This program was written after experimenting with unprivileged LXC containers.

LXC is a great tool but it also means your filesystem is filling with strangely numbered ids of files which will only work for a specific user using his alternative namespace. So, I sought a way to address this issue and thought it was also a great chance to brush up on the C language, and improve my knowledge of maintaining file systems.

At first I just thought of shifting the user and group id's, but then I realized that extended posix ACL's are also something to be reckoned with. They store numeric ids inside extended attributes. And I added a complete run through to check for showstoppers in the combination of parameters and files present.

Some usage info then:

./fs-offset -b base [-n] [-f] -- treeroot(single absolute path of dir)
-b : new base uid for tree based at basepath
-n : only do preliminary tests and print statistics (dryrun)
-f : force, ignore safety questions
-p : posix, alter posix acl qualifier ids
treeroot : the root of the tree to rebase.

CAUTION: The filesystem under the treeroot must not be accessed when running this program.

To compile you need to install the headers for libacl and libe2p on your system and architecture, and link them.

Special thanks go to:

• The people that support GNU for their great toolchain.
• The crews of You Complete Me and vim.
• The people participating in the clang set of tools.
• All the contributors to stackoverflow and its sister sites
• The great Michael Kerrisk for the terrific man-pages support.
• The contributors to LXC, namespaces and cgroups.