It turns out that this problem is a side-effect of the approach used to implement multi-user storage in Android 4.2. I've explained the problem in detail in the commit log for a Gerrit issue we're reviewing for CyanogenMod 10.1 that addresses the problem:
Zygote: Restrict slave mountspace so Dalvik apps can mount system-wide volumes
Android 4.2 implements multi-user storage using per-process mount namespaces. Originally, 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. Thus, apps like CifsManager are effectively broken in Android 4.2, since its cifs mounts are only visible to the CifsManager app itself. All other apps see empty mountpoints instead of the mounted volume. Furthermore, Linux provides no provision for a process to "escape" a recursive-slave mountspace in versions prior to Linux 3.8 (setns syscall).
Here, we restrict the slave mountspace to /storage (and, due to a possible kernel bug, /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.
This change requires that a tmpfs volume is mounted as /storage in init.rc. If this volume is unavailable, then Zygote falls back to the previous behavior of marking the entire filesystem hierarchy as slave. It also implicitly requires that EMULATED_STORAGE_TARGET is path-prefixed by (part of the subhierarchy of) ANDROID_STORAGE, which is the typical case.
- Dalvik change: Zygote: Restrict slave mountspace so Dalvik apps can mount system-wide volumes
- init.rc change: init.rc: Create /storage mountpoint so Dalvik can mark as slave in zygotes
Alternatively, ROMs that can't/prefer not to use a source-build Dalvik (libdvm), I've also provided a kernel patch that implements a similar workaround within the kernel. It also requires the above init.rc modification:
- Kernel commit: Restrict slave mountspace so Dalvik apps can mount system-wide volumes
- init.rc change: init.rc: Create /storage mountpoint so Dalvik can mark as slave in zygotes
With either of the above two fixes, CifsManager et al. should work when using a mountpoint outside /storage (and /mnt/shell/emulated). I'd recommend using "/mnt/cifs" or something similar. Attempting to mount inside /storage retains the previous behavior where the mount can not be seen by other apps.
Note that ROMs only need one of the above two fixes, although they are compatible with each other and will function correctly if both are present. The Dalvik approach is preferred over the kernel workaround where feasible. Attached are the three patches referenced in the issues/commits.