系统应用根据Uri授予权限方法详解

int checkGrantUriPermission(int callingUid, String targetPkg, GrantUri grantUri,
        final int modeFlags, int lastTargetUid) {
   ....
    // Bail early if system is trying to hand out permissions directly; it
    // must always grant permissions on behalf of someone explicit.
    final int callingAppId = UserHandle.getAppId(callingUid);
    if ((callingAppId == SYSTEM_UID) || (callingAppId == ROOT_UID)) {
        if ("com.android.settings.files".equals(grantUri.uri.getAuthority())
                || "com.android.settings.module_licenses".equals(grantUri.uri.getAuthority())) {
            // Exempted authority for
            // 1. cropping user photos and sharing a generated license html
            //    file in Settings app
            // 2. sharing a generated license html file in TvSettings app
            // 3. Sharing module license files from Settings app
        } else {
            Slog.w(TAG, "For security reasons, the system cannot issue a Uri permission"
                    + " grant to " + grantUri + "; use startActivityAsCaller() instead");
            return -1;
        }
    }
    ....
}
复制代码

 private void enforceCallingPermissionInternal(Uri uri, boolean forWrite) {
          ... // 省略部分代码
          // First, check to see if caller has direct write access
          if (forWrite) {
              final sqliteQueryBuilder qb = getQueryBuilder(TYPE_UPDATE, uri, table, null);
              try (Cursor c = qb.query(db, new String[0], null, null, null, null, null)) {
                  if (c.moveToFirst()) {
                      // Direct write access granted, yay!
                      return;
                  }
              }
          }
          ... // 省略部分代码
          // Second, check to see if caller has direct read access
          final SQLiteQueryBuilder qb = getQueryBuilder(TYPE_QUERY, uri, table, null);
          try (Cursor c = qb.query(db, new String[0], null, null, null, null, null)) {
              if (c.moveToFirst()) {
                  if (!forWrite) {
                      // Direct read access granted, yay!
                      return;
                  } else if (allowUserGrant) {
                      // Caller has read access, but they wanted to write, and
                      // they'll need to get the user to grant that access
                      final Context context = getContext();
                      final PendingIntent intent = PendingIntent.getActivity(context, 42,
                              new Intent(null, uri, context, PermissionActivity.class),
                              FLAG_ONE_SHOT | FLAG_CANCEL_CURRENT | FLAG_IMMUTABLE);
 
                      final Icon icon = getCollectionIcon(uri);
                      final RemoteAction action = new RemoteAction(icon,
                              context.getText(R.string.permission_required_action),
                              context.getText(R.string.permission_required_action),
                              intent);
 
                      throw new RecoverableSecurityException(new SecurityException(
                              getCallingPackageOrSelf() + " has no access to " + uri),
                              context.getText(R.string.permission_required), action);
                  }
              }
          }
 
          throw new SecurityException(getCallingPackageOrSelf() + " has no access to " + uri);
      }