OpenWRT tips

Feb 17 2012 Published by under howto

“Undeleting” files from the /rom filesystem…

This post describes in gory detail how to recover files from the ROM image in case you accidentally removed a base package from OpenWRT.
In brief, locate files called ‘META_*’ on /overlay and edit them to remove deletions from the overlay filesystem. For detail, read on…

TL;DR

When first installed, the OpenWRT image is built into a squashfs read-only filesystem, mounted at /rom and underlying the root (/).  Writes to the root filesystem persist on a jffs2 filesystem mounted at /overlay and merged at the root using mini_fo.  Ignoring operations occurring actually on /rom/ and /overlay paths, if a file is overwritten, the file is actually stored in /overlay at the same relative location, and mini_fo makes the newer file appear to the system form that point.  Operations in /overlay/ itself are obviously filtered by mini_fo.  /tmp, /dev, etc. are all mounted on tmpfs outside of this process.

So how are deleted files handled? It turns out there is a special file made in each directory in /overlay, that mini_fo hides from view in the applicable root level directory, that records files on /rom that are deleted.

The special files are prefixed with ‘META_’, for  example: META_dAfFgHE39ktF3HD2sr, the content is simply one file per line, two fields, the letter D followed by the deleted file:

Undelete files by simply removing the entry from the META_ file.

In the event you remove a package that was in the /rom image using ipkg, it is easy enough to get back by finding editing all the special files in /overlay and removing the deletion references.

Hint: although there are various to choose from, I always start with the smallest /rom image, as that allows easier reconfiguration by adding and later removing packages, and provides a bigger R/W jffs2 partition at /overlay.  If you need to delete a package that was in /rom, it doesn’t actually give you any extra space on the small SSD…

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