Divide-by-zero in loader

[John Baldwin]

On Friday, January 28, 2011 12:41:08 pm Matthew Fleming wrote:
> I spent a few days chasing down a bug and I'm wondering if a loader
> change would be appropriate.
> 
> So we have these new front-panel LCDs, and like everything these days
> it's a SoC.  Normally it presents to FreeBSD as a USB communications
> device (ucom), but when the SoC is sitting in its own boot loader, it
> presents as storage (umass).  If the box is rebooted in this state,
> the reboot gets into /boot/loader and then reboots itself.  (It took a
> few days just to figure out I was getting into /boot/loader, since the
> only prompt I could definitively stop at was boot2).
> 
> Anyways, I eventually debugged it to the device somehow presenting
> itself to /boot/loader with a geometry of 1024/256/0, and since od_sec
> is 0 that causes a divide-by-zero error in bd_io() while the loader is
> trying to figure out if this is GPT or MBR formatted.  We're still
> trying to figure out why the loader sees this incorrect geometry.
> 
> But meanwhile, this patch fixes the issue, and I wonder if it would be
> a useful safety-belt for other devices where an incorrect geometry can
> be seen?

That's probably fine.  A sector count of zero is invalid for CHS.  However, 
probably we should not even be using C/H/S at all if the device claims to 
support EDD.  We already use raw LBAs if it supports EDD, and we should 
probably just ignore C/H/S altogether if it supports EDD.

-- 
John Baldwin