vufs.md

vufs: VUOS virtual filesystems

Files and file system subtrees can be moved, merges, can be made virtually writable, it is possible to change ownership and permission, create virtual devices using vufs

This table summarizes the four modes of vufs. A short description of each mode follows.

vumount -t vufs -o mode A B

mode	A	B	comments:
bind (default)	R/W	not accessible	A substitutes B
merge	Read Only	R/W	merge A on B, updates in B
cow	R/W	Read Only	merge A on B, updates in A, copy-on-write
mincow	R/W	R/W	merge A on B, updates in B, copy-on-write on A if not permitted

bind mode

This is the default mode (the option -o bind can be omitted.

$$ vu_insmod vufs
$$ vumount -t vufs /tmp/a /tmp/b

or

$$ vumount -t vufs -o bind /tmp/a /tmp/b

The contents of /tmp/a appear in /tmp/b too. The original contents of /dev/b cannot be accessed.

merge mode

$$ vumount -t vufs -o merge /tmp/a /tmp/b

The contents of /tmp/a is merged with the contents of /tmp/b. More precisely the contents of /tmp/a are added to those of /tmp/b. If a file or directory exists both in /tmp/a and /tmp/b the one in /tmp/a prevails. The files and directories coming from /tmp/a are in read-only mode. New files and directories are created in /tmp/b, any modification takes place in /tmp/b.

copy-on-write (cow) mode

$$ vumount -t vufs -o cow /tmp/a /tmp/b

The contents of /tmp/a is merged with the contents of /tmp/b. The original contents of /tmp/b are not modified. All new files and directories are created in /tmp/a. Changes in files originally in /tmp/b causes the file to be copied in /tmp/a and then modified.

$$ vumount -t vufs -o mincow /tmp/a /tmp/b

minimal copy-on-write (cow) mode

Minimal means that files are copied only when it is not possible to update the original files. So in this mode the contents of /tmp/a is merged with the contents of /tmp/b. vufs tries to perform the actions requested on /tmp/b. If then it discovers that the action is not permitted on /tmp/b it works on /tmp/a copying the files when needed.

Some magic: disappearing directories

Here is the content of /var and /var/spool:

$$ ls /var
backups  cache  lib  local  lock  log  mail  opt  run  spool  tmp
$$ ls -l /var/spool
total 8
drwxr-xr-x 3 root root 4096 Jun 20 04:30 cron
lrwxrwxrwx 1 root root    7 Jun 20 04:28 mail -> ../mail
drwx------ 2 root root 4096 Apr 28 16:28 rsyslog

Let us not set up a directory in /tmp/var and an empty subdir /tmp/var/spool

$$ mkdir /tmp/var
$$ mkdir /tmp/var/spool

And now using vufs, bind mode and an exception all the subdirectories of /var/ disappear (except /var/spool):

$$ vumount -t vufs -o bind,except=/spool /tmp/var /var
$$ ls /var
spool
$$ ls -l /var/spool
total 8
drwxr-xr-x 3 root root 4096 Jun 20 04:30 cron
lrwxrwxrwx 1 root root    7 Jun 20 04:28 mail -> ../mail
drwx------ 2 root root 4096 Apr 28 16:28 rsyslog

(virtually) modify system files

Create a empty directory and mount it in mincow mode:

$$ mkdir /tmp/newroot
$$ vumount -t vufs -o mincow,except=/tmp,except=/proc /tmp/newroot /

Now using your favorite editor (e.g. vi or nano) delete the line of teh user user from /etc/passwd

$$ nano /etc/passwd
# .... delete the line and save
$$ whoami
whoami: cannot find name for user ID 1000

Now delete a file in /etc

$$ vu_insmod vufs
$$ vumount -t vufs -o mincow /tmp/newroot/ /
$$ ls -l /etc/protocols
-rw-r--r-- 1 root root 2932 Apr  1  2013 /etc/protocols
$$ rm /etc/protocols
rm: remove write-protected regular file '/etc/protocols'? y
$$ ls -l /etc/protocols
ls: cannot access '/etc/protocols': No such file or directory

It is also possible to (virtually) change ownership or permissions of files.

$$ ls -l /etc/passwd
-rw-r--r-- 1 root root 1689 Jul 22 16:10 /etc/passwd
$$ chown user /etc/passwd
$$ chmod 600 /etc/passwd
$$ ls -l /etc/passwd
-rw------- 1 user root 1689 Jul 22 16:10 /etc/passwd