mount_unionfs(8)

NAME

mount_unionfs - mount union file systems

SYNOPSIS

mount_unionfs [-br] [-o options] directory uniondir

DESCRIPTION

The mount_unionfs utility attaches directory above uniondir

in such a way
that the contents of both directory trees remain visible.

By default,
directory becomes the upper layer and uniondir becomes the

lower layer.

The options are as follows:

-b Invert the default position, so that directory be

comes the lower

layer and uniondir becomes the upper layer. Howev

er, uniondir
remains the mount point.

-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.

-r Hide the lower layer completely in the same way as

mounting with

mount_nullfs(8).

To enforce file system security, the user mounting the file

system must
be superuser or else have write permission on the mounted-on

directory.
In addition, the vfs.usermount sysctl(8) variable must be

set to 1 to
permit file system mounting by ordinary users.

Filenames are looked up in the upper layer and then in the

lower layer.
If a directory is found in the lower layer, and there is no

entry in the
upper layer, then a shadow directory will be created in the

upper layer.
It will be owned by the user who originally did the union

mount, with
mode ``rwxrwxrwx'' (0777) modified by the umask in effect at

that time.

If a file exists in the upper layer then there is no way to

access a file
with the same name in the lower layer. If necessary, a com

bination of
loopback and union mounts can be made which will still allow

the lower
files to be accessed by a different pathname.

Except in the case of a directory, access to an object is

granted via the
normal file system access checks. For directories, the cur

rent user must
have access to both the upper and lower directories (should

they both
exist).

Requests to create or modify objects in uniondir are passed

to the upper
layer with the exception of a few special cases. An attempt

to open for
writing a file which exists in the lower layer causes a copy

of the
entire file to be made to the upper layer, and then for the

upper layer
copy to be opened. Similarly, an attempt to truncate a low

er layer file
to zero length causes an empty file to be created in the up

per layer.
Any other operation which would ultimately require modifica

tion to the
lower layer fails with EROFS.

The union file system manipulates the namespace, rather than

individual
file systems. The union operation applies recursively down

the directory
tree now rooted at uniondir. Thus any file systems which

are mounted
under uniondir will take part in the union operation. This

differs from
the union option to mount(8) which only applies the union

operation to
the mount point itself, and then only for lookups.

EXAMPLES

The commands

mount -t cd9660 -o ro /dev/cd0a /usr/src
mount -t unionfs /var/obj /usr/src

mount the CD-ROM drive /dev/cd0a on /usr/src and then at

taches /var/obj
on top. For most purposes the effect of this is to make the

source tree
appear writable even though it is stored on a CD-ROM.

The command

mount -t unionfs -o -b /sys $HOME/sys

attaches the system source tree below the sys directory in

the user's
home directory. This allows individual users to make pri

vate changes to
the source, and build new kernels, without those changes be

coming visible
to other users. Note that the files in the lower layer re

main accessible
via /sys.

SEE ALSO

intro(2), mount(2), unmount(2), fstab(5), mount(8),

mount_nullfs(8)

HISTORY

The mount_unionfs utility first appeared in 4.4BSD. It

first worked in
FreeBSD-(fill this in).

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.

Without whiteout support from the file system backing the

upper layer,
there is no way that delete and rename operations on lower

layer objects
can be done. EROFS is returned for this kind of operations

along with
any others which would make modifications to the lower lay

er, such as
chmod(1).

Running find(1) over a union tree has the side-effect of

creating a tree
of shadow directories in the upper layer.

BSD March 27, 1994

BSD