mount_umapfs -- sample file system layer
mount_umapfs [-o options] -u uid-mapfile -g gid-mapfile target
The mount_umapfs utility is used to mount a sub-tree of an existing file
system that uses a different set of uids and gids than the local system.
Such a file system could be mounted from a remote site via NFS or it
could be a file system on removable media brought from some foreign location
that uses a different password file.
The mount_umapfs utility uses a set of files provided by the user to make
correspondences between uids and gids in the sub-tree's original environment
and some other set of ids in the local environment. For instance,
user smith might have uid 1000 in the original environment, while having
uid 2000 in the local environment. The mount_umapfs utility allows the
subtree from smith's original environment to be mapped in such a way that
all files with owning uid 1000 look like they are actually owned by uid
The options are as follows:
-o 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.
target Should be the current location of the sub-tree in the local system's
Should be a directory where the mapped subtree is to be placed.
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.
THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM. USE AT YOUR OWN
RISK. BEWARE OF DOG. SLIPPERY WHEN WET.
This code also needs an owner in order to be less dangerous - serious
hackers can apply by sending mail to <hackers@FreeBSD.org> and announcing
their intent to take it over.
The mount_umapfs utility first appeared in 4.4BSD.
FreeBSD 5.2.1 May 1, 1995 FreeBSD 5.2.1 [ Back ]