From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Sat,  7 Jan 2012 20:22:13 +0100
From: tlaronde@polynum.com
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Message-ID: <20120107192213.GA4971@polynum.com>
References: <20120107182908.GA3674@polynum.com>
	<1c3ba45416e47236eedd6ca05863d751@chula.quanstro.net>
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Subject: Re: [9fans] BUG: disk/fdisk.c: corrupts MBR
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On Sat, Jan 07, 2012 at 02:04:51PM -0500, erik quanstrom wrote:
> On Sat Jan  7 13:30:33 EST 2012, tlaronde@polynum.com wrote:
> > 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;
> >

> can you just set sec2cyl=1?

This wouls probably do the trick. But the code is clean and setting just
this is not---to my taste.

The ideal would be to simply patch the existing with changed values (no
need to be efficient for 4 * 16 bytes), and leave clearly alone values
set by others.

And to verify the magic signature for, at least, the active partition
(it would have spot the problem).
--
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
                      http://www.kergis.com/
Key fingerprint = 0FF7 E906 FBAF FE95 FD89  250D 52B1 AE95 6006 F40C