Merge tag 'vfs-7.0-rc1.nullfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Merge tag 'vfs-7.0-rc1.nullfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs nullfs update from Christian Brauner:
 "Add a completely catatonic minimal pseudo filesystem called "nullfs"
  and make pivot_root() work in the initramfs.

  Currently pivot_root() does not work on the real rootfs because it
  cannot be unmounted. Userspace has to recursively delete initramfs
  contents manually before continuing boot, using the fragile
  switch_root sequence (overmount + chroot).

  Add nullfs, a minimal immutable filesystem that serves as the true
  root of the mount hierarchy. The mutable rootfs (tmpfs/ramfs) is
  mounted on top of it. This allows userspace to simply:

      chdir(new_root);
      pivot_root(".", ".");
      umount2(".", MNT_DETACH);

  without the traditional switch_root workarounds. systemd already
  handles this correctly. It tries pivot_root() first and falls back
  to MS_MOVE only when that fails.

  This also means rootfs mounts in unprivileged namespaces no longer
  need MNT_LOCKED, since the immutable nullfs guarantees nothing can be
  revealed by unmounting the covering mount.

  nullfs is a single-instance filesystem (get_tree_single()) marked
  SB_NOUSER | SB_I_NOEXEC | SB_I_NODEV with an immutable empty root
  directory. This means sooner or later it can be used to overmount
  other directories to hide their contents without any additional
  protection needed.

  We enable it unconditionally. If we see any real regression we'll
  hide it behind a boot option.

  nullfs has extensions beyond this in the future. It will serve as a
  concept to support the creation of completely empty mount namespaces -
  which is work coming up in the next cycle"

* tag 'vfs-7.0-rc1.nullfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  fs: use nullfs unconditionally as the real rootfs
  docs: mention nullfs
  fs: add immutable rootfs
  fs: add init_pivot_root()
  fs: ensure that internal tmpfs mount gets mount id zero