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     THE LOOP DEVICE
       One further possible type is a mount via the loop device. For  example,
       the command

         mount /tmp/fdimage /mnt -t msdos -o loop=/dev/loop3,blocksize=1024

       will  set  up  the  loop  device  /dev/loop3  to correspond to the file
       /tmp/fdimage, and then mount this device on /mnt.

       This type of mount knows about three options, namely loop,  offset  and
       encryption,  that  are  really  options  to  losetup(8).   If the mount
       requires a passphrase, you will be prompted for one unless you  specify
       a  file  descriptor  to  read  from  instead with the --pass-fd option.
       (These options can be  used  in  addition  to  those  specific  to  the
       filesystem type.)

       If  no  explicit loop device is mentioned (but just an option `-o loop'
       is given), then mount will try to find some unused loop device and  use
       that.  If you are not so unwise as to make /etc/mtab a symbolic link to
       /proc/mounts then any loop device allocated by mount will be  freed  by
       umount.   You  can also free a loop device by hand, using `losetup -d',
       see losetup(8).