Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

Andy Lutomirski [off-list ref] writes:

Why should MNT_LOCKED on submounts be enforced?

Is it because, if you retain a reference to the detached tree, then
you can see under the submounts?  

Yes. MNT_DETACH is a recursive operation that detaches all of the mount
and all of it's submounts.  Which means you can see under the submounts
if you have a reference to a detached mount.

If so, let's fix *that*.  Because
otherwise the whole model of pivot_root + detach will break.

I am not certain what you are referring to.  pivot_root doesn't
manipulate the mount tree so you can see under anything.

What I believe is the appropriate fix is to fail umount2(...,MNT_DETACH)
if there are any referenced mount points being detached that have a
locked submount.

Also, damn it, we need change_the_ns_root instead of pivot_root.  I
doubt that any container programs actually want to keep the old root
attached after pivot_root.

Shrug.  Except for chroot_fs_refs() pivot_root is a cheap.

I'm not particularly in favor of merging pivot_root and umount2.  The
number of weird cases in the current api are high.  A merged piece of
code would just make them higher.

I am hoping that one more round of bug fixing will at least get the bugs
for having unprivileged mounts fixed in the current API.

Eric