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!
