Subject: Default disklabel for cd(4) (PR 25108)
To: None <>
From: Quentin Garnier <>
List: tech-kern
Date: 05/06/2004 09:45:00
Hi all,

There is a bug in the way readdisklabel() interacts with device label
generation routines in lower level driver.  It is clearly apparent with
cd(4), but might as well happen with other devices for which it can be
completely normal to have neither a disklabel nor a MBR.

kern/subr_disk_mbr.c is not used by all archs, only by amd64, i386 and
ibmnws as it seems.  So the issue as I will describe it only happens on
those archs.  That doesn't mean some other archs don't have their
readdisklabel() bugged, too.

When cd(4) wants to generate a disklabel, it calls cdgetdisklabel().
That function first tries to call readdisklabel(), and if that function
returns an error, it outputs it on the console and use a default label.

On i386 (I confirm it does about the same on -1-6, too), readdisklabel()
tries to find a MBR first, then tries to directly read a disklabel on
the device.

The difference between -1-6 and -current from after July, 7th, 2003, is
that the MI readdisklabel() does not return an error if it couldn't read
neither a MBR not a disklabel.

That avoids the message "cd0: no disk label" we can see on our -1-6
hosts, but that makes cdgetdisklabel immediately return, not generating
a correct label.

The reason why disklabel shows 4.2BSD for 'a' is that the MI
readdisklabel sets that by default, in case it doesn't find a MBR or a

I think this is not a correct behaviour, and even though I agree we
should avoid printing an error message when the case of actually having
a MBR or a label on a CD-ROM would be very unexpected.

My proposal is the following:  making readdisklabel() return an empty
string in case no relevant data was found while no error happened. Then
the device routine that calls readdisklabel() can check the length of
the returned message and not print it if it is empty.

Somehow I wonder what happens for devices that first calls their default
labelling function, and then call readdisklabel(), which inconditionally
overwrites type for 'a'.  I'd like some input on what to do here, too.


Quentin Garnier.