Re: more breakage there (was Re: [RFC] move_mount(2): still breakage around new mount detection)

[Christian Brauner <brauner@kernel.org>]

On Thu, May 08, 2025 at 06:56:10AM +0100, Al Viro wrote:
> On Mon, Apr 28, 2025 at 07:53:18PM +0100, Al Viro wrote:
> 
> > Up to you; propagation calculations *are* hard-serialized (on namespace_sem)
> > and changing that is too much pain to consider, so I have no problem with
> > globals in that specific case (note several such in propagate_mnt()
> > machinery; that was a deliberate decision to avoid shitloads of arguments
> > that would have to be passed around otherwise), but...
> 
> OK, now I finally understand what felt fishy about either solution.
> 
> Back when the checks had been IS_MNT_NEW, we were guaranteed that
> anything on the the slave lists of new mount would be new as well.
> 
> No amount of copy_tree() could change ->mnt_master of existing mounts,
> so anything predating the beginning of propagate_mnt() would still
> have ->mnt_master pointing to old mounts - no operations other than
> copy_tree() had been done since we have taken namespace_sem.

Yes.

> 
> That's where your IS_MNT_PROPAGATED breaks.  It mixes "nothing useful
> to be found in this direction" with "don't mount anything on this one".
> And these are not the same now.
> 
> Suppose you have mounts A, B and C, A propagating to B, B - to C.
> 
> If you made B private, propagation would go directly from A to C,
> and mount on A/foo would result in a copy on C/foo.
> 
> Suppose you've done open_tree B with OPEN_TREE_CLONE before making
> B private.  After open_tree your propagation graph is
> 	A -> [B <-> B'] -> C
> with new mount B' being in your anon_ns.  Making B private leaves you
> with

I cannot describe how much I hate mount propagation and how much I would
like to burn it from the face of this earth.

> 	A -> B' -> C
> and mount on A/foo still propagates to C/foo, along with foo in your
> anon_ns.
> 
> So far, so good, but what happens if you move_mount the root of your
> anon_ns to A/foo?  Sure, you want to suppress copying it to foo in B',
> but you will end suppressing the copy on C/foo as well.  propagation_next()
> will not visit C at all - when it reaches B', it'll see IS_MNT_PROPAGATED
> and refuse to look what B' might be propagating to.

I regret that I reenabled propagation into anonymous mount namepaces in
the first place.

> 
> IOW, IS_MNT_PROPAGATED in propagate_one() is fine, but in propagation_next(),
> skip_propagation_subtree() and next_group() we really need IS_MNT_NEW.
> And the check in propagate_one() should be
> 
> 	/* skip ones added by this propagate_mnt() */
> 	if (IS_MNT_NEW(m))
>                 return 0;
>         /* skip if mountpoint is outside of subtree seen in m */
> 	if (!is_subdir(dest_mp->m_dentry, m->mnt.mnt_root))
> 		return 0;
> 	/* skip if m is in the anon_ns we are emptying */
> 	if (m->mnt_ns->mntns_flags & MNTNS_PROPAGATING)
> 		return 0;
> That part of check is really about the validity of this particular
> location, not the cutoff for further propagation.  IS_MNT_NEW(),
> OTOH, is a hard cutoff.
> 
> FWIW, I would take the last remaining IS_MNT_PROPAGATED() (in
> propagation_would_overmount()) as discussed in this thread -
> with
> -       if (propagation_would_overmount(parent_mnt_to, mnt_from, mp))
> +       if (check_mnt(mnt_from) &&
> +           propagation_would_overmount(parent_mnt_to, mnt_from, mp))
> in can_move_mount_beneath() and lose the one in propagation_would_overmount()

Yes.

> 
> I'll cook something along those lines (on top of "do_move_mount(): don't
> leak MNTNS_PROPAGATING on failures") and if it survives overnight tests
> post it tomorrow^Win the morning...

Thanks!