losetup silently fails to attach small files

1) # lsb_release -rd
Description: Ubuntu 12.04.4 LTS
Release: 12.04

2) # apt-cache policy util-linux
util-linux:
  Installed: 2.20.1-1ubuntu3
  Candidate: 2.20.1-1ubuntu3
  Version table:
 *** 2.20.1-1ubuntu3 0
        500 http://archive.ubuntu.com//ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status

3) losetup -f /srv/swift.img
attach the file to a free loop device

4) losetup -f /srv/swift.img returned 0, but did not attach the file to a loop device.

Specifically, the target file was very small, 20 bytes. The following sequence was run:

truncate --size 20 /srv/swift.img
stat /srv/swift.img
losetup -f /srv/swift.img
losetup -a

When the file is small (20 bytes) losetup returns zero, but does not setup the file. Through some investigation, it appears any file of size < 512 will fail in this way:

root@hayward-42:~# truncate --size 511 /srv/swift.img
root@hayward-42:~# stat /srv/swift.img
  File: `/srv/swift.img'
  Size: 511 Blocks: 0 IO Block: 4096 regular file
Device: 801h/2049d Inode: 5767170 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2014-06-03 19:24:58.213512482 +0000
Modify: 2014-06-03 19:26:52.593509603 +0000
Change: 2014-06-03 19:26:52.593509603 +0000
 Birth: -
root@hayward-42:~# losetup --find /srv/swift.img
root@hayward-42:~# echo $?
0
root@hayward-42:~# losetup -a

Here is a successful sequence with size=512:

root@hayward-42:~# truncate --size 512 /srv/swift.img
root@hayward-42:~# stat /srv/swift.img
  File: `/srv/swift.img'
  Size: 512 Blocks: 0 IO Block: 4096 regular file
Device: 801h/2049d Inode: 5767170 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2014-06-03 19:24:58.213512482 +0000
Modify: 2014-06-03 19:27:33.029508499 +0000
Change: 2014-06-03 19:27:33.029508499 +0000
 Birth: -
root@hayward-42:~# losetup --find /srv/swift.img
root@hayward-42:~# echo $?
0
root@hayward-42:~# losetup -a
/dev/loop7: [0801]:5767170 (/srv/swift.img)

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: mount 2.20.1-1ubuntu3
ProcVersionSignature: Ubuntu 3.2.0-60.91-generic 3.2.55
Uname: Linux 3.2.0-60-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Tue Jun 3 19:16:57 2014
MarkForUpload: True
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: util-linux
UpgradeStatus: No upgrade log present (probably fresh install)