Pull underlying network MTU using Java APIs

This change ensures that if the VCN's underlying network MTU is not
reported via the LinkProperties APIs (as in the case of Carrier WiFi),
the kernel values will be used (as possible).

Bug: 204813618
Test: Manual testing
Test: atest FrameworksVcnTests CtsVcnTestCases
Original-Change: https://android-review.googlesource.com/1877796
Change-Id: If291aa6b915e4da39152a679bbe3e40216b42f44
Merged-In: If291aa6b915e4da39152a679bbe3e40216b42f44

2 files changed, 91 insertions, 1 deletions

diff --git a/services/core/java/com/android/server/vcn/VcnGatewayConnection.java b/services/core/java/com/android/server/vcn/VcnGatewayConnection.java
index 51c3b33b0e79..055012b1dad4 100644
--- a/services/core/java/com/android/server/vcn/VcnGatewayConnection.java
+++ b/services/core/java/com/android/server/vcn/VcnGatewayConnection.java
@@ -99,6 +99,7 @@ import java.io.IOException;
 import java.net.Inet4Address;
 import java.net.Inet6Address;
 import java.net.InetAddress;
+import java.net.NetworkInterface;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
@@ -2061,7 +2062,8 @@ public class VcnGatewayConnection extends StateMachine {
         return builder.build();
     }
 
-    private static LinkProperties buildConnectedLinkProperties(
+    @VisibleForTesting(visibility = Visibility.PRIVATE)
+    LinkProperties buildConnectedLinkProperties(
             @NonNull VcnGatewayConnectionConfig gatewayConnectionConfig,
             @NonNull IpSecTunnelInterface tunnelIface,
             @NonNull VcnChildSessionConfiguration childConfig,
@@ -2089,6 +2091,13 @@ public class VcnGatewayConnection extends StateMachine {
 
             lp.setTcpBufferSizes(underlyingLp.getTcpBufferSizes());
             underlyingMtu = underlyingLp.getMtu();
+
+            // WiFi LinkProperties uses DHCP as the sole source of MTU information, and as a result
+            // often lists MTU as 0 (see b/184678973). Use the interface MTU as retrieved by
+            // NetworkInterface APIs.
+            if (underlyingMtu == 0 && underlyingLp.getInterfaceName() != null) {
+                underlyingMtu = mDeps.getUnderlyingIfaceMtu(underlyingLp.getInterfaceName());
+            }
         } else {
             Slog.wtf(
                     TAG,
@@ -2436,6 +2445,17 @@ public class VcnGatewayConnection extends StateMachine {
         public long getElapsedRealTime() {
             return SystemClock.elapsedRealtime();
         }
+
+        /** Gets the MTU for the given underlying interface. */
+        public int getUnderlyingIfaceMtu(String ifaceName) {
+            try {
+                final NetworkInterface underlyingIface = NetworkInterface.getByName(ifaceName);
+                return underlyingIface == null ? 0 : underlyingIface.getMTU();
+            } catch (IOException e) {
+                Slog.d(TAG, "Could not get MTU of underlying network", e);
+                return 0;
+            }
+        }
     }
 
     /**
diff --git a/tests/vcn/java/com/android/server/vcn/VcnGatewayConnectionTest.java b/tests/vcn/java/com/android/server/vcn/VcnGatewayConnectionTest.java
index a7001713533c..b8eefabea3c6 100644
--- a/tests/vcn/java/com/android/server/vcn/VcnGatewayConnectionTest.java
+++ b/tests/vcn/java/com/android/server/vcn/VcnGatewayConnectionTest.java
@@ -16,6 +16,7 @@
 
 package com.android.server.vcn;
 
+import static android.net.IpSecManager.IpSecTunnelInterface;
 import static android.net.NetworkCapabilities.NET_CAPABILITY_DUN;
 import static android.net.NetworkCapabilities.NET_CAPABILITY_INTERNET;
 import static android.net.NetworkCapabilities.NET_CAPABILITY_NOT_METERED;
@@ -24,6 +25,8 @@ import static android.net.NetworkCapabilities.NET_CAPABILITY_NOT_VCN_MANAGED;
 import static android.net.NetworkCapabilities.TRANSPORT_CELLULAR;
 import static android.net.NetworkCapabilities.TRANSPORT_WIFI;
 
+import static com.android.server.vcn.VcnGatewayConnection.DUMMY_ADDR;
+import static com.android.server.vcn.VcnGatewayConnection.VcnChildSessionConfiguration;
 import static com.android.server.vcn.VcnGatewayConnection.VcnIkeSession;
 import static com.android.server.vcn.VcnGatewayConnection.VcnNetworkAgent;
 
@@ -36,8 +39,11 @@ import static org.mockito.Matchers.eq;
 import static org.mockito.Mockito.CALLS_REAL_METHODS;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
+import android.net.IpSecManager;
+import android.net.LinkAddress;
 import android.net.LinkProperties;
 import android.net.Network;
 import android.net.NetworkCapabilities;
@@ -59,8 +65,11 @@ import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
+import java.net.InetAddress;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.UUID;
 
@@ -70,6 +79,8 @@ import java.util.UUID;
 public class VcnGatewayConnectionTest extends VcnGatewayConnectionTestBase {
     private static final int TEST_UID = Process.myUid() + 1;
 
+    private static final String LOOPBACK_IFACE = "lo";
+
     private static final ParcelUuid TEST_PARCEL_UUID = new ParcelUuid(UUID.randomUUID());
     private static final int TEST_SIM_SLOT_INDEX = 1;
     private static final int TEST_SUBSCRIPTION_ID_1 = 2;
@@ -77,6 +88,12 @@ public class VcnGatewayConnectionTest extends VcnGatewayConnectionTestBase {
     private static final int TEST_SUBSCRIPTION_ID_2 = 3;
     private static final SubscriptionInfo TEST_SUBINFO_2 = mock(SubscriptionInfo.class);
     private static final Map<Integer, ParcelUuid> TEST_SUBID_TO_GROUP_MAP;
+    private static final String TEST_TCP_BUFFER_SIZES = "1,2,3,4,5,6";
+    private static final int TEST_MTU = 1300;
+    private static final int TEST_MTU_DELTA = 64;
+    private static final List<LinkAddress> TEST_INTERNAL_ADDRESSES =
+            Arrays.asList(new LinkAddress(DUMMY_ADDR, 16));
+    private static final List<InetAddress> TEST_DNS_ADDRESSES = Arrays.asList(DUMMY_ADDR);
 
     private static final int TEST_UPSTREAM_BANDWIDTH = 1234;
     private static final int TEST_DOWNSTREAM_BANDWIDTH = 2345;
@@ -166,6 +183,59 @@ public class VcnGatewayConnectionTest extends VcnGatewayConnectionTestBase {
     }
 
     @Test
+    public void testBuildLinkProperties() throws Exception {
+        final IpSecTunnelInterface tunnelIface =
+                mContext.getSystemService(IpSecManager.class)
+                        .createIpSecTunnelInterface(
+                                DUMMY_ADDR, DUMMY_ADDR, TEST_UNDERLYING_NETWORK_RECORD_1.network);
+
+        final LinkProperties underlyingLp = new LinkProperties();
+        underlyingLp.setInterfaceName(LOOPBACK_IFACE);
+        underlyingLp.setTcpBufferSizes(TEST_TCP_BUFFER_SIZES);
+        doReturn(TEST_MTU).when(mDeps).getUnderlyingIfaceMtu(LOOPBACK_IFACE);
+
+        final VcnChildSessionConfiguration childSessionConfig =
+                mock(VcnChildSessionConfiguration.class);
+        doReturn(TEST_INTERNAL_ADDRESSES).when(childSessionConfig).getInternalAddresses();
+        doReturn(TEST_DNS_ADDRESSES).when(childSessionConfig).getInternalDnsServers();
+
+        UnderlyingNetworkRecord record =
+                new UnderlyingNetworkRecord(
+                        mock(Network.class, CALLS_REAL_METHODS),
+                        new NetworkCapabilities.Builder().build(),
+                        underlyingLp,
+                        false);
+
+        final LinkProperties vcnLp1 =
+                mGatewayConnection.buildConnectedLinkProperties(
+                        VcnGatewayConnectionConfigTest.buildTestConfig(),
+                        tunnelIface,
+                        childSessionConfig,
+                        record);
+
+        verify(mDeps).getUnderlyingIfaceMtu(LOOPBACK_IFACE);
+
+        // Instead of having to recalculate the final MTU (after accounting for IPsec overhead),
+        // calculate another set of Link Properties with a lower MTU, and calculate the delta.
+        doReturn(TEST_MTU - TEST_MTU_DELTA).when(mDeps).getUnderlyingIfaceMtu(LOOPBACK_IFACE);
+
+        final LinkProperties vcnLp2 =
+                mGatewayConnection.buildConnectedLinkProperties(
+                        VcnGatewayConnectionConfigTest.buildTestConfig(),
+                        tunnelIface,
+                        childSessionConfig,
+                        record);
+
+        verify(mDeps, times(2)).getUnderlyingIfaceMtu(LOOPBACK_IFACE);
+
+        assertEquals(tunnelIface.getInterfaceName(), vcnLp1.getInterfaceName());
+        assertEquals(TEST_INTERNAL_ADDRESSES, vcnLp1.getLinkAddresses());
+        assertEquals(TEST_DNS_ADDRESSES, vcnLp1.getDnsServers());
+        assertEquals(TEST_TCP_BUFFER_SIZES, vcnLp1.getTcpBufferSizes());
+        assertEquals(TEST_MTU_DELTA, vcnLp1.getMtu() - vcnLp2.getMtu());
+    }
+
+    @Test
     public void testSubscriptionSnapshotUpdateNotifiesUnderlyingNetworkTracker() {
         verifyWakeLockSetUp();