[BUG] propagation graph breakage by MOVE_MOUNT_SET_GROUP move_mount(2)

[Al Viro <viro@zeniv.linux.org.uk>]

AFAICS, 9ffb14ef61ba "move_mount: allow to add a mount into an existing
group" breaks assertions on ->mnt_share/->mnt_slave.  For once, the data
structures in question are actually documented.

Documentation/filesystem/sharedsubtree.rst:
        All vfsmounts in a peer group have the same ->mnt_master.  If it is
	non-NULL, they form a contiguous (ordered) segment of slave list.

fs/pnode.c:
 * Note that peer groups form contiguous segments of slave lists.

fs/namespace.c:do_set_group():
        if (IS_MNT_SLAVE(from)) {
                struct mount *m = from->mnt_master;

                list_add(&to->mnt_slave, &m->mnt_slave_list);
                to->mnt_master = m;
        }

        if (IS_MNT_SHARED(from)) {
                to->mnt_group_id = from->mnt_group_id;
                list_add(&to->mnt_share, &from->mnt_share);
                lock_mount_hash();
                set_mnt_shared(to);
                unlock_mount_hash();
        }

Note that 'to' goes right after 'from' in ->mnt_share (i.e. peer group
list) and into the beginning of the slave list 'from' belongs to.  IOW,
contiguity gets broken if 'from' is both IS_MNT_SLAVE and IS_MNT_SHARED.
Which is what happens when the peer group 'from' is in gets propagation
from somewhere.

It's not hard to fix - something like

        if (IS_MNT_SHARED(from)) {
		to->mnt_group_id = from->mnt_group_id;
                list_add(&to->mnt_share, &from->mnt_share);
		if (IS_MNT_SLAVE(from))
			list_add(&to->mnt_slave, &from->mnt_slave);
		to->mnt_master = from->mnt_master;
                lock_mount_hash();
                set_mnt_shared(to);
                unlock_mount_hash();
        } else if (IS_MNT_SLAVE(from)) {
		to->mnt_master = from->mnt_master;
		list_add(&to->mnt_slave, &from->mnt_master->mnt_slave_list);
	}

ought to do it.  I'm nowhere near sufficiently awake right now to put
together a regression test, but unless I'm missing something subtle, it
should be possible to get a fairly obvious breakage of propagate_mnt()
out of that...