Android上如何使用UID获取用户名？

我有几个这样的UID：
10022、10011、1000

其实我知道他们的用户名是u0_a22, u0_a11, system.

但问题是，如何使用UID获取用户名呢？没有/etc/passwd根本没有文件。

我编写了一个实用程序类，通过硬编码值来获取 UID/GID 名称android_filesystem_config.h https://android.googlesource.com/platform/system/core/+/master/include/private/android_filesystem_config.h.

Usage:

String uid = AndroidFilesystemConfig.getNameForId(1000);

AndroidFilesystemConfig.java

import android.os.Build;
import android.util.SparseArray;

import java.util.Locale;

/**
 * <p>System Users and Groups for the Android platform as specified in
 * <a href="https://android.googlesource.com/platform/system/core/+/master/include/private/android_filesystem_config.h">android_filesystem_config.h</a>.</p>
 *
 * <p>Last updated: April 20th, 2016</p>
 *
 * <p><b>Note:</b> Some OEMs may have specific UIDs for other system users not in this class.</p>
 */
public class AndroidFilesystemConfig {

  /* first app user */
  private static final int AID_APP = 10000;

  /* offset for uid ranges for each user */
  private static final int AID_USER = 100000;

  /* start of gids for apps in each user to share */
  private static final int AID_SHARED_GID_START = 50000;

  /* end of gids for apps in each user to share */
  private static final int AID_SHARED_GID_END = 59999;

  private static final SparseArray<String> SYSTEM_IDS = new SparseArray<>();

  static {
    putSystemId(0, "root"); /* traditional unix root user */
    putSystemId(1000, "system"); /* system server */
    putSystemId(1001, "radio"); /* telephony subsystem, RIL */
    putSystemId(1002, "bluetooth"); /* bluetooth subsystem */
    putSystemId(1003, "graphics"); /* graphics devices */
    putSystemId(1004, "input"); /* input devices */
    putSystemId(1005, "audio"); /* audio devices */
    putSystemId(1006, "camera"); /* camera devices */
    putSystemId(1007, "log"); /* log devices */
    putSystemId(1008, "compass"); /* compass device */
    putSystemId(1009, "mount"); /* mountd socket */
    putSystemId(1010, "wifi"); /* wifi subsystem */
    putSystemId(1011, "adb"); /* android debug bridge (adbd) */
    putSystemId(1012, "install"); /* group for installing packages */
    putSystemId(1013, "media"); /* mediaserver process */
    putSystemId(1014, "dhcp"); /* dhcp client */
    putSystemId(1015, "sdcard_rw"); /* external storage write access */
    putSystemId(1016, "vpn"); /* vpn system */
    putSystemId(1017, "keystore"); /* keystore subsystem */
    putSystemId(1018, "usb"); /* USB devices */
    putSystemId(1019, "drm"); /* DRM server */
    putSystemId(1020, "mdnsr"); /* MulticastDNSResponder (service discovery) */
    putSystemId(1021, "gps"); /* GPS daemon */
    // 1022 is deprecated and not used.
    putSystemId(1023, "media_rw"); /* internal media storage write access */
    putSystemId(1024, "mtp"); /* MTP USB driver access */
    // 1025 is deprecated and not used.
    putSystemId(1026, "drmrpc"); /* group for drm rpc */
    putSystemId(1027, "nfc"); /* nfc subsystem */
    putSystemId(1028, "sdcard_r"); /* external storage read access */
    putSystemId(1029, "clat"); /* clat part of nat464 */
    putSystemId(1030, "loop_radio"); /* loop radio devices */
    putSystemId(1031, "mediadrm"); /* MediaDrm plugins */
    putSystemId(1032, "package_info"); /* access to installed package details */
    putSystemId(1033, "sdcard_pics"); /* external storage photos access */
    putSystemId(1034, "sdcard_av"); /* external storage audio/video access */
    putSystemId(1035, "sdcard_all"); /* access all users external storage */
    putSystemId(1036, "logd"); /* log daemon */
    putSystemId(1037, "shared_relro"); /* creator of shared GNU RELRO files */
    putSystemId(1038, "dbus"); /* dbus-daemon IPC broker process */
    putSystemId(1039, "tlsdate"); /* tlsdate unprivileged user */
    putSystemId(1040, "mediaex"); /* mediaextractor process */
    putSystemId(1041, "audioserver"); /* audioserver process */
    putSystemId(1042, "metrics_coll"); /* metrics_collector process */
    putSystemId(1043, "metricsd"); /* metricsd process */
    putSystemId(1044, "webserv"); /* webservd process */
    putSystemId(1045, "debuggerd"); /* debuggerd unprivileged user */
    putSystemId(1046, "mediacodec"); /* mediacodec process */
    putSystemId(1047, "cameraserver"); /* cameraserver process */
    putSystemId(1048, "firewall"); /* firewalld process */
    putSystemId(1049, "trunks"); /* trunksd process (TPM daemon) */
    putSystemId(1050, "nvram"); /* Access-controlled NVRAM */
    putSystemId(1051, "dns"); /* DNS resolution daemon (system: netd) */
    putSystemId(1052, "dns_tether"); /* DNS resolution daemon (tether: dnsmasq) */
    putSystemId(1053, "webview_zygote"); /* WebView zygote process */
    putSystemId(1054, "vehicle_network"); /* Vehicle network service */
    putSystemId(1055, "media_audio"); /* GID for audio files on internal media storage */
    putSystemId(1056, "media_video"); /* GID for video files on internal media storage */
    putSystemId(1057, "media_image"); /* GID for image files on internal media storage */

    putSystemId(2000, "shell"); /* adb and debug shell user */
    putSystemId(2001, "cache"); /* cache access */
    putSystemId(2002, "diag"); /* access to diagnostic resources */

    /* The range 2900-2999 is reserved for OEMs */

    // The 3000 series are intended for use as supplemental group id's only. They indicate
    // special Android capabilities that the kernel is aware of.
    putSystemId(3001, "net_bt_admin"); /* bluetooth: get any socket */
    putSystemId(3002, "net_bt"); /* bluetooth: get sco, rfcomm or l2cap sockets */
    putSystemId(3003, "inet"); /* can get AF_INET and AF_INET6 sockets */
    putSystemId(3004, "net_raw"); /* can get raw INET sockets */
    putSystemId(3005, "net_admin"); /* can configure interfaces and routing tables. */
    putSystemId(3006, "net_bw_stats"); /* read bandwidth statistics */
    putSystemId(3007, "net_bw_acct"); /* change bandwidth statistics accounting */
    putSystemId(3008, "net_bt_stack"); /* bluetooth: access config files */
    putSystemId(3009, "readproc"); /* Allow /proc read access */
    putSystemId(3010, "wakelock"); /* Allow system wakelock read/write access */

    /* The range 5000-5999 is also reserved for OEMs. */

    putSystemId(9997, "everybody"); /* shared between all apps in the same profile */
    putSystemId(9998, "misc"); /* access to misc storage */
    putSystemId(9999, "nobody");
  }

  private static void putSystemId(int id, String name) {
    // Check if the uid exists before adding it so we don't add unsupported ids.
    if (android.os.Process.getUidForName(name) != id) {
      // Not valid on this system. Most likely due to a lower API.
      return;
    }
    SYSTEM_IDS.put(id, name);
  }

  /**
   * @return An array of system UIDs
   */
  public static SparseArray<String> getSystemIds() {
    return SYSTEM_IDS;
  }

  /**
   * Returns the UID/GID name assigned to a particular id, or {@code null} if there is none.
   *
   * @param id
   *     The UID/GID of a process or file
   * @return the name of the UID/GID or {@code null} if the id is unrecognized.
   */
  public static String getNameForId(int id) {
    String name = SYSTEM_IDS.get(id);
    if (name == null) {
      if (id >= AID_SHARED_GID_START && id <= AID_SHARED_GID_END) {
        name = String.format(Locale.ENGLISH, "all_a%d", id - AID_SHARED_GID_START);
      } else {
        int appId = id - AID_APP;
        int userId = 0;
        // loop until we get the correct user id.
        // 100000 is the offset for each user.
        while (appId > AID_USER) {
          appId -= AID_USER;
          userId++;
        }
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
          // u{user_id}_a{app_id} is used on API 17+ for multiple user account support.
          name = String.format(Locale.ENGLISH, "u%d_a%d", userId, appId);
        } else {
          // app_{app_id} is used below API 17.
          name = String.format(Locale.ENGLISH, "app_%d", appId);
        }
      }
    }
    return name;
  }

  private AndroidFilesystemConfig() {
    throw new AssertionError("no instances");
  }

}

使用反射：

受 odexide's 答案的启发，这里有一些非常丑陋的反射，我不建议使用它来获取 UID/GID 名称获取pwuid(uid_t) http://androidxref.com/5.0.0_r2/xref/bionic/libc/bionic/stubs.cpp#347:

public static String getNameForUid(int id) {
  try {
    Class<?> clazz = Class.forName("libcore.io.Libcore");
    Field field = clazz.getDeclaredField("os");
    if (!field.isAccessible()) {
      field.setAccessible(true);
    }
    Object os = field.get(null);
    if (os != null) {
      Method getpwuid = os.getClass().getMethod("getpwuid", int.class);
      if (getpwuid != null) {
        if (!getpwuid.isAccessible()) {
          getpwuid.setAccessible(true);
        }
        Object passwd = getpwuid.invoke(os, id);
        if (passwd != null) {
          Field pw_name = passwd.getClass().getDeclaredField("pw_name");
          if (!pw_name.isAccessible()) {
            pw_name.setAccessible(true);
          }
          return (String) pw_name.get(passwd);
        }
      }
    }
  } catch (Exception ignored) {
  }
  return null;
}