// 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>
#include <linux/udp.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.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 != bpf_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 != bpf_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 (bpf_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 = bpf_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 = bpf_htons(bpf_ntohs(ip6->payload_len) + sizeof(struct iphdr)), // u16
.id = 0, // u16
.frag_off = bpf_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, bpf_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 != bpf_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 != bpf_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 (bpf_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 (bpf_ntohs(ip4->tot_len) < sizeof(*ip4))
return TC_ACT_OK;
// We are incapable of dealing with IPv4 fragments
if (ip4->frag_off & ~bpf_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 = bpf_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 = bpf_htons(bpf_ntohs(ip4->tot_len) - 20), // __be16
.nexthdr = ip4->protocol, // __u8
.hop_limit = ip4->ttl, // __u8
};
ip6.saddr.in6_u.u6_addr32[0] = bpf_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] = bpf_htonl(1);
ip6.daddr.in6_u.u6_addr32[0] = bpf_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] = bpf_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, bpf_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");