Divide-by-zero in loader

[Matthew Fleming]

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?

Thanks,
matthew

Index: i386/libi386/biosdisk.c
===================================================================
--- i386/libi386/biosdisk.c	(.../head/src/sys/boot)	(revision 172978)
+++ i386/libi386/biosdisk.c	(.../branches/BR_BUG_73454/src/sys/boot)	(revision
172978)
@@ -2064,30 +2064,38 @@ bd_getgeom(struct open_disk *od)
     v86.addr = 0x13;
     v86.eax = 0x800;
     v86.edx = od->od_unit;
     v86int();

     if ((v86.efl & 0x1) ||				/* carry set */
 	((v86.edx & 0xff) <= (unsigned)(od->od_unit & 0x7f)))	/* unit # bad */
 	return(1);

     /* convert max cyl # -> # of cylinders */
     od->od_cyl = ((v86.ecx & 0xc0) << 2) + ((v86.ecx & 0xff00) >> 8) + 1;
     /* convert max head # -> # of heads */
     od->od_hds = ((v86.edx & 0xff00) >> 8) + 1;
     od->od_sec = v86.ecx & 0x3f;

+    if (od->od_sec == 0) {
+	printf("Bad disk geometry on unit %d, bios unit %d, chs %d/%d/%d\n",
+	    od->od_dkunit, od->od_unit, od->od_cyl, od->od_hds, od->od_sec);
+	return (1);
+    }
+
     DEBUG("unit 0x%x geometry %d/%d/%d", od->od_unit, od->od_cyl,
od->od_hds, od->od_sec);
     return(0);
 }

 /*
  * Return the BIOS geometry of a given "fixed drive" in a format
  * suitable for the legacy bootinfo structure.  Since the kernel is
  * expecting raw int 0x13/0x8 values for N_BIOS_GEOM drives, we
  * prefer to get the information directly, rather than rely on being
  * able to put it together from information already maintained for
  * different purposes and for a probably different number of drives.
  *
  * For valid drives, the geometry is expected in the format (31..0)
  * "000000cc cccccccc hhhhhhhh 00ssssss"; and invalid drives are
  * indicated by returning the geometry of a "1.2M" PC-format floppy