failed++;
                return TC_ACT_SHOT;
        }

        if (bpf_skb_pull_data(skb, sizeof(encap_gre_t))) {
                metrics->errors_total_encap_buffer_too_small++;
                return TC_ACT_SHOT;
        }

        buf_t pkt = {
                .skb = skb,
                .head = (uint8_t *)(long)skb->data,
                .tail = (uint8_t *)(long)skb->data_end,
        };

        encap_gre_t *encap_gre = buf_assign(&pkt, sizeof(encap_gre_t), NULL);
        if (encap_gre == NULL) {
                metrics->errors_total_encap_buffer_too_small++;
                return TC_ACT_SHOT;
        }

        encap_gre->ip.protocol = IPPROTO_GRE;
        encap_gre->ip.daddr = next_hop->s_addr;
        encap_gre->ip.saddr = ENCAPSULATION_IP;
        encap_gre->ip.tot_len =
                bpf_htons(bpf_ntohs(encap_gre->ip.tot_len) + delta);
        encap_gre->gre.flags = 0;
        encap_gre->gre.protocol = bpf_htons(proto);
        pkt_ipv4_checksum((void *)&encap_gre->ip);

        return bpf_redirect(skb->ifindex, 0);
}

static INLINING ret_t forward_to_next_hop(struct __sk_buff *skb, encap_headers_t *encap,
                                          struct in_addr *next_hop, metrics_t *metrics)
{
        /* swap L2 addresses */
        /* This assumes that packets are received from a router.
         * So just swapping the MAC addresses here will make the packet go back to
         * the router, which will send it to the appropriate machine.
         */
        unsigned char temp[ETH_ALEN];
        memcpy(temp, encap->eth.h_dest, sizeof(temp));
        memcpy(encap->eth.h_dest, encap->eth.h_source,
               sizeof(encap->eth.h_dest));
        memcpy(encap->eth.h_source, temp, sizeof(encap->eth.h_source));

        if (encap->unigue.next_hop == encap->unigue.hop_count - 1 &&
            encap->unigue.last_hop_gre) {
                return forward_with_gre(skb, encap, next_hop, metrics);
        }

        metrics->forwarded_packets_total_gue++;
        uint32_t old_saddr = encap->ip.saddr;
        encap->ip.saddr = encap->ip.daddr;
        encap->ip.daddr = next_hop->s_addr;
        if (encap->unigue.next_hop < encap->unigue.hop_count) {
                encap->unigue.next_hop++;
        }

        /* Remove ip->saddr, add next_hop->s_addr */
        const uint64_t off = offsetof(typeof(*encap), ip.check);
        int ret = bpf_l3_csum_replace(skb, off, old_saddr, next_hop->s_addr, 4);
        if (ret < 0) {
                return TC_ACT_SHOT;
        }

        return bpf_redirect(skb->ifindex, 0);
}

static INLINING ret_t skip_next_hops(buf_t *pkt, int n)
{
        switch (n) {
        case 1:
                if (!buf_skip(pkt, sizeof(struct in_addr)))
                        return TC_ACT_SHOT;
        case 0:
                return CONTINUE_PROCESSING;

        default:
                return TC_ACT_SHOT;
        }
}

/* Get the next hop from the GLB header.
 *
 * Sets next_hop->s_addr to 0 if there are no more hops left.
 * pkt is positioned just after the variable length GLB header
 * iff the call is successful.
 */
static INLINING ret_t get_next_hop(buf_t *pkt, encap_headers_t *encap,
                                   struct in_addr *next_hop)
{
        if (encap->unigue.next_hop > encap->unigue.hop_count) {
                return TC_ACT_SHOT;
        }

        /* Skip "used" next hops. */
        MAYBE_RETURN(skip_next_hops(pkt, encap->unigue.next_hop));

        if (encap->unigue.next_hop == encap->unigue.hop_count) {
                /* No more next hops, we are at the end of the GLB header. */
                next_hop->s_addr = 0;
                return CONTINUE_PROCESSING;
        }

        if (!buf_copy(pkt, next_hop, sizeof(*next_hop))) {
                return TC_ACT_SHOT;
        }

        /* Skip the remainig next hops (may be zero). */
        return skip_next_hops(pkt, encap->unigue.hop_count -
                                           encap->unigue.next_hop - 1);
}

/* Fill a bpf_sock_tuple to be used with the socket lookup functions.
 * This is a kludge that let's us work around verifier limitations:
 *
 *    fill_tuple(&t, foo, sizeof(struct iphdr), 123, 321)
 *
 * clang will substitue a costant for sizeof, which allows the verifier
 * to track it's value. Based on this, it can figure out the constant
 * return value, and calling code works while still being "generic" to
 * IPv4 and IPv6.
 */
static INLINING uint64_t fill_tuple(struct bpf_sock_tuple *tuple, void *iph,
                                    uint64_t iphlen, uint16_t sport, uint16_t dport)
{
        switch (iphlen) {
        case sizeof(struct iphdr): {
                struct iphdr *ipv4 = (struct iphdr *)iph;
                tuple->ipv4.daddr = ipv4->daddr;
                tuple->ipv4.saddr = ipv4->saddr;
                tuple->ipv4.sport = sport;
                tuple->ipv4.dport = dport;
                return sizeof(tuple->ipv4);
        }

        case sizeof(struct ipv6hdr): {
                struct ipv6hdr *ipv6 = (struct ipv6hdr *)iph;
                memcpy(&tuple->ipv6.daddr, &ipv6->daddr,
                       sizeof(tuple->ipv6.daddr));
                memcpy(&tuple->ipv6.saddr, &ipv6->saddr,
                       sizeof(tuple->ipv6.saddr));
                tuple->ipv6.sport = sport;
                tuple->ipv6.dport = dport;
                return sizeof(tuple->ipv6);
        }

        default:
                return 0;
        }
}

static INLINING verdict_t classify_tcp(struct __sk_buff *skb,
                                       struct bpf_sock_tuple *tuple, uint64_t tuplen,
                                       void *iph, struct tcphdr *tcp)
{
        struct bpf_sock *sk =
                bpf_skc_lookup_tcp(skb, tuple, tuplen, BPF_F_CURRENT_NETNS, 0);
        if (sk == NULL) {
                return UNKNOWN;
        }

        if (sk->state != BPF_TCP_LISTEN) {
                bpf_sk_release(sk);
                return ESTABLISHED;
        }

        if (iph != NULL && tcp != NULL) {
                /* Kludge: we've run out of arguments, but need the length of the ip header. */
                uint64_t iphlen = sizeof(struct iphdr);
                if (tuplen == sizeof(tuple->ipv6)) {
                        iphlen = sizeof(struct ipv6hdr);
                }

                if (bpf_tcp_check_syncookie(sk, iph, iphlen, tcp,
                                            sizeof(*tcp)) == 0) {
                        bpf_sk_release(sk);
                        return SYN_COOKIE;
                }
        }

        bpf_sk_release(sk);
        return UNKNOWN;
}

static INLINING verdict_t classify_udp(struct __sk_buff *skb,
                                       struct bpf_sock_tuple *tuple, uint64_t tuplen)
{
        struct bpf_sock *sk =
                bpf_sk_lookup_udp(skb, tuple, tuplen, BPF_F_CURRENT_NETNS, 0);
        if (sk == NULL) {
                return UNKNOWN;
        }

        if (sk->state == BPF_TCP_ESTABLISHED) {
                bpf_sk_release(sk);
                return ESTABLISHED;
        }

        bpf_sk_release(sk);
        return UNKNOWN;
}

static INLINING verdict_t classify_icmp(struct __sk_buff *skb, uint8_t proto,
                                        struct bpf_sock_tuple *tuple, uint64_t tuplen,
                                        metrics_t *metrics)
{
        switch (proto) {
        case IPPROTO_TCP:
                return classify_tcp(skb, tuple, tuplen, NULL, NULL);

        case IPPROTO_UDP:
                return classify_udp(skb, tuple, tuplen);

        default:
                metrics->errors_total_malformed_icmp++;
                return INVALID;
        }
}

static INLINING verdict_t process_icmpv4(buf_t *pkt, metrics_t *metrics)
{
        struct icmphdr icmp;
        if (!buf_copy(pkt, &icmp, sizeof(icmp))) {
                metrics->errors_total_malformed_icmp++;
                return INVALID;
        }

        /* We should never receive encapsulated echo replies. */
        if (icmp.type == ICMP_ECHOREPLY) {
                metrics->errors_total_icmp_echo_replies++;
                return INVALID;
        }

        if (icmp.type == ICMP_ECHO) {
                return ECHO_REQUEST;
        }

        if (icmp.type != ICMP_DEST_UNREACH || icmp.code != ICMP_FRAG_NEEDED) {
                metrics->errors_total_unwanted_icmp++;
                return INVALID;
        }

        struct iphdr _ip4;
        const struct iphdr *ipv4 = pkt_parse_ipv4(pkt, &_ip4);
        if (ipv4 == NULL) {
                metrics->errors_total_malformed_icmp_pkt_too_big++;
                return INVALID;
        }

        /* The source address in the outer IP header is from the entity that
         * originated the ICMP message. Use the original IP header to restore
         * the correct flow tuple.
         */
        struct bpf_sock_tuple tuple;
        tuple.ipv4.saddr = ipv4->daddr;
        tuple.ipv4.daddr = ipv4->saddr;

        if (!pkt_parse_icmp_l4_ports(pkt, (flow_ports_t *)&tuple.ipv4.sport)) {
                metrics->errors_total_malformed_icmp_pkt_too_big++;
                return INVALID;
        }

        return classify_icmp(pkt->skb, ipv4->protocol, &tuple,
                             sizeof(tuple.ipv4), metrics);
}

static INLINING verdict_t process_icmpv6(buf_t *pkt, metrics_t *metrics)
{
        struct icmp6hdr icmp6;
        if (!buf_copy(pkt, &icmp6, sizeof(icmp6))) {
                metrics->errors_total_malformed_icmp++;
                return INVALID;
        }

        /* We should never receive encapsulated echo replies. */
        if (icmp6.icmp6_type == ICMPV6_ECHO_REPLY) {
                metrics->errors_total_icmp_echo_replies++;
                return INVALID;
        }

        if (icmp6.icmp6_type == ICMPV6_ECHO_REQUEST) {
                return ECHO_REQUEST;
        }

        if (icmp6.icmp6_type != ICMPV6_PKT_TOOBIG) {
                metrics->errors_total_unwanted_icmp++;
                return INVALID;
        }

        bool is_fragment;
        uint8_t l4_proto;
        struct ipv6hdr _ipv6;
        const struct ipv6hdr *ipv6 =
                pkt_parse_ipv6(pkt, &_ipv6, &l4_proto, &is_fragment);
        if (ipv6 == NULL) {
                metrics->errors_total_malformed_icmp_pkt_too_big++;
                return INVALID;
        }

        if (is_fragment) {
                metrics->errors_total_fragmented_ip++;
                return INVALID;
        }

        /* Swap source and dest addresses. */
        struct bpf_sock_tuple tuple;
        memcpy(&tuple.ipv6.saddr, &ipv6->daddr, sizeof(tuple.ipv6.saddr));
        memcpy(&tuple.ipv6.daddr, &ipv6->saddr, sizeof(tuple.ipv6.daddr));

        if (!pkt_parse_icmp_l4_ports(pkt, (flow_ports_t *)&tuple.ipv6.sport)) {
                metrics->errors_total_malformed_icmp_pkt_too_big++;
                return INVALID;
        }

        return classify_icmp(pkt->skb, l4_proto, &tuple, sizeof(tuple.ipv6),
                             metrics);
}

static INLINING verdict_t process_tcp(buf_t *pkt, void *iph, uint64_t iphlen,
                                      metrics_t *metrics)
{
        metrics->l4_protocol_packets_total_tcp++;

        struct tcphdr _tcp;
        struct tcphdr *tcp = buf_assign(pkt, sizeof(_tcp), &_tcp);
        if (tcp == NULL) {
                metrics->errors_total_malformed_tcp++;
                return INVALID;
        }

        if (tcp->syn) {
                return SYN;
        }

        struct bpf_sock_tuple tuple;
        uint64_t tuplen =
                fill_tuple(&tuple, iph, iphlen, tcp->source, tcp->dest);
        return classify_tcp(pkt->skb, &tuple, tuplen, iph, tcp);
}

static INLINING verdict_t process_udp(buf_t *pkt, void *iph, uint64_t iphlen,
                                      metrics_t *metrics)
{
        metrics->l4_protocol_packets_total_udp++;

        struct udphdr _udp;
        struct udphdr *udph = buf_assign(pkt, sizeof(_udp), &_udp);
        if (udph == NULL) {
                metrics->errors_total_malformed_udp++;
                return INVALID;
        }

        struct bpf_sock_tuple tuple;
        uint64_t tuplen =
                fill_tuple(&tuple, iph, iphlen, udph->source, udph->dest);
        return classify_udp(pkt->skb, &tuple, tuplen);
}

static INLINING verdict_t process_ipv4(buf_t *pkt, metrics_t *metrics)
{
        metrics->l3_protocol_packets_total_ipv4++;

        struct iphdr _ip4;
        struct iphdr *ipv4 = pkt_parse_ipv4(pkt, &_ip4);
        if (ipv4 == NULL) {
                metrics->errors_total_malformed_ip++;
                return INVALID;
        }

        if (ipv4->version != 4) {
                metrics->errors_total_malformed_ip++;
                return INVALID;
        }

        if (ipv4_is_fragment(ipv4)) {
                metrics->errors_total_fragmented_ip++;
                return INVALID;
        }

        switch (ipv4->protocol) {
        case IPPROTO_ICMP:
                return process_icmpv4(pkt, metrics);

        case IPPROTO_TCP:
                return process_tcp(pkt, ipv4, sizeof(*ipv4), metrics);

        case IPPROTO_UDP:
                return process_udp(pkt, ipv4, sizeof(*ipv4), metrics);

        default:
                metrics->errors_total_unknown_l4_proto++;
                return INVALID;
        }
}

static INLINING verdict_t process_ipv6(buf_t *pkt, metrics_t *metrics)
{
        metrics->l3_protocol_packets_total_ipv6++;

        uint8_t l4_proto;
        bool is_fragment;
        struct ipv6hdr _ipv6;
        struct ipv6hdr *ipv6 =
                pkt_parse_ipv6(pkt, &_ipv6, &l4_proto, &is_fragment);
        if (ipv6 == NULL) {
                metrics->errors_total_malformed_ip++;
                return INVALID;
        }

        if (ipv6->version != 6) {
                metrics->errors_total_malformed_ip++;
                return INVALID;
        }

        if (is_fragment) {
                metrics->errors_total_fragmented_ip++;
                return INVALID;
        }

        switch (l4_proto) {
        case IPPROTO_ICMPV6:
                return process_icmpv6(pkt, metrics);

        case IPPROTO_TCP:
                return process_tcp(pkt, ipv6, sizeof(*ipv6), metrics);

        case IPPROTO_UDP:
                return process_udp(pkt, ipv6, sizeof(*ipv6), metrics);

        default:
                metrics->errors_total_unknown_l4_proto++;
                return INVALID;
        }
}

SEC("classifier/cls_redirect")
int cls_redirect(struct __sk_buff *skb)
{
        metrics_t *metrics = get_global_metrics();
        if (metrics == NULL) {
                return TC_ACT_SHOT;
        }

        metrics->processed_packets_total++;

        /* Pass bogus packets as long as we're not sure they're
         * destined for us.
         */
        if (skb->protocol != bpf_htons(ETH_P_IP)) {
                return TC_ACT_OK;
        }

        encap_headers_t *encap;

        /* Make sure that all encapsulation headers are available in
         * the linear portion of the skb. This makes it easy to manipulate them.
         */
        if (bpf_skb_pull_data(skb, sizeof(*encap))) {
                return TC_ACT_OK;
        }

        buf_t pkt = {
                .skb = skb,
                .head = (uint8_t *)(long)skb->data,
                .tail = (uint8_t *)(long)skb->data_end,
        };

        encap = buf_assign(&pkt, sizeof(*encap), NULL);
        if (encap == NULL) {
                return TC_ACT_OK;
        }

        if (encap->ip.ihl != 5) {
                /* We never have any options. */
                return TC_ACT_OK;
        }

        if (encap->ip.daddr != ENCAPSULATION_IP ||
            encap->ip.protocol != IPPROTO_UDP) {
                return TC_ACT_OK;
        }

        /* TODO Check UDP length? */
        if (encap->udp.dest != ENCAPSULATION_PORT) {
                return TC_ACT_OK;
        }

        /* We now know that the packet is destined to us, we can
         * drop bogus ones.
         */
        if (ipv4_is_fragment((void *)&encap->ip)) {
                metrics->errors_total_fragmented_ip++;
                return TC_ACT_SHOT;
        }

        if (encap->gue.variant != 0) {
                metrics->errors_total_malformed_encapsulation++;
                return TC_ACT_SHOT;
        }

        if (encap->gue.control != 0) {
                metrics->errors_total_malformed_encapsulation++;
                return TC_ACT_SHOT;
        }

        if (encap->gue.flags != 0) {
                metrics->errors_total_malformed_encapsulation++;
                return TC_ACT_SHOT;
        }

        if (encap->gue.hlen !=
            sizeof(encap->unigue) / 4 + encap->unigue.hop_count) {
                metrics->errors_total_malformed_encapsulation++;
                return TC_ACT_SHOT;
        }

        if (encap->unigue.version != 0) {
                metrics->errors_total_malformed_encapsulation++;
                return TC_ACT_SHOT;
        }

        if (encap->unigue.reserved != 0) {
                return TC_ACT_SHOT;
        }

        struct in_addr next_hop;
        MAYBE_RETURN(get_next_hop(&pkt, encap, &next_hop));

        if (next_hop.s_addr == 0) {
                metrics->accepted_packets_total_last_hop++;
                return accept_locally(skb, encap);
        }

        verdict_t verdict;
        switch (encap->gue.proto_ctype) {
        case IPPROTO_IPIP:
                verdict = process_ipv4(&pkt, metrics);
                break;

        case IPPROTO_IPV6:
                verdict = process_ipv6(&pkt, metrics);
                break;

        default:
                metrics->errors_total_unknown_l3_proto++;
                return TC_ACT_SHOT;
        }

        switch (verdict) {
        case INVALID:
                /* metrics have already been bumped */
                return TC_ACT_SHOT;

        case UNKNOWN:
                return forward_to_next_hop(skb, encap, &next_hop, metrics);

        case ECHO_REQUEST:
                metrics->accepted_packets_total_icmp_echo_request++;
                break;

        case SYN:
                if (encap->unigue.forward_syn) {
                        return forward_to_next_hop(skb, encap, &next_hop,
                                                   metrics);
                }

                metrics->accepted_packets_total_syn++;
                break;

        case SYN_COOKIE:
                metrics->accepted_packets_total_syn_cookies++;
                break;

        case ESTABLISHED:
                metrics->accepted_packets_total_established++;
                break;
        }

        return accept_locally(skb, encap);
}