Add util to add an ARP table entry

This is to be used by the new DhcpServer to add ARP entries with new
addresses before sending unicast responses.

Test: manual: cat /proc/net/arp with implementation based on this
Bug: b/109584964
Change-Id: I3559893583aa3c49b188ad689a41ee2f3e9d9bf3

core/java/android/net/NetworkUtils.java

@@ -22,6 +22,7 @@ import android.util.Log;
 import android.util.Pair;
 
 import java.io.FileDescriptor;
+import java.io.IOException;
 import java.math.BigInteger;
 import java.net.Inet4Address;
 import java.net.Inet6Address;
@@ -130,6 +131,17 @@ public class NetworkUtils {
      */
     public native static boolean queryUserAccess(int uid, int netId);
 
+    /**
+     * Add an entry into the ARP cache.
+     */
+    public static void addArpEntry(Inet4Address ipv4Addr, MacAddress ethAddr, String ifname,
+            FileDescriptor fd) throws IOException {
+        addArpEntry(ethAddr.toByteArray(), ipv4Addr.getAddress(), ifname, fd);
+    }
+
+    private static native void addArpEntry(byte[] ethAddr, byte[] netAddr, String ifname,
+            FileDescriptor fd) throws IOException;
+
     /**
      * @see #intToInet4AddressHTL(int)
      * @deprecated Use either {@link #intToInet4AddressHTH(int)}

core/jni/android_net_NetUtils.cpp

@@ -323,6 +323,55 @@ static jboolean android_net_utils_queryUserAccess(JNIEnv *env, jobject thiz, jin
     return (jboolean) !queryUserAccess(uid, netId);
 }
 
+static bool checkLenAndCopy(JNIEnv* env, const jbyteArray& addr, int len, void* dst)
+{
+    if (env->GetArrayLength(addr) != len) {
+        return false;
+    }
+    env->GetByteArrayRegion(addr, 0, len, reinterpret_cast<jbyte*>(dst));
+    return true;
+}
+
+static void android_net_utils_addArpEntry(JNIEnv *env, jobject thiz, jbyteArray ethAddr,
+        jbyteArray ipv4Addr, jstring ifname, jobject javaFd)
+{
+    struct arpreq req = {};
+    struct sockaddr_in& netAddrStruct = *reinterpret_cast<sockaddr_in*>(&req.arp_pa);
+    struct sockaddr& ethAddrStruct = req.arp_ha;
+
+    ethAddrStruct.sa_family = ARPHRD_ETHER;
+    if (!checkLenAndCopy(env, ethAddr, ETH_ALEN, ethAddrStruct.sa_data)) {
+        jniThrowException(env, "java/io/IOException", "Invalid ethAddr length");
+        return;
+    }
+
+    netAddrStruct.sin_family = AF_INET;
+    if (!checkLenAndCopy(env, ipv4Addr, sizeof(in_addr), &netAddrStruct.sin_addr)) {
+        jniThrowException(env, "java/io/IOException", "Invalid ipv4Addr length");
+        return;
+    }
+
+    int ifLen = env->GetStringLength(ifname);
+    // IFNAMSIZ includes the terminating NULL character
+    if (ifLen >= IFNAMSIZ) {
+        jniThrowException(env, "java/io/IOException", "ifname too long");
+        return;
+    }
+    env->GetStringUTFRegion(ifname, 0, ifLen, req.arp_dev);
+
+    req.arp_flags = ATF_COM;  // Completed entry (ha valid)
+    int fd = jniGetFDFromFileDescriptor(env, javaFd);
+    if (fd < 0) {
+        jniThrowExceptionFmt(env, "java/io/IOException", "Invalid file descriptor");
+        return;
+    }
+    // See also: man 7 arp
+    if (ioctl(fd, SIOCSARP, &req)) {
+        jniThrowExceptionFmt(env, "java/io/IOException", "ioctl error: %s", strerror(errno));
+        return;
+    }
+}
+
 
 // ----------------------------------------------------------------------------
 
@@ -337,6 +386,7 @@ static const JNINativeMethod gNetworkUtilMethods[] = {
     { "bindSocketToNetwork", "(II)I", (void*) android_net_utils_bindSocketToNetwork },
     { "protectFromVpn", "(I)Z", (void*)android_net_utils_protectFromVpn },
     { "queryUserAccess", "(II)Z", (void*)android_net_utils_queryUserAccess },
+    { "addArpEntry", "([B[BLjava/lang/String;Ljava/io/FileDescriptor;)V", (void*) android_net_utils_addArpEntry },
     { "attachDhcpFilter", "(Ljava/io/FileDescriptor;)V", (void*) android_net_utils_attachDhcpFilter },
     { "attachRaFilter", "(Ljava/io/FileDescriptor;I)V", (void*) android_net_utils_attachRaFilter },
     { "attachControlPacketFilter", "(Ljava/io/FileDescriptor;I)V", (void*) android_net_utils_attachControlPacketFilter },