Android-vold源码分析之挂载SD卡(8)

这里续上一篇文章，讲解挂载磁盘的操作：

3.挂载磁盘。这里都有一个const char *类型的参数，这参数保存着每个磁盘的标签信息，比如sd卡的label是sdcard。

int VolumeManager::mountVolume(const char *label) {
    Volume *v = lookupVolume(label);

    if (!v) {
        errno = ENOENT;
        return -1;
    }

    return v->mountVol();
}

lookupVolume函数寻找与label匹配的对象：

Volume *VolumeManager::lookupVolume(const char *label) {
    VolumeCollection::iterator i;

    for (i = mVolumes->begin(); i != mVolumes->end(); ++i) {
        if (label[0] == '/') {
            if (!strcmp(label, (*i)->getMountpoint()))
                return (*i);
        } else {
            if (!strcmp(label, (*i)->getLabel()))
                return (*i);
        }
    }
    return NULL;
}

如果找到，直接返回磁盘对象Volume*，挂载操作在mountVol函数里面，该函数内容有点多，贴源码:

int Volume::mountVol() {
    dev_t deviceNodes[4];
    int n, i, rc = 0;
    char errmsg[255];

    if (getState() == Volume::State_NoMedia) {
        snprintf(errmsg, sizeof(errmsg),
                 "Volume %s %s mount failed - no media",
                 getLabel(), getMountpoint());
        mVm->getBroadcaster()->sendBroadcast(
                                         ResponseCode::VolumeMountFailedNoMedia,
                                         errmsg, false);
        errno = ENODEV;
        return -1;
    } else if (getState() != Volume::State_Idle) {
        errno = EBUSY;
        return -1;
    }
    /*判断该挂载点是否已经挂载，若已经挂载，则直接返回。*/
    if (isMountpointMounted(getMountpoint())) {
        SLOGW("Volume is idle but appears to be mounted - fixing");
        /*这里的setState函数用得很频繁，这函数就是将状态通知给framework*/
        setState(Volume::State_Mounted);
        // mCurrentlyMountedKdev = XXX
        return 0;
    }
    /*获取磁盘的设备号与分区数量,在下面说明*/
    n = getDeviceNodes((dev_t *) &deviceNodes, 4);
    if (!n) {
        SLOGE("Failed to get device nodes (%s)\n", strerror(errno));
        return -1;
    }
    /*将循环挂载n个分区，但从代码上看，只适用于挂载一个分区*/
    for (i = 0; i < n; i++) {
        char devicePath[255];
        /*这里看到了吧，用这种方式来使用磁盘的设备节点很方便，直接用主次设备号来命令*/
        sprintf(devicePath, "/dev/block/vold/%d:%d", MAJOR(deviceNodes[i]),
                MINOR(deviceNodes[i]));

        SLOGI("%s being considered for volume %s\n", devicePath, getLabel());

        errno = 0;
        setState(Volume::State_Checking);
        /*挂载之前先检测一下该分区是否是fat分区，如果不是fat格式，返回-1*/
        if (Fat::check(devicePath)) {
            if (errno == ENODATA) {
                SLOGW("%s does not contain a FAT filesystem\n", devicePath);
                continue;
            }
            errno = EIO;
            /* Badness - abort the mount */
            SLOGE("%s failed FS checks (%s)", devicePath, strerror(errno));
            setState(Volume::State_Idle);
            return -1;
        }

        /*
         * Mount the device on our internal staging mountpoint so we can
         * muck with it before exposing it to non priviledged users.
         */
        errno = 0;
        /*将设备节点挂载在/mnt/secure/staging目录*/
        if (Fat::doMount(devicePath, "/mnt/secure/staging", false, false, 1000, 1015, 0702, true)) {
            SLOGE("%s failed to mount via VFAT (%s)\n", devicePath, strerror(errno));
            continue;
        }

        SLOGI("Device %s, target %s mounted @ /mnt/secure/staging", devicePath, getMountpoint());

        protectFromAutorunStupidity();
        /*挂载一个只有root用户能够访问的目录，这函数挂载了两次
    将/mnt/secure/staging/.android_secure 挂载在 /mnt/secure/asec，
    将tmpfs 挂载在 /mnt/secure/staging/.android_secure*/
        if (createBindMounts()) {
            SLOGE("Failed to create bindmounts (%s)", strerror(errno));
            umount("/mnt/secure/staging");
            setState(Volume::State_Idle);
            return -1;
        }

        /*
         * Now that the bindmount trickery is done, atomically move the
         * whole subtree to expose it to non priviledged users.
         */
        /*将挂载点挂载的目录再挂载到最终的目录/mnt/sdcard*/
        if (doMoveMount("/mnt/secure/staging", getMountpoint(), false)) {
            SLOGE("Failed to move mount (%s)", strerror(errno));
            umount("/mnt/secure/staging");
            setState(Volume::State_Idle);
            return -1;
        }
        setState(Volume::State_Mounted);
        mLastMountedKdev = mCurrentlyMountedKdev = deviceNodes[i];
        return 0;
    }

    SLOGE("Volume %s found no suitable devices for mounting :(\n", getLabel());
    setState(Volume::State_Idle);

    return -1;
}

这个挂载函数看起来，会发现很繁琐，好几个目录的挂载关系，有以下挂载目录：
/dev/block/vold/8:1 挂载在-> /mnt/secure/staging
/mnt/secure/staging/.android_secure 挂载在-> /mnt/secure/asec
tmpfs 挂载在-> /mnt/secure/staging/.android_secure
/mnt/secure/staging 挂载在-> /mnt/sdcard
从程序的注释看，这样的目的是挂载一个只有root用户能查看的目录，具体还是没搞清楚谷歌为什么要这样挂载，
还是有疑问，希望有清楚的高手指点一下。
sd卡的挂载比较清楚，中间多了一个中介，将设备节点8:1挂载在/mnt/secure/staging,最后又将该目录挂载在/mnt/sdcard，
这目录就是最终用户能够看到文件的目录。
函数里面涉及到几个函数：
getDeviceNodes函数获取挂载设备的设备号与分区数量，是Volume类的一个纯虚函数，在子类DirectVolume中实现，源码：

int DirectVolume::getDeviceNodes(dev_t *devs, int max) {
    if (mPartIdx == -1) {
        // If the disk has no partitions, try the disk itself
        if (!mDiskNumParts) {
            devs[0] = MKDEV(mDiskMajor, mDiskMinor);
            return 1;
        }

        int i;
        for (i = 0; i < mDiskNumParts; i++) {
            if (i == max)
                break;
            devs[i] = MKDEV(mDiskMajor, mPartMinors[i]);
        }
        return mDiskNumParts;
    }
    devs[0] = MKDEV(mDiskMajor, mPartMinors[mPartIdx -1]);
    return 1;
}

下面贴一些mountVol里面挂载的源码：

int Fat::doMount(const char *fsPath, const char *mountPoint,
                 bool ro, bool remount, int ownerUid, int ownerGid,
                 int permMask, bool createLost) {
    int rc;
    unsigned long flags;
    char mountData[255];

    flags = MS_NODEV | MS_NOEXEC | MS_NOSUID | MS_DIRSYNC;

    flags |= (ro ? MS_RDONLY : 0);
    flags |= (remount ? MS_REMOUNT : 0);

    /*
     * Note: This is a temporary hack. If the sampling profiler is enabled,
     * we make the SD card world-writable so any process can write snapshots.
     *
     * TODO: Remove this code once we have a drop box in system_server.
     */
    char value[PROPERTY_VALUE_MAX];
    property_get("persist.sampling_profiler", value, "");
    if (value[0] == '1') {
        SLOGW("The SD card is world-writable because the"
            " 'persist.sampling_profiler' system property is set to '1'.");
        permMask = 0;
    }

    sprintf(mountData,
            "utf8,uid=%d,gid=%d,fmask=%o,dmask=%o,shortname=mixed",
            ownerUid, ownerGid, permMask, permMask);

    rc = mount(fsPath, mountPoint, "vfat", flags, mountData);

    if (rc && errno == EROFS) {
        SLOGE("%s appears to be a read only filesystem - retrying mount RO", fsPath);
        flags |= MS_RDONLY;
        rc = mount(fsPath, mountPoint, "vfat", flags, mountData);
    }

    if (rc == 0 && createLost) {
        char *lost_path;
        asprintf(&lost_path, "%s/LOST.DIR", mountPoint);
        if (access(lost_path, F_OK)) {
            /*
             * Create a LOST.DIR in the root so we have somewhere to put
             * lost cluster chains (fsck_msdos doesn't currently do this)
             */
            if (mkdir(lost_path, 0755)) {
                SLOGE("Unable to create LOST.DIR (%s)", strerror(errno));
            }
        }
        free(lost_path);
    }

    return rc;
}

int Volume::createBindMounts() {
    unsigned long flags;

    /*
     * Rename old /android_secure -> /.android_secure
     */
    if (!access("/mnt/secure/staging/android_secure", R_OK | X_OK) &&
         access(SEC_STG_SECIMGDIR, R_OK | X_OK)) {
        if (rename("/mnt/secure/staging/android_secure", SEC_STG_SECIMGDIR)) {
            SLOGE("Failed to rename legacy asec dir (%s)", strerror(errno));
        }
    }

    /*
     * Ensure that /android_secure exists and is a directory
     */
    if (access(SEC_STG_SECIMGDIR, R_OK | X_OK)) {
        if (errno == ENOENT) {
            if (mkdir(SEC_STG_SECIMGDIR, 0777)) {
                SLOGE("Failed to create %s (%s)", SEC_STG_SECIMGDIR, strerror(errno));
                return -1;
            }
        } else {
            SLOGE("Failed to access %s (%s)", SEC_STG_SECIMGDIR, strerror(errno));
            return -1;
        }
    } else {
        struct stat sbuf;

        if (stat(SEC_STG_SECIMGDIR, &sbuf)) {
            SLOGE("Failed to stat %s (%s)", SEC_STG_SECIMGDIR, strerror(errno));
            return -1;
        }
        if (!S_ISDIR(sbuf.st_mode)) {
            SLOGE("%s is not a directory", SEC_STG_SECIMGDIR);
            errno = ENOTDIR;
            return -1;
        }
    }

    /*
     * Bind mount /mnt/secure/staging/android_secure -> /mnt/secure/asec so we'll
     * have a root only accessable mountpoint for it.
     */
    if (mount(SEC_STG_SECIMGDIR, SEC_ASECDIR, "", MS_BIND, NULL)) {
        SLOGE("Failed to bind mount points %s -> %s (%s)",
                SEC_STG_SECIMGDIR, SEC_ASECDIR, strerror(errno));
        return -1;
    }

    /*
     * Mount a read-only, zero-sized tmpfs  on <mountpoint>/android_secure to
     * obscure the underlying directory from everybody - sneaky eh? ;)
     */
    if (mount("tmpfs", SEC_STG_SECIMGDIR, "tmpfs", MS_RDONLY, "size=0,mode=000,uid=0,gid=0")) {
        SLOGE("Failed to obscure %s (%s)", SEC_STG_SECIMGDIR, strerror(errno));
        umount("/mnt/asec_secure");
        return -1;
    }

    return 0;
}

int Volume::doMoveMount(const char *src, const char *dst, bool force) {
    unsigned int flags = MS_MOVE;
    int retries = 5;

    while(retries--) {
        if (!mount(src, dst, "", flags, NULL)) {
            if (mDebug) {
                SLOGD("Moved mount %s -> %s sucessfully", src, dst);
            }
            return 0;
        } else if (errno != EBUSY) {
            SLOGE("Failed to move mount %s -> %s (%s)", src, dst, strerror(errno));
            return -1;
        }
        int action = 0;

        if (force) {
            if (retries == 1) {
                action = 2; // SIGKILL
            } else if (retries == 2) {
                action = 1; // SIGHUP
            }
        }
        SLOGW("Failed to move %s -> %s (%s, retries %d, action %d)",
                src, dst, strerror(errno), retries, action);
        Process::killProcessesWithOpenFiles(src, action);
        usleep(1000*250);
    }

    errno = EBUSY;
    SLOGE("Giving up on move %s -> %s (%s)", src, dst, strerror(errno));
    return -1;
}

篇幅有点长了，挂载点也挂载了好几个，mountVol处理的事情最多的，也最繁琐，但都是简单的，主要是有点摸不着头脑，
谷歌为啥要这样挂载，也许自己太菜了。。
下一篇文章继续。。