[CLATJ#1] libclat: move makeChecksumNeutral() from netd to apex
Introduce the library libclat to implement clat functions for jni.
It is helpful for unit test.
makeChecksumNeutral() and its test are moved from ClatdController
without behavior change.
Bug: 212345928
Test: build and boot
atest libclat_test
Change-Id: I1c0981d98141fc1cc07b2d3a0f3cbddf38683ff3
diff --git a/service/native/libs/libclat/clatutils.cpp b/service/native/libs/libclat/clatutils.cpp
new file mode 100644
index 0000000..2a53ba8
--- /dev/null
+++ b/service/native/libs/libclat/clatutils.cpp
@@ -0,0 +1,51 @@
+// 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.
+
+#include "libclat/clatutils.h"
+
+#include <stdlib.h>
+
+extern "C" {
+#include "checksum.h"
+}
+
+namespace android {
+namespace net {
+namespace clat {
+
+// Alters the bits in the IPv6 address to make them checksum neutral with v4 and nat64Prefix.
+void makeChecksumNeutral(in6_addr* v6, const in_addr v4, const in6_addr& nat64Prefix) {
+ // Fill last 8 bytes of IPv6 address with random bits.
+ arc4random_buf(&v6->s6_addr[8], 8);
+
+ // Make the IID checksum-neutral. That is, make it so that:
+ // checksum(Local IPv4 | Remote IPv4) = checksum(Local IPv6 | Remote IPv6)
+ // in other words (because remote IPv6 = NAT64 prefix | Remote IPv4):
+ // checksum(Local IPv4) = checksum(Local IPv6 | NAT64 prefix)
+ // Do this by adjusting the two bytes in the middle of the IID.
+
+ uint16_t middlebytes = (v6->s6_addr[11] << 8) + v6->s6_addr[12];
+
+ uint32_t c1 = ip_checksum_add(0, &v4, sizeof(v4));
+ uint32_t c2 = ip_checksum_add(0, &nat64Prefix, sizeof(nat64Prefix)) +
+ ip_checksum_add(0, v6, sizeof(*v6));
+
+ uint16_t delta = ip_checksum_adjust(middlebytes, c1, c2);
+ v6->s6_addr[11] = delta >> 8;
+ v6->s6_addr[12] = delta & 0xff;
+}
+
+} // namespace clat
+} // namespace net
+} // namespace android