use blkid -p on luks loopback
util-linux 2.22.2 causes blkid to mysteriously fail ~half the time on loopback files unless you pass -p
I don't understand it but solves a long-standing problem that kept me from upgrading util-linux (and cryptsetup on the linodes)
loopfile needs to be world-readable now, but that shouldn't matter security-wise as it's encrypted