android 為了 sdcard/usbdisk 使用 Fuse filesystem，寫了一個 tool : sdcard

/system/core/sdcard

其中 option 的部份：

    while ((opt = getopt(argc, argv, "u:g:U:mw")) != -1) {
        switch (opt) {
            case 'u':
                uid = strtoul(optarg, NULL, 10);
                break;
            case 'g':
                gid = strtoul(optarg, NULL, 10);
                break;
            case 'U':
                userid = strtoul(optarg, NULL, 10);
                break;
            case 'm':
                multi_user = true;
                break;
            case 'w':
                full_write = true;
                break;
            case '?':
            default:
                return usage();
        }
    }

其中 multi-user 的意思：

    if (multi_user) {
        /* Multi-user storage is fully isolated per user, so "other"
         * permissions are completely masked off. */
        if (fuse_setup(&fuse_default, AID_SDCARD_RW, 0006)
                || fuse_setup(&fuse_read, AID_EVERYBODY, 0027)
                || fuse_setup(&fuse_write, AID_EVERYBODY, full_write ? 0007 : 0027)) {
            ERROR("failed to fuse_setup\n");
            exit(1);
        }
    } else {
        /* Physical storage is readable by all users on device, but
         * the Android directories are masked off to a single user
         * deep inside attr_from_stat(). */
        if (fuse_setup(&fuse_default, AID_SDCARD_RW, 0006)
                || fuse_setup(&fuse_read, AID_EVERYBODY, full_write ? 0027 : 0022)
                || fuse_setup(&fuse_write, AID_EVERYBODY, full_write ? 0007 : 0022)) {
            ERROR("failed to fuse_setup\n");
            exit(1);
        }
    }