Andy Lutomirski [off-list ref] writes:

quoted

On Aug 10, 2018, at 7:05 AM, Eric W. Biederman [off-list ref] wrote:


There is a serious problem with mount options today that fsopen does not
address.  The problem is that mount options are ignored for block based
filesystems, and any other type of filesystem that follows the same
pattern.

quoted

/dev/loop0 /root/loop0-noacl-noquota-nouser_xattr ext4 rw,relatime,nouser_xattr,noacl 0 0
/dev/loop0 /root/loop0-acl-quota-user_xattr ext4 rw,relatime,nouser_xattr,noacl 0 0

To make sure I understand correctly: the problem is that the second
mount ignored the options because the device was already mounted,
right?

Yes.

For the new API, I think the only remotely sane approach is to refuse
to mount or init or whatever you call it an already mounted bdev. If
user code genuinely needs to bind-mount an existing mount that is
known only by its bdev, we can add a specific API just for that.

Eric