From mboxrd@z Thu Jan  1 00:00:00 1970
From: erik quanstrom <quanstro@quanstro.net>
Date: Sat,  7 Jan 2012 14:04:51 -0500
To: 9fans@9fans.net
Message-ID: <1c3ba45416e47236eedd6ca05863d751@chula.quanstro.net>
In-Reply-To: <20120107182908.GA3674@polynum.com>
References: <20120107182908.GA3674@polynum.com>
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Subject: Re: [9fans] BUG: disk/fdisk.c: corrupts MBR
Topicbox-Message-UUID: 5639d4e8-ead7-11e9-9d60-3106f5b1d025

On Sat Jan  7 13:30:33 EST 2012, tlaronde@polynum.com wrote:
> Hello,
>
> Since I'm playing with the installation, I have been hit once again
> with disk/fdisk corrupting the MBR and rendering other systems
> unbootable.
>
> The explanation is arithmetic truncation. disk/fdisk.c does this:
>
> p->start = lba/sec2cyl;
>
> and, if modifications are done (for example creating a Plan9 partition,
> of marking a primary partition active), when written back, the entries
> are "restored" by:
>
> *endlba = (vlong)p->start*sec2cyl;
>
> Of course, if the values were not integer multiples of sec2cyl, the
> entry point for the PBS is corrupt. (For example, on *BSD, the principle
> is the same as for Plan9: a "primary" partition is a sequence of
> adjacent bytes, furthermore subdivided; but one special subdivision
> [a] is at the same offset (at start) and holds the bootcode and a
> disklabel describing the subdivision; if the MBR is corrupt, neither
> boot nor disklabel so user can conclude: everything lost). And the first
> travel in the flying saucer will also be the last...

can you just set sec2cyl=1?

- erik