Disklabel question (versus bug)

[walt]

Perhaps a little background first:

I've been using both DragonFly-current and FreeBSD-current from
extended(logical) slices for about a year or so.  This is possible
only because the DragonFly /boot/loader was patched to parse the
DOS partition table correctly (unlike the FBSD version).

Now -- can any of you gurus explain why the DragonFly disklabel
program behaves this way when reading a FreeBSD disklabel:

# disklabel ad1s8  [this is FBSD on an extended/logical slice]
disklabel: ioctl DIOCGDINFO: Invalid argument

BUT, if I add the -r flag, it works normally:

# disklabel -r ad1s8
# /dev/ad1s8c:
type: unknown
disk: amnesiac
label:
flags:
bytes/sector: 512
sectors/track: 63
tracks/cylinder: 16
sectors/cylinder: 1008
cylinders: 16654
sectors/unit: 16787862
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0           # milliseconds
track-to-track seek: 0  # milliseconds
drivedata: 0

8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  a: 16273746   514080    4.2BSD     2048 16384 25600   # (Cyl.  510 - 16654*)
  b:   514080        0      swap                        # (Cyl.    0 - 509)
  c: 16787862        0    unused        0     0         # (Cyl.    0 - 16654*)
super block size 0

Is this a bug in 'disklabel'?  Or not.

Thanks for any clues!