verify CLAT_MARK matches aidl constant.

Bug: 235778483
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ife3ad35b2e5451727a3795a0e78dd820293c0eae

bpf_progs/clat_mark.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2022 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.
+ */
+
+#pragma once
+
+/* -=-=-=-=-= WARNING -=-=-=-=-=-
+ *
+ * DO *NOT* *EVER* CHANGE THIS CONSTANT
+ *
+ * This is aidl::android::net::INetd::CLAT_MARK but we can't use that from
+ * pure C code (ie. the eBPF clat program).
+ *
+ * It must match the iptables rules setup by netd on Android T.
+ *
+ * This mark value is used by the eBPF clatd program to mark ingress non-offloaded clat
+ * packets for later dropping in ip6tables bw_raw_PREROUTING.
+ * They need to be dropped *after* the clat daemon (via receive on an AF_PACKET socket)
+ * sees them and thus cannot be dropped from the bpf program itself.
+ */
+static const uint32_t CLAT_MARK = 0xDEADC1A7;

bpf_progs/clatd.c

@@ -36,16 +36,11 @@
 #include "bpf_helpers.h"
 #include "bpf_net_helpers.h"
 #include "bpf_shared.h"
+#include "clat_mark.h"
 
 // From kernel:include/net/ip.h
 #define IP_DF 0x4000  // Flag: "Don't Fragment"
 
-// Used for iptables drops ingress clat packet. Beware of clat mark change may break the device
-// which is using the old clat mark in netd platform code. The reason is that the clat mark is a
-// mainline constant since T+ but netd iptable rules (ex: bandwidth control, firewall, and so on)
-// are set in stone.
-#define CLAT_MARK 0xdeadc1a7
-
 DEFINE_BPF_MAP_GRW(clat_ingress6_map, HASH, ClatIngress6Key, ClatIngress6Value, 16, AID_SYSTEM)
 
 static inline __always_inline int nat64(struct __sk_buff* skb, bool is_ethernet) {

service/native/include/Common.h

@@ -17,9 +17,12 @@
 #pragma once
 // TODO: deduplicate with the constants in NetdConstants.h.
 #include <aidl/android/net/INetd.h>
+#include "clat_mark.h"
 
 using aidl::android::net::INetd;
 
+static_assert(INetd::CLAT_MARK == CLAT_MARK, "must be 0xDEADC1A7");
+
 enum FirewallRule { ALLOW = INetd::FIREWALL_RULE_ALLOW, DENY = INetd::FIREWALL_RULE_DENY };
 
 // ALLOWLIST means the firewall denies all by default, uids must be explicitly ALLOWed