Why is /dev on a different filesystem ? [Kernel 2.6.20.3]

["Paul Rolland" <rol@as2917.net>]

Hello,

I was trying to backup a machine using tar and the --one-file-system option
and I was getting an archive without /dev, but tar was spitting :
/dev: file is on a different filesystem; not dumped

So, I had a look at the code in tar, and the comparison is done on the
stat.st_dev field... 
I then wrote a simple program to stat("/", ...) and stat("/dev", ...),
and got :

For / :
st_dev   : 769
st_ino   : 2
st_mode  : 16877
st_nlink : 23
st_uid   : 0
st_gid   : 0
st_rdev  : 0
st_size  : 4096

For /dev :
st_dev   : 15
st_ino   : 1117
st_mode  : 16877
st_nlink : 23
st_uid   : 0
st_gid   : 0
st_rdev  : 0
st_size  : 165580

So, obviously, tar is right...
man 2 stat says :
       The st_dev field describes the device on which this file resides.

and df -a reports :
[root@host-181 src]# df -a
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda1              7936256   3105556   4421048  42% /
proc                         0         0         0   -  /proc
sysfs                        0         0         0   -  /sys
devpts                       0         0         0   -  /dev/pts
tmpfs                   517632         0    517632   0% /dev/shm
/dev/hda5             61787140    237360  58360480   1% /usr/local/witbe
/dev/hda2              4956316    142292   4558192   4% /var/log
none                         0         0         0   -
/proc/sys/fs/binfmt_misc

So, obviously, /dev is on /, but the stat(2) says no.
Who is right, and where is the bug ?

Kernel 2.4 had it right : /dev was on /, no doubt.

Regards,
Paul