mount_umapfs(8)

NAME

mount_umapfs - sample file system layer

SYNOPSIS

mount_umapfs [-o  options]  -u  uid-mapfile  -g  gid-mapfile
target
             mount-point

DESCRIPTION

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
2000.

The options are as follows:

-o Options are specified with a -o flag followed by a

comma sepa

rated 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 sys

tem's name space.

mount-point

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 identi

fiers. 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 origi

nal 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 cor

rect.

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.

SEE ALSO

mount(8), mount_nullfs(8)

HISTORY

The mount_umapfs utility first appeared in 4.4BSD.

BUGS

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.

BSD May 1, 1995

BSD