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+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * This code is taken from the Android Open Source Project and the author
+ * (Maciej Żenczykowski) has gave permission to relicense it under the
+ * GPLv2. Therefore this program is free software;
+ * You can redistribute it and/or modify it under the terms of the GNU
+ * General Public License version 2 as published by the Free Software
+ * Foundation
+
+ * The original headers, including the original license headers, are
+ * included below for completeness.
+ *
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include <linux/bpf.h>
+#include <linux/if.h>
+#include <linux/if_ether.h>
+#include <linux/if_packet.h>
+#include <linux/in.h>
+#include <linux/in6.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <linux/pkt_cls.h>
+#include <linux/swab.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+// bionic kernel uapi linux/udp.h header is munged...
+#define __kernel_udphdr udphdr
+#include <linux/udp.h>
+
+#include <bpf/bpf_helpers.h>
+
+#define htons(x) (__builtin_constant_p(x) ? ___constant_swab16(x) : __builtin_bswap16(x))
+#define htonl(x) (__builtin_constant_p(x) ? ___constant_swab32(x) : __builtin_bswap32(x))
+#define ntohs(x) htons(x)
+#define ntohl(x) htonl(x)
+
+// From kernel:include/net/ip.h
+#define IP_DF 0x4000 // Flag: "Don't Fragment"
+
+SEC("schedcls/ingress6/nat_6")
+int sched_cls_ingress6_nat_6_prog(struct __sk_buff *skb)
+{
+
+ const int l2_header_size = sizeof(struct ethhdr);
+ void *data = (void *)(long)skb->data;
+ const void *data_end = (void *)(long)skb->data_end;
+ const struct ethhdr * const eth = data; // used iff is_ethernet
+ const struct ipv6hdr * const ip6 = (void *)(eth + 1);
+
+ // Require ethernet dst mac address to be our unicast address.
+ if (skb->pkt_type != PACKET_HOST)
+ return TC_ACT_OK;
+
+ // Must be meta-ethernet IPv6 frame
+ if (skb->protocol != htons(ETH_P_IPV6))
+ return TC_ACT_OK;
+
+ // Must have (ethernet and) ipv6 header
+ if (data + l2_header_size + sizeof(*ip6) > data_end)
+ return TC_ACT_OK;
+
+ // Ethertype - if present - must be IPv6
+ if (eth->h_proto != htons(ETH_P_IPV6))
+ return TC_ACT_OK;
+
+ // IP version must be 6
+ if (ip6->version != 6)
+ return TC_ACT_OK;
+ // Maximum IPv6 payload length that can be translated to IPv4
+ if (ntohs(ip6->payload_len) > 0xFFFF - sizeof(struct iphdr))
+ return TC_ACT_OK;
+ switch (ip6->nexthdr) {
+ case IPPROTO_TCP: // For TCP & UDP the checksum neutrality of the chosen IPv6
+ case IPPROTO_UDP: // address means there is no need to update their checksums.
+ case IPPROTO_GRE: // We do not need to bother looking at GRE/ESP headers,
+ case IPPROTO_ESP: // since there is never a checksum to update.
+ break;
+ default: // do not know how to handle anything else
+ return TC_ACT_OK;
+ }
+
+ struct ethhdr eth2; // used iff is_ethernet
+
+ eth2 = *eth; // Copy over the ethernet header (src/dst mac)
+ eth2.h_proto = htons(ETH_P_IP); // But replace the ethertype
+
+ struct iphdr ip = {
+ .version = 4, // u4
+ .ihl = sizeof(struct iphdr) / sizeof(__u32), // u4
+ .tos = (ip6->priority << 4) + (ip6->flow_lbl[0] >> 4), // u8
+ .tot_len = htons(ntohs(ip6->payload_len) + sizeof(struct iphdr)), // u16
+ .id = 0, // u16
+ .frag_off = htons(IP_DF), // u16
+ .ttl = ip6->hop_limit, // u8
+ .protocol = ip6->nexthdr, // u8
+ .check = 0, // u16
+ .saddr = 0x0201a8c0, // u32
+ .daddr = 0x0101a8c0, // u32
+ };
+
+ // Calculate the IPv4 one's complement checksum of the IPv4 header.
+ __wsum sum4 = 0;
+
+ for (int i = 0; i < sizeof(ip) / sizeof(__u16); ++i)
+ sum4 += ((__u16 *)&ip)[i];
+
+ // Note that sum4 is guaranteed to be non-zero by virtue of ip.version == 4
+ sum4 = (sum4 & 0xFFFF) + (sum4 >> 16); // collapse u32 into range 1 .. 0x1FFFE
+ sum4 = (sum4 & 0xFFFF) + (sum4 >> 16); // collapse any potential carry into u16
+ ip.check = (__u16)~sum4; // sum4 cannot be zero, so this is never 0xFFFF
+
+ // Calculate the *negative* IPv6 16-bit one's complement checksum of the IPv6 header.
+ __wsum sum6 = 0;
+ // We'll end up with a non-zero sum due to ip6->version == 6 (which has '0' bits)
+ for (int i = 0; i < sizeof(*ip6) / sizeof(__u16); ++i)
+ sum6 += ~((__u16 *)ip6)[i]; // note the bitwise negation
+
+ // Note that there is no L4 checksum update: we are relying on the checksum neutrality
+ // of the ipv6 address chosen by netd's ClatdController.
+
+ // Packet mutations begin - point of no return, but if this first modification fails
+ // the packet is probably still pristine, so let clatd handle it.
+ if (bpf_skb_change_proto(skb, htons(ETH_P_IP), 0))
+ return TC_ACT_OK;
+ bpf_csum_update(skb, sum6);
+
+ data = (void *)(long)skb->data;
+ data_end = (void *)(long)skb->data_end;
+ if (data + l2_header_size + sizeof(struct iphdr) > data_end)
+ return TC_ACT_SHOT;
+
+ struct ethhdr *new_eth = data;
+
+ // Copy over the updated ethernet header
+ *new_eth = eth2;
+
+ // Copy over the new ipv4 header.
+ *(struct iphdr *)(new_eth + 1) = ip;
+ return bpf_redirect(skb->ifindex, BPF_F_INGRESS);
+}
+SEC("schedcls/egress4/snat4")
+int sched_cls_egress4_snat4_prog(struct __sk_buff *skb)
+{
+ const int l2_header_size = sizeof(struct ethhdr);
+ void *data = (void *)(long)skb->data;
+ const void *data_end = (void *)(long)skb->data_end;
+ const struct ethhdr *const eth = data; // used iff is_ethernet
+ const struct iphdr *const ip4 = (void *)(eth + 1);
+
+
+ // Must be meta-ethernet IPv4 frame
+ if (skb->protocol != htons(ETH_P_IP))
+ return TC_ACT_OK;
+
+ // Must have ipv4 header
+ if (data + l2_header_size + sizeof(struct ipv6hdr) > data_end)
+ return TC_ACT_OK;
+
+ // Ethertype - if present - must be IPv4
+ if (eth->h_proto != htons(ETH_P_IP))
+ return TC_ACT_OK;
+
+ // IP version must be 4
+ if (ip4->version != 4)
+ return TC_ACT_OK;
+
+ // We cannot handle IP options, just standard 20 byte == 5 dword minimal IPv4 header
+ if (ip4->ihl != 5)
+ return TC_ACT_OK;
+
+ // Maximum IPv6 payload length that can be translated to IPv4
+ if (htons(ip4->tot_len) > 0xFFFF - sizeof(struct ipv6hdr))
+ return TC_ACT_OK;
+
+ // Calculate the IPv4 one's complement checksum of the IPv4 header.
+ __wsum sum4 = 0;
+
+ for (int i = 0; i < sizeof(*ip4) / sizeof(__u16); ++i)
+ sum4 += ((__u16 *)ip4)[i];
+
+ // Note that sum4 is guaranteed to be non-zero by virtue of ip4->version == 4
+ sum4 = (sum4 & 0xFFFF) + (sum4 >> 16); // collapse u32 into range 1 .. 0x1FFFE
+ sum4 = (sum4 & 0xFFFF) + (sum4 >> 16); // collapse any potential carry into u16
+ // for a correct checksum we should get *a* zero, but sum4 must be positive, ie 0xFFFF
+ if (sum4 != 0xFFFF)
+ return TC_ACT_OK;
+
+ // Minimum IPv4 total length is the size of the header
+ if (ntohs(ip4->tot_len) < sizeof(*ip4))
+ return TC_ACT_OK;
+
+ // We are incapable of dealing with IPv4 fragments
+ if (ip4->frag_off & ~htons(IP_DF))
+ return TC_ACT_OK;
+
+ switch (ip4->protocol) {
+ case IPPROTO_TCP: // For TCP & UDP the checksum neutrality of the chosen IPv6
+ case IPPROTO_GRE: // address means there is no need to update their checksums.
+ case IPPROTO_ESP: // We do not need to bother looking at GRE/ESP headers,
+ break; // since there is never a checksum to update.
+
+ case IPPROTO_UDP: // See above comment, but must also have UDP header...
+ if (data + sizeof(*ip4) + sizeof(struct udphdr) > data_end)
+ return TC_ACT_OK;
+ const struct udphdr *uh = (const struct udphdr *)(ip4 + 1);
+ // If IPv4/UDP checksum is 0 then fallback to clatd so it can calculate the
+ // checksum. Otherwise the network or more likely the NAT64 gateway might
+ // drop the packet because in most cases IPv6/UDP packets with a zero checksum
+ // are invalid. See RFC 6935. TODO: calculate checksum via bpf_csum_diff()
+ if (!uh->check)
+ return TC_ACT_OK;
+ break;
+
+ default: // do not know how to handle anything else
+ return TC_ACT_OK;
+ }
+ struct ethhdr eth2; // used iff is_ethernet
+
+ eth2 = *eth; // Copy over the ethernet header (src/dst mac)
+ eth2.h_proto = htons(ETH_P_IPV6); // But replace the ethertype
+
+ struct ipv6hdr ip6 = {
+ .version = 6, // __u8:4
+ .priority = ip4->tos >> 4, // __u8:4
+ .flow_lbl = {(ip4->tos & 0xF) << 4, 0, 0}, // __u8[3]
+ .payload_len = htons(ntohs(ip4->tot_len) - 20), // __be16
+ .nexthdr = ip4->protocol, // __u8
+ .hop_limit = ip4->ttl, // __u8
+ };
+ ip6.saddr.in6_u.u6_addr32[0] = htonl(0x20010db8);
+ ip6.saddr.in6_u.u6_addr32[1] = 0;
+ ip6.saddr.in6_u.u6_addr32[2] = 0;
+ ip6.saddr.in6_u.u6_addr32[3] = htonl(1);
+ ip6.daddr.in6_u.u6_addr32[0] = htonl(0x20010db8);
+ ip6.daddr.in6_u.u6_addr32[1] = 0;
+ ip6.daddr.in6_u.u6_addr32[2] = 0;
+ ip6.daddr.in6_u.u6_addr32[3] = htonl(2);
+
+ // Calculate the IPv6 16-bit one's complement checksum of the IPv6 header.
+ __wsum sum6 = 0;
+ // We'll end up with a non-zero sum due to ip6.version == 6
+ for (int i = 0; i < sizeof(ip6) / sizeof(__u16); ++i)
+ sum6 += ((__u16 *)&ip6)[i];
+
+ // Packet mutations begin - point of no return, but if this first modification fails
+ // the packet is probably still pristine, so let clatd handle it.
+ if (bpf_skb_change_proto(skb, htons(ETH_P_IPV6), 0))
+ return TC_ACT_OK;
+
+ // This takes care of updating the skb->csum field for a CHECKSUM_COMPLETE packet.
+ // In such a case, skb->csum is a 16-bit one's complement sum of the entire payload,
+ // thus we need to subtract out the ipv4 header's sum, and add in the ipv6 header's sum.
+ // However, we've already verified the ipv4 checksum is correct and thus 0.
+ // Thus we only need to add the ipv6 header's sum.
+ //
+ // bpf_csum_update() always succeeds if the skb is CHECKSUM_COMPLETE and returns an error
+ // (-ENOTSUPP) if it isn't. So we just ignore the return code (see above for more details).
+ bpf_csum_update(skb, sum6);
+
+ // bpf_skb_change_proto() invalidates all pointers - reload them.
+ data = (void *)(long)skb->data;
+ data_end = (void *)(long)skb->data_end;
+
+ // I cannot think of any valid way for this error condition to trigger, however I do
+ // believe the explicit check is required to keep the in kernel ebpf verifier happy.
+ if (data + l2_header_size + sizeof(ip6) > data_end)
+ return TC_ACT_SHOT;
+
+ struct ethhdr *new_eth = data;
+
+ // Copy over the updated ethernet header
+ *new_eth = eth2;
+ // Copy over the new ipv4 header.
+ *(struct ipv6hdr *)(new_eth + 1) = ip6;
+ return TC_ACT_OK;
+}
+
+char _license[] SEC("license") = ("GPL");
+