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Date: Wed, 11 Nov 2009 20:55:24 -0800
From: Julian Elischer <julian@elischer.org>
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Cc: Marcel Moolenaar <xcllnt@mac.com>, current@freebsd.org
Subject: Re: How to confuse geom_part_mbr
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Dag-Erling Smørgrav wrote:
> Marcel Moolenaar <xcllnt@mac.com> writes:
>> Dag-Erling Smørgrav <des@des.no> writes:
>>> The machine won't boot, even though you have a valid partition table
>>> on ad0 that points to a valid bsdlabel in ad0s1.
>> No, you don't have a valid partition table on ad0, because
>> you didn't remove the BSD disklabel in sector 2 on ad0.
> 
> Yes, I do have a valid partition table.  It is exactly byte-by-byte
> identical to the one I get after I zero sector two and re-run fdisk.
> The fact that there is unwanted data in sector 2 does *not* make it any
> less valid.
> 
> What's more, this could have easily been avoided if geom_whatever gave
> the partition table precedence over the label it found in sector 2.
> 
> DES

The dummy MBR on a disklabel can be relatively easily identified, and 
the regular MDR tasting code should note it and give it a lower 
priority than a real MBR. (and a disklabel)