stat inconsistency with overlayfs

[Atom2 <ariel.atom2@web2web.at>]

I am using find with its printf "%D" option (provides the same 
information as stats device information "%d" - device number in decimal) 
to figure out whether a file system entry resides in the r/o lowerdir or 
the r/w upperdir of an overlayfs mounted filesystem. I distinguish 
between the two by getting the device number from a (plain) file know to 
be in the upperdir.

The use case behind that is to be able to backup only files from the 
upperdir for several systems sharing a common lowerdir filesystem. I 
have used that (scripted approach via rsync) now for quiet some time and 
a few kernels back and it seemed to have worked very well.

Currently I am using kernel 3.17.7 on gentoo and I seem to observe a 
strange behaviour (which I do not recall to have seen before on 3.13 and 
3.11) with my approach as follows:

.) plain files still work and the device number is correct
.) directories, however, always seem to reside in the lowerdir - even 
thoguh they do not exist there; in fact there's not a single file in the 
whole filesystem hierarchy that, according to stat/find, seems to reside 
in the upperdir:

please see the stat output for a file and a directory, both residing in 
the same (parent) directory which is completely located in the upperdir 
(and does not at all exist in the lowerdir):
# stat serial
   File: ‘serial’
   Size: 17              Blocks: 8          IO Block: 4096   regular file
Device: ca03h/51715d    Inode: 88          Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2014-02-19 19:01:56.278161346 +0100
Modify: 2014-02-19 19:01:56.278161346 +0100
Change: 2014-02-19 19:01:56.278161346 +0100
  Birth: -
#
# stat certs/
   File: ‘certs/’
   Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: dh/13d  Inode: 331140      Links: 2
Access: (0700/drwx------)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2014-04-26 14:08:47.337968562 +0200
Modify: 2014-02-19 18:55:58.458161346 +0100
Change: 2014-02-19 18:55:58.458161346 +0100
  Birth: -

For comparision, please see the stat of /bin which only resides in the 
lowerdir and does not exist in the upperdir:
# stat /bin
   File: ‘/bin’
   Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: dh/13d  Inode: 401777      Links: 2
Access: (0755/drwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2015-02-20 17:50:24.066368607 +0100
Modify: 2015-02-09 17:51:18.000000000 +0100
Change: 2015-02-09 23:58:06.011825328 +0100
  Birth: -

I do not think that this is the expected behaviour and I am pretty 
confident that this was different on older kernels - or am I missing 
anything/doing anything wrong here?

Thanks and regards Atom2
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