Options are specified with a -o flag followed by a comma separated string of options. See the mount(8) man page for possible options and their meanings.
Should be the current location of the sub-tree in the local systems name space.
Should be a directory where the mapped subtree is to be placed.
-u uid-mapfile -g gid-mapfile
Describe the mappings to be made between identifiers. Briefly, the format of these files is a count of the number of mappings on the first line, with each subsequent line containing a single mapping. Each of these mappings consists of an id in the local environment and the corresponding id from the original environment, separated by white space. Uid-mapfile should contain all uid mappings, and gid-mapfile should contain all gid mappings. Any uids not mapped in uid-mapfile will be treated as user NOBODY, and any gids not mapped in gid-mapfile will be treated as group NULLGROUP. At most 64 uids can be mapped for a given subtree, and at most 16 groups can be mapped by a given subtree.
The mapfiles can be located anywhere in the file hierarchy, but they must be owned by root, and they must be writable only by root. The mount_umapfs utility will refuse to map the sub-tree if the ownership or permissions on these files are improper. It will also balk if the count of mappings in the first line of the map files is not correct.
The layer created by the mount_umapfs utility is meant to serve as a simple example of file system layering. It is not meant for production use. The implementation is not very sophisticated.