# Copyright (C) 2005-2014 Junjiro R. Okajima Branch Manipulation Since aufs supports dynamic branch manipulation, ie. add/remove a branch and changing its permission/attribute, there are a lot of works to do. Add a Branch ---------------------------------------------------------------------- o Confirm the adding dir exists outside of aufs, including loopback mount. - and other various attributes... o Initialize the xino file and whiteout bases if necessary. See struct.txt. o Check the owner/group/mode of the directory When the owner/group/mode of the adding directory differs from the existing branch, aufs issues a warning because it may impose a security risk. For example, when a upper writable branch has a world writable empty top directory, a malicious user can create any files on the writable branch directly, like copy-up and modify manually. If something like /etc/{passwd,shadow} exists on the lower readonly branch but the upper writable branch, and the writable branch is world-writable, then a malicious guy may create /etc/passwd on the writable branch directly and the infected file will be valid in aufs. I am afraid it can be a security issue, but nothing to do except producing a warning. Delete a Branch ---------------------------------------------------------------------- o Confirm the deleting branch is not busy To be general, there is one merit to adopt "remount" interface to manipulate branches. It is to discard caches. At deleting a branch, aufs checks the still cached (and connected) dentries and inodes. If there are any, then they are all in-use. An inode without its corresponding dentry can be alive alone (for example, inotify/fsnotify case). For the cached one, aufs checks whether the same named entry exists on other branches. If the cached one is a directory, because aufs provides a merged view to users, as long as one dir is left on any branch aufs can show the dir to users. In this case, the branch can be removed from aufs. Otherwise aufs rejects deleting the branch. If any file on the deleting branch is opened by aufs, then aufs rejects deleting. Modify the Permission of a Branch ---------------------------------------------------------------------- o Re-initialize or remove the xino file and whiteout bases if necessary. See struct.txt. o rw --> ro: Confirm the modifying branch is not busy Aufs rejects the request if any of these conditions are true. - a file on the branch is mmap-ed. - a regular file on the branch is opened for write and there is no same named entry on the upper branch.