>Number:         18165
>Category:       kern
>Synopsis:       zeroing disklabel can prove difficult due to kernel label protection
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Sep 04 04:20:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     grant beattie
>Release:        NetBSD 1.6F
>Organization:
>Environment:
System: NetBSD mofo 1.6F NetBSD 1.6F (MOFO) #1: Tue Aug 20 12:21:00 EST 2002 grant@mofo:/raid0/netbsd/current/src/sys/arch/i386/compile/MOFO i386
Architecture: i386
Machine: i386
>Description:
difficulty was experienced while attempting to blow away the NetBSD
disklabel on a disk, due to the default behavior of the kernel which
attempts to protect the label area of the disk.

the disk in question is wd1 on i386, and it has both a msdos and a
NetBSD mbr partition.

the disklabel currently looks like this:

#        size    offset     fstype  [fsize bsize cpg/sgs]
 a:    204800   4209030     4.2BSD      0     0     0   # (Cyl. 4175*- 4378*)
 c:   8389584   4209030     unused      0     0         # (Cyl. 4175*- 12498*)
 d:  78177792         0     unused      0     0         # (Cyl.    0 - 77557*)

>How-To-Repeat:
# dd if=/dev/zero of=/dev/rwd1a bs=1024 count=1
dd: /dev/rwd1a: Read-only file system
1+0 records in
0+0 records out
0 bytes transferred in 0.001 secs (0 bytes/sec)

# dd if=/dev/zero of=/dev/rwd1c bs=1024 count=1
dd: /dev/rwd1c: Device not configured

>Fix:
workaround is first enable writing to the label area:

# disklabel -W wd1

then create a partition starting at the same sector as 'c', eg.

#        size    offset     fstype  [fsize bsize cpg/sgs]
 a:    204800   4209030     4.2BSD      0     0     0   # (Cyl. 4175*- 4378*)
 c:   8389584   4209030     unused      0     0         # (Cyl. 4175*- 12498*)

and then overwrite the label area:

# dd if=/dev/zero of=/dev/rwd1a bs=1024 count=1

this is suboptimal.

the user should be able to use the raw 'c' partition for this purpose,
and there should be a way to query the status of kernel disklabel
protection.

the kernel should also log a message when label protection results
in EROFS, and hint at "disklabel -W".

>Release-Note:
>Audit-Trail:
>Unformatted: