Merge branch 'cloudflare-prog'

Lorenz Bauer says:

====================
We've been developing an in-house L4 load balancer based on XDP
and TC for a while. Following Alexei's call for more up-to-date examples of
production BPF in the kernel tree [1], Cloudflare is making this available
under dual GPL-2.0 or BSD 3-clause terms.

The code requires at least v5.3 to function correctly.

1: https://lore.kernel.org/bpf/20200326210719.den5isqxntnoqhmv@ast-mbp/
====================

Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/cls_redirect.c

···
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+// Copyright (c) 2020 Cloudflare
+
+#define _GNU_SOURCE
+
+#include <arpa/inet.h>
+#include <string.h>
+
+#include <linux/pkt_cls.h>
+
+#include <test_progs.h>
+
+#include "progs/test_cls_redirect.h"
+#include "test_cls_redirect.skel.h"
+
+#define ENCAP_IP INADDR_LOOPBACK
+#define ENCAP_PORT (1234)
+
+struct addr_port {
+	in_port_t port;
+	union {
+		struct in_addr in_addr;
+		struct in6_addr in6_addr;
+	};
+};
+
+struct tuple {
+	int family;
+	struct addr_port src;
+	struct addr_port dst;
+};
+
+static int start_server(const struct sockaddr *addr, socklen_t len, int type)
+{
+	int fd = socket(addr->sa_family, type, 0);
+	if (CHECK_FAIL(fd == -1))
+		return -1;
+	if (CHECK_FAIL(bind(fd, addr, len) == -1))
+		goto err;
+	if (type == SOCK_STREAM && CHECK_FAIL(listen(fd, 128) == -1))
+		goto err;
+
+	return fd;
+
+err:
+	close(fd);
+	return -1;
+}
+
+static int connect_to_server(const struct sockaddr *addr, socklen_t len,
+			     int type)
+{
+	int fd = socket(addr->sa_family, type, 0);
+	if (CHECK_FAIL(fd == -1))
+		return -1;
+	if (CHECK_FAIL(connect(fd, addr, len)))
+		goto err;
+
+	return fd;
+
+err:
+	close(fd);
+	return -1;
+}
+
+static bool fill_addr_port(const struct sockaddr *sa, struct addr_port *ap)
+{
+	const struct sockaddr_in6 *in6;
+	const struct sockaddr_in *in;
+
+	switch (sa->sa_family) {
+	case AF_INET:
+		in = (const struct sockaddr_in *)sa;
+		ap->in_addr = in->sin_addr;
+		ap->port = in->sin_port;
+		return true;
+
+	case AF_INET6:
+		in6 = (const struct sockaddr_in6 *)sa;
+		ap->in6_addr = in6->sin6_addr;
+		ap->port = in6->sin6_port;
+		return true;
+
+	default:
+		return false;
+	}
+}
+
+static bool set_up_conn(const struct sockaddr *addr, socklen_t len, int type,
+			int *server, int *conn, struct tuple *tuple)
+{
+	struct sockaddr_storage ss;
+	socklen_t slen = sizeof(ss);
+	struct sockaddr *sa = (struct sockaddr *)&ss;
+
+	*server = start_server(addr, len, type);
+	if (*server < 0)
+		return false;
+
+	if (CHECK_FAIL(getsockname(*server, sa, &slen)))
+		goto close_server;
+
+	*conn = connect_to_server(sa, slen, type);
+	if (*conn < 0)
+		goto close_server;
+
+	/* We want to simulate packets arriving at conn, so we have to
+	 * swap src and dst.
+	 */
+	slen = sizeof(ss);
+	if (CHECK_FAIL(getsockname(*conn, sa, &slen)))
+		goto close_conn;
+
+	if (CHECK_FAIL(!fill_addr_port(sa, &tuple->dst)))
+		goto close_conn;
+
+	slen = sizeof(ss);
+	if (CHECK_FAIL(getpeername(*conn, sa, &slen)))
+		goto close_conn;
+
+	if (CHECK_FAIL(!fill_addr_port(sa, &tuple->src)))
+		goto close_conn;
+
+	tuple->family = ss.ss_family;
+	return true;
+
+close_conn:
+	close(*conn);
+	*conn = -1;
+close_server:
+	close(*server);
+	*server = -1;
+	return false;
+}
+
+static socklen_t prepare_addr(struct sockaddr_storage *addr, int family)
+{
+	struct sockaddr_in *addr4;
+	struct sockaddr_in6 *addr6;
+
+	switch (family) {
+	case AF_INET:
+		addr4 = (struct sockaddr_in *)addr;
+		memset(addr4, 0, sizeof(*addr4));
+		addr4->sin_family = family;
+		addr4->sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+		return sizeof(*addr4);
+	case AF_INET6:
+		addr6 = (struct sockaddr_in6 *)addr;
+		memset(addr6, 0, sizeof(*addr6));
+		addr6->sin6_family = family;
+		addr6->sin6_addr = in6addr_loopback;
+		return sizeof(*addr6);
+	default:
+		fprintf(stderr, "Invalid family %d", family);
+		return 0;
+	}
+}
+
+static bool was_decapsulated(struct bpf_prog_test_run_attr *tattr)
+{
+	return tattr->data_size_out < tattr->data_size_in;
+}
+
+enum type {
+	UDP,
+	TCP,
+	__NR_KIND,
+};
+
+enum hops {
+	NO_HOPS,
+	ONE_HOP,
+};
+
+enum flags {
+	NONE,
+	SYN,
+	ACK,
+};
+
+enum conn {
+	KNOWN_CONN,
+	UNKNOWN_CONN,
+};
+
+enum result {
+	ACCEPT,
+	FORWARD,
+};
+
+struct test_cfg {
+	enum type type;
+	enum result result;
+	enum conn conn;
+	enum hops hops;
+	enum flags flags;
+};
+
+static int test_str(void *buf, size_t len, const struct test_cfg *test,
+		    int family)
+{
+	const char *family_str, *type, *conn, *hops, *result, *flags;
+
+	family_str = "IPv4";
+	if (family == AF_INET6)
+		family_str = "IPv6";
+
+	type = "TCP";
+	if (test->type == UDP)
+		type = "UDP";
+
+	conn = "known";
+	if (test->conn == UNKNOWN_CONN)
+		conn = "unknown";
+
+	hops = "no hops";
+	if (test->hops == ONE_HOP)
+		hops = "one hop";
+
+	result = "accept";
+	if (test->result == FORWARD)
+		result = "forward";
+
+	flags = "none";
+	if (test->flags == SYN)
+		flags = "SYN";
+	else if (test->flags == ACK)
+		flags = "ACK";
+
+	return snprintf(buf, len, "%s %s %s %s (%s, flags: %s)", family_str,
+			type, result, conn, hops, flags);
+}
+
+static struct test_cfg tests[] = {
+	{ TCP, ACCEPT, UNKNOWN_CONN, NO_HOPS, SYN },
+	{ TCP, ACCEPT, UNKNOWN_CONN, NO_HOPS, ACK },
+	{ TCP, FORWARD, UNKNOWN_CONN, ONE_HOP, ACK },
+	{ TCP, ACCEPT, KNOWN_CONN, ONE_HOP, ACK },
+	{ UDP, ACCEPT, UNKNOWN_CONN, NO_HOPS, NONE },
+	{ UDP, FORWARD, UNKNOWN_CONN, ONE_HOP, NONE },
+	{ UDP, ACCEPT, KNOWN_CONN, ONE_HOP, NONE },
+};
+
+static void encap_init(encap_headers_t *encap, uint8_t hop_count, uint8_t proto)
+{
+	const uint8_t hlen =
+		(sizeof(struct guehdr) / sizeof(uint32_t)) + hop_count;
+	*encap = (encap_headers_t){
+		.eth = { .h_proto = htons(ETH_P_IP) },
+		.ip = {
+			.ihl = 5,
+			.version = 4,
+			.ttl = IPDEFTTL,
+			.protocol = IPPROTO_UDP,
+			.daddr = htonl(ENCAP_IP)
+		},
+		.udp = {
+			.dest = htons(ENCAP_PORT),
+		},
+		.gue = {
+			.hlen = hlen,
+			.proto_ctype = proto
+		},
+		.unigue = {
+			.hop_count = hop_count
+		},
+	};
+}
+
+static size_t build_input(const struct test_cfg *test, void *const buf,
+			  const struct tuple *tuple)
+{
+	in_port_t sport = tuple->src.port;
+	encap_headers_t encap;
+	struct iphdr ip;
+	struct ipv6hdr ipv6;
+	struct tcphdr tcp;
+	struct udphdr udp;
+	struct in_addr next_hop;
+	uint8_t *p = buf;
+	int proto;
+
+	proto = IPPROTO_IPIP;
+	if (tuple->family == AF_INET6)
+		proto = IPPROTO_IPV6;
+
+	encap_init(&encap, test->hops == ONE_HOP ? 1 : 0, proto);
+	p = mempcpy(p, &encap, sizeof(encap));
+
+	if (test->hops == ONE_HOP) {
+		next_hop = (struct in_addr){ .s_addr = htonl(0x7f000002) };
+		p = mempcpy(p, &next_hop, sizeof(next_hop));
+	}
+
+	proto = IPPROTO_TCP;
+	if (test->type == UDP)
+		proto = IPPROTO_UDP;
+
+	switch (tuple->family) {
+	case AF_INET:
+		ip = (struct iphdr){
+			.ihl = 5,
+			.version = 4,
+			.ttl = IPDEFTTL,
+			.protocol = proto,
+			.saddr = tuple->src.in_addr.s_addr,
+			.daddr = tuple->dst.in_addr.s_addr,
+		};
+		p = mempcpy(p, &ip, sizeof(ip));
+		break;
+	case AF_INET6:
+		ipv6 = (struct ipv6hdr){
+			.version = 6,
+			.hop_limit = IPDEFTTL,
+			.nexthdr = proto,
+			.saddr = tuple->src.in6_addr,
+			.daddr = tuple->dst.in6_addr,
+		};
+		p = mempcpy(p, &ipv6, sizeof(ipv6));
+		break;
+	default:
+		return 0;
+	}
+
+	if (test->conn == UNKNOWN_CONN)
+		sport--;
+
+	switch (test->type) {
+	case TCP:
+		tcp = (struct tcphdr){
+			.source = sport,
+			.dest = tuple->dst.port,
+		};
+		if (test->flags == SYN)
+			tcp.syn = true;
+		if (test->flags == ACK)
+			tcp.ack = true;
+		p = mempcpy(p, &tcp, sizeof(tcp));
+		break;
+	case UDP:
+		udp = (struct udphdr){
+			.source = sport,
+			.dest = tuple->dst.port,
+		};
+		p = mempcpy(p, &udp, sizeof(udp));
+		break;
+	default:
+		return 0;
+	}
+
+	return (void *)p - buf;
+}
+
+static void close_fds(int *fds, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++)
+		if (fds[i] > 0)
+			close(fds[i]);
+}
+
+void test_cls_redirect(void)
+{
+	struct test_cls_redirect *skel = NULL;
+	struct bpf_prog_test_run_attr tattr = {};
+	int families[] = { AF_INET, AF_INET6 };
+	struct sockaddr_storage ss;
+	struct sockaddr *addr;
+	socklen_t slen;
+	int i, j, err;
+
+	int servers[__NR_KIND][ARRAY_SIZE(families)] = {};
+	int conns[__NR_KIND][ARRAY_SIZE(families)] = {};
+	struct tuple tuples[__NR_KIND][ARRAY_SIZE(families)];
+
+	skel = test_cls_redirect__open();
+	if (CHECK_FAIL(!skel))
+		return;
+
+	skel->rodata->ENCAPSULATION_IP = htonl(ENCAP_IP);
+	skel->rodata->ENCAPSULATION_PORT = htons(ENCAP_PORT);
+
+	if (CHECK_FAIL(test_cls_redirect__load(skel)))
+		goto cleanup;
+
+	addr = (struct sockaddr *)&ss;
+	for (i = 0; i < ARRAY_SIZE(families); i++) {
+		slen = prepare_addr(&ss, families[i]);
+		if (CHECK_FAIL(!slen))
+			goto cleanup;
+
+		if (CHECK_FAIL(!set_up_conn(addr, slen, SOCK_DGRAM,
+					    &servers[UDP][i], &conns[UDP][i],
+					    &tuples[UDP][i])))
+			goto cleanup;
+
+		if (CHECK_FAIL(!set_up_conn(addr, slen, SOCK_STREAM,
+					    &servers[TCP][i], &conns[TCP][i],
+					    &tuples[TCP][i])))
+			goto cleanup;
+	}
+
+	tattr.prog_fd = bpf_program__fd(skel->progs.cls_redirect);
+	for (i = 0; i < ARRAY_SIZE(tests); i++) {
+		struct test_cfg *test = &tests[i];
+
+		for (j = 0; j < ARRAY_SIZE(families); j++) {
+			struct tuple *tuple = &tuples[test->type][j];
+			char input[256];
+			char tmp[256];
+
+			test_str(tmp, sizeof(tmp), test, tuple->family);
+			if (!test__start_subtest(tmp))
+				continue;
+
+			tattr.data_out = tmp;
+			tattr.data_size_out = sizeof(tmp);
+
+			tattr.data_in = input;
+			tattr.data_size_in = build_input(test, input, tuple);
+			if (CHECK_FAIL(!tattr.data_size_in))
+				continue;
+
+			err = bpf_prog_test_run_xattr(&tattr);
+			if (CHECK_FAIL(err))
+				continue;
+
+			if (tattr.retval != TC_ACT_REDIRECT) {
+				PRINT_FAIL("expected TC_ACT_REDIRECT, got %d\n",
+					   tattr.retval);
+				continue;
+			}
+
+			switch (test->result) {
+			case ACCEPT:
+				if (CHECK_FAIL(!was_decapsulated(&tattr)))
+					continue;
+				break;
+			case FORWARD:
+				if (CHECK_FAIL(was_decapsulated(&tattr)))
+					continue;
+				break;
+			default:
+				PRINT_FAIL("unknown result %d\n", test->result);
+				continue;
+			}
+		}
+	}
+
+cleanup:
+	test_cls_redirect__destroy(skel);
+	close_fds((int *)servers, sizeof(servers) / sizeof(servers[0][0]));
+	close_fds((int *)conns, sizeof(conns) / sizeof(conns[0][0]));
+}

tools/testing/selftests/bpf/progs/test_cls_redirect.c

···
+// 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"
+
+#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.
+ */
+static volatile const __be16 ENCAPSULATION_PORT;
+static 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;
+} 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 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 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 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 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 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 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 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 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 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 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 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 inline __attribute__((__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 bpf_map_def metrics_map SEC("maps") = {
+	.type = BPF_MAP_TYPE_PERCPU_ARRAY,
+	.key_size = sizeof(unsigned int),
+	.value_size = sizeof(metrics_t),
+	.max_entries = 1,
+};
+
+static metrics_t *get_global_metrics(void)
+{
+	uint64_t key = 0;
+	return bpf_map_lookup_elem(&metrics_map, &key);
+}
+
+static 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)) {
+		return TC_ACT_SHOT;
+	}
+
+	return bpf_redirect(skb->ifindex, BPF_F_INGRESS);
+}
+
+static 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;
+
+	/* 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_skb_adjust_room(skb, delta, BPF_ADJ_ROOM_NET,
+				BPF_F_ADJ_ROOM_FIXED_GSO)) {
+		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 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 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 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 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 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 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 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 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 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 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 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 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 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);
+}

tools/testing/selftests/bpf/progs/test_cls_redirect.h

···
+/* SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause */
+/* Copyright 2019, 2020 Cloudflare */
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <linux/if_ether.h>
+#include <linux/in.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <linux/udp.h>
+
+struct gre_base_hdr {
+	uint16_t flags;
+	uint16_t protocol;
+} __attribute__((packed));
+
+struct guehdr {
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	uint8_t hlen : 5, control : 1, variant : 2;
+#else
+	uint8_t variant : 2, control : 1, hlen : 5;
+#endif
+	uint8_t proto_ctype;
+	uint16_t flags;
+};
+
+struct unigue {
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	uint8_t _r : 2, last_hop_gre : 1, forward_syn : 1, version : 4;
+#else
+	uint8_t version : 4, forward_syn : 1, last_hop_gre : 1, _r : 2;
+#endif
+	uint8_t reserved;
+	uint8_t next_hop;
+	uint8_t hop_count;
+	// Next hops go here
+} __attribute__((packed));
+
+typedef struct {
+	struct ethhdr eth;
+	struct iphdr ip;
+	struct gre_base_hdr gre;
+} __attribute__((packed)) encap_gre_t;
+
+typedef struct {
+	struct ethhdr eth;
+	struct iphdr ip;
+	struct udphdr udp;
+	struct guehdr gue;
+	struct unigue unigue;
+} __attribute__((packed)) encap_headers_t;

tools/testing/selftests/bpf/test_progs.h

···
 } __packed;
 extern struct ipv6_packet pkt_v6;
 
+#define PRINT_FAIL(format...)                                                  \
+	({                                                                     \
+		test__fail();                                                  \
+		fprintf(stdout, "%s:FAIL:%d ", __func__, __LINE__);            \
+		fprintf(stdout, ##format);                                     \
+	})
+
 #define _CHECK(condition, tag, duration, format...) ({			\
 	int __ret = !!(condition);					\
 	int __save_errno = errno;					\