commit a987d621d8ddc54cdf32c5f69c8e9873f0c3a199
author Milton D. Miller II <miltonm@us.ibm.com> Mon Feb 29 21:34:43 2016 -0600
committer Milton D. Miller II <miltonm@us.ibm.com> Fri Mar 04 10:51:05 2016 -0600
tree bed0b702b57f1205f136fe0877f8836b25a20a45
parent c3697de01b798f1ed69652d8463a355e6b9534cd

update: Save whitelist files from RAM or mounted read-write filesystem

Skip mounting and unmounting the read-write file system when
saving files if the upper directory exists.  Instead interpret
this as a sign the system either is or was running from RAM (or
from some future alternative and maybe temporary mounted file
system) and save the files from that upper directory.

It is possible some other filesystem is mounted on the read-write
mountpoint that is not a mtd device, either a tmpfs or some
other future media, so only try to unmount the file system if
this update script mounted it.

Each boot an empty /run filesytem is created and populated by init
with selected directories and mount points for the read-only and
read-write file systems and mounts them on these points.

The upper directory is under the read-write mount point, so if it
exists then either a file system is mounted there or the system
was prepared to run from RAM using the run tmpfs file system and
updates may have been made there.  In either case the files to
be saved exist in that directory.

Background on overlayfs:

The upper directory contains files and directory entries that were
either opened for write or had meta data changed.  Before this
happens the overlayfs copies each file or directory by name into
the work directory then atomically moves it into the corresponding
upper directory.

To form a read-write overlayfs mount, the upper directory must be,
by definition, in a read-write file system along with the work
directory, and both are required to be in the same filesystem.

Signed-off-by: Milton Miller <miltonm@us.ibm.com>