Comment 8 for bug 1214500

one simple little test that i see differing results depending on kernel is 'mkfifo foo && ln foo bar'.
Heres a test script that demos that, and output on precise, raring, saucy (no containers used).

== raring: 3.8.0-27-generic ==
$ ls -ld .
drwxrwxrwt 5 root root 4096 Aug 21 19:16 .
$ sudo rm -Rf lower upper mp
$ mkdir lower upper mp
$ sh -c 'echo hi > lower/f1.txt'
$ sudo mount -t overlayfs -o lowerdir=/tmp/lower,upperdir=/tmp/upper overlay /tmp/mp
$ sh -c 'cd mp && touch foo && ln foo bar'
file link: user: PASS
$ sudo sh -c 'cd mp && touch foo && ln foo bar'
file link: root: PASS
$ sh -c 'cd mp && mkfifo foo && ln foo bar'
fifo link: user: PASS
$ sudo sh -c 'cd mp && mkfifo foo && ln foo bar'
fifo link: user: PASS

== saucy: 3.11.0-2-generic ==
$ ls -ld .
drwxrwxrwt 17 root root 32768 Aug 21 15:17 .
$ sudo rm -Rf lower upper mp
[sudo] password for smoser:
$ mkdir lower upper mp
$ sh -c 'echo hi > lower/f1.txt'
$ sudo mount -t overlayfs -o lowerdir=/tmp/lower,upperdir=/tmp/upper overlay /tmp/mp
$ sh -c 'cd mp && touch foo && ln foo bar'
file link: user: PASS
$ sudo sh -c 'cd mp && touch foo && ln foo bar'
file link: root: PASS
$ sh -c 'cd mp && mkfifo foo && ln foo bar'
fifo link: user: PASS
$ sudo sh -c 'cd mp && mkfifo foo && ln foo bar'
fifo link: user: PASS

== precise: 3.2.0-51-virtual ==
$ ls -ld .
drwxrwxrwt 8 root root 4096 Aug 21 19:18 .
$ sudo rm -Rf lower upper mp
$ mkdir lower upper mp
$ sh -c 'echo hi > lower/f1.txt'
$ sudo mount -t overlayfs -o lowerdir=/tmp/lower,upperdir=/tmp/upper overlay /tmp/mp
$ sh -c 'cd mp && touch foo && ln foo bar'
file link: user: PASS
$ sudo sh -c 'cd mp && touch foo && ln foo bar'
file link: root: PASS
$ sh -c 'cd mp && mkfifo foo && ln foo bar'
ln: failed to create hard link `bar' => `foo': Operation not permitted
fifo link: user: FAIL[1]
$ sudo sh -c 'cd mp && mkfifo foo && ln foo bar'
fifo link: user: PASS