shared: use move_pivot_root() for services

shared: use move_pivot_root() for services

Currently, services use mount_move_root() in order to setup the root
directory of services using a mount namespace. This relies on MS_MOVE
and chroot(). However, this has serious drawbacks even for relatively
simple mount propagation scenarios.

What systemd currently does is roughly equivalent to the following shell
code:

  unshare --mount --propagation=shared
  cd /
  mount --make-rslave /
  mkdir /new-root
  mount --rbind / /new-root
  cd /new-root
  mount --move /new-root /
  chroot .

This looks simple enough but has the consequence that two separate mount
trees exist for the lifetime of the service. The first one was created
when the mount namespace was created, and the second one when a new
mount for the rootfs was created. The first mount tree sticks around as
a shadow mount tree. Both mount trees are dependent mounts with the host
rootfs as their dominating mount.

Now, when mount propagation is triggered by the host by e.g.,

   mount --bind /opt /mnt

it means that two propagation events are generated. I'm skipping over
the exact kernel details as they aren't that important. The gist is that
for every propagation event that is generated a second one is generated
for the shadow mount tree. In other words, the kernel creates two copies
for each mount that is propagated instead of one.

This isn't necessary. We can simply change the sequence above to:

  unshare --mount --propagation=shared
  cd /
  mount --make-rslave /
  mkdir /new-root
  # stash fd to old rootfs
  # stash fd to new rootfs
  mount --rbind / /new-root
  mkdir /new-root
  cd /new-root
  pivot_root . .
  # new root is tucked under old root
  # chdir into old rootfs via stashed fd
  umount -l /old-root

The pivot_root allows us to get rid of the old mount tree that was
created when the mount namespace was created. So after this sequence
only one mount tree is alive. Plus, it's safer and nicer. Moving mounts
isn't pleasnt.

This patch doesn't convert nspawn yet as the requirements are more
tricky given that it wants to preserve the rootfs as a shared mount
which goes against pivot_root() requirements.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>