Re: [bug report] A filesystem abnormal mount issue

[Zizhi Wo <wozizhi@huawei.com>]

在 2025/7/17 17:39, Christian Brauner 写道:
> On Thu, Jul 17, 2025 at 05:11:50PM +0800, Zizhi Wo wrote:
>> Currently, we have the following test scenario:
>>
>> disk_container=$(
>>      ${docker} run...kata-runtime...io.kubernets.docker.type=container...
>> )
>> docker_id=$(
>>      ${docker} run...kata-runtime...io.kubernets.docker.type=container...
>>      io.katacontainers.disk_share="{"src":"/dev/sdb","dest":"/dev/test"}"...
>> )
>>
>> ${docker} stop "$disk_container"
>> ${docker} exec "$docker_id" mount /dev/test /tmp -->success!!
>>
>> When the "disk_container" is started, a series of block devices are
>> created. During the startup of "docker_id", /dev/test is created using
>> mknod. After "disk_container" is stopped, the created sda/sdb/sdc disks
>> are deleted, but mounting /dev/test still succeeds.
>>
>> The reason is that runc calls unshare, which triggers clone_mnt(),
>> increasing the "sb->s_active" reference count. As long as the "docker_id"
>> does not exit, the superblock still has a reference count.
>>
>> So when mounting, the old superblock is reused in sget_fc(), and the mount
>> succeeds, even if the actual device no longer exists. The whole process can
>> be simplified as follows:
>>
>> mkfs.ext4 -F /dev/sdb
>> mount /dev/sdb /mnt
>> mknod /dev/test b 8 16    # [sdb 8:16]
>> echo 1 > /sys/block/sdb/device/delete
>> mount /dev/test /mnt1    # -> mount success
>>
>> The overall change was introduced by: aca740cecbe5 ("fs: open block device
>> after superblock creation"). Previously, we would open the block device
>> once. Now, if the old superblock can be reused, the block device won't be
>> opened again.
>>
>> Would it be possible to additionally open the block device in read-only
>> mode in super_s_dev_test() for verification? Or is there any better way to
>> avoid this issue?
> 
> As long as you use the new mount api you should pass
> FSCONFIG_CMD_CREATE_EXCL which will refuse to mount if a superblock for
> the device already exists. IOW, it ensure that you cannot silently reuse
> a superblock.

Yes, it is indeed exclusive.

> 
> Other than that I think a blkdev_get_no_open(dev, false) after
> lookup_bdev() should sort the issue out. Christoph?

Oh, I didn't consider it before. blkdev_get_no_open() is sufficient.
Thanks for suggestion!

Thanks,
Zizhi Wo