Cifs: CONFIG_RESTRICT_ROOTFS_SLAVE.

Android 4.2 and up implements multi-user storage using per-process mount namespaces.  Everything under "/" (the entire filesystem hierarchy) is marked as a recursive-slave mountspace for all zygote instances.  This is done so that user-storage sandbox mounts under /storage/emulated are hidden from other apps and users.  Unfortunately this means that any Dalvik app (actually, any program whose clone/fork ancestry includes a Dalvik zygote, which is everything except services spawned directly from init) cannot mount system-wide volumes.

This option restricts rootfs-slave calls to /storage (and/mnt/shell/emulated) so that Dalvik apps can mount system-wide volumes elsewhere (with appropriate permission, as in earlier versions of Android), while retaining full multi-user storage compatibility.  It is made available as a kernel-based workaround for instances where libdvm can not be modified.