cryptsetup documentation on where /dev/mapper/ is needed/not needed not ideal

Binary package hint: cryptsetup

I've just been setting up encrypted /home/ and /var/ on my laptop, and was caught out by an extra /dev/mapper/ entry in /etc/crypttab, which led to very odd errors

If you setup your /etc/crypttab along the lines of:
   /dev/mapper/enc_home /dev/sda7 none luks
(instead of just "enc_home /dev/sda7 none luks")

Then when "/etc/rcS.d/S28cryptdisks start" runs, after unlocking your key, you'll get an error something like:
   dm_task_set_name /dev/mapper/enc_home: device not found

There's nothing in the header of /etc/crypttab to make it clear that you shouldn't prefix the target with /dev/mapper/ (which is required for /etc/fstab, so it's not too silly a mistake). The error message you get is really odd and cryptic, and ideally should be made clearer (eg it should issue a warning if you try to create an entry starting with "/dev/mapper" under /dev/mapper)

This is running 8.10 (rc), and cryptsetup is 2:1.0.6-6ubuntu2