Re: [9fans] BUG: disk/fdisk.c: corrupts MBR

[tlaronde@polynum.com]

On Sat, Jan 07, 2012 at 02:30:52PM -0500, erik quanstrom wrote:
>
> the code assumes c/h/s.  and if it (libdisk) can't find the geometry, it makes stuff
> up.  that seems like a dubious assumption.  not very clean at all.
>
> i was thinking that setting cyc2cyl=1 should cleanly remove this feature.
> it could then be reenabled with a command-line flag if necessary for
> some really ancient h/w.

We should be careful, because bootloaders whether take the c/h/s, or the
as is lba values (and the incorrect assumption is that c/h/s == lba
since this will defeat the purpose of lba).

But in the code, there are p->ctlstart and p->ctlend, that have the
correct values... and are not used. Perhaps simply using these values
when writing will suffice.

But whether I or another have/has to look carefully: doing blunders in
Plan9 zone is unfortunate but... Messing others' areas must be
forbidden. disk/fdisk(8) is the program that impacts others.

The MBR is the BIOS entry point so it shall not be botched. Furthermore,
with a booloader that has not the capability to select the primary to
boot, if the MBR is partially wrong, and the active partition not the
correct one, you have to have... a plan B (but perhaps not the well
known one ;).
--
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
                      http://www.kergis.com/
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