MOUNT:UNMOUNTED-BELOW: stop calling 'mount --make-rslave'

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>

1 files changed, 31 insertions, 7 deletions

diff --git a/src/property/mount.lisp b/src/property/mount.lisp
index fe805ad..72eb8d7 100644
--- a/src/property/mount.lisp
+++ b/src/property/mount.lisp
@@ -19,14 +19,42 @@
 (named-readtables:in-readtable :consfigurator)
 
 (defprop unmounted-below :posix (dir)
-  "Unmount anything mounted at or below DIR."
+  "Unmount anything mounted at or below DIR.
+
+Not aware of shared subtrees, so you might need to use the --make-rslave
+option to mount(1) first.  For example, if you did 'mount --rbind /dev
+chroot/dev' then unless you also execute 'mount --make-rslave chroot/dev',
+this property will empty /dev, breaking all kinds of things."
   (:desc #?"${dir} unmounted")
   (:hostattrs
-   (declare (ignore dir))
-   ;; findmnt(1) & arguments to mount(1)/umount(1) we use are from util-linux
+   ;; findmnt(1) & --recursive argument to umount(1) are from util-linux
    (os:required 'os:linux))
   (:apply
    (with-change-if-changes-file-content ("/proc/mounts")
+     ;; We used to call --make-rslave as we worked through, but for mounts
+     ;; which were *not* made using the --rbind option to mount(1) or similar,
+     ;; doing that can can get us into a state where we can unmount everything
+     ;; we can see under DIR but the kernel will still consider the block
+     ;; device to be in use.  That's a bit much for this property to deal
+     ;; with, so we require that the caller call --make-rslave as appropriate.
+     ;;
+     ;; In addition to that problem, we would also require multiple unmount
+     ;; passes; for example if we
+     ;;
+     ;;   % mount --bind chroot chroot
+     ;;   % mount proc chroot/proc -t proc
+     ;;   % mount --make-rslave chroot
+     ;;
+     ;; then we would need
+     ;;
+     ;;   % umount chroot/proc
+     ;;   % umount chroot
+     ;;   % umount chroot/proc
+     ;;
+     ;; because the --make-rslave leaves us with two independent mounts of
+     ;; /proc, and the second can't be removed until the bind mount is
+     ;; removed.  (This situation arises because :CHROOT.FORK connections bind
+     ;; mount the chroot on itself if it is not already a mount point.)
      (let* ((dir (ensure-directory-pathname dir))
             (all-mounts
               (mapcar #'ensure-directory-pathname
@@ -36,10 +64,6 @@
        (loop as next = (pop sorted)
              while next
              do (loop while (subpathp (car sorted) next) do (pop sorted))
-                ;; If any of the mounts were made with --rbind then unmounting
-                ;; will unmount the source filesystems too (e.g. things
-                ;; mounted under /dev), so use --make-rslave to prevent that.
-                (mrun "mount" "--make-rslave" next)
                 (mrun "umount" "--recursive" next))))))
 
 (defproplist unmounted-below-and-removed :posix (dir)