[Bug 235380] gptzfsboot does not boot ZFS pool made from whole disks (regression)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235380

            Bug ID: 235380
           Summary: gptzfsboot does not boot ZFS pool made from whole
                    disks (regression)
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: reto.haeuptli at infinox.ch

I used to boot zfs on whole disk(s) from a USB stick with a GPT partition
scheme and gptzfsboot on it since FreeBSD version 9 on many testing machines
and even production servers without any problems. With the release 12 this
feature is broken. 

I found the reason in the source code: In the file
/head/stand/i386/libi386/biosdisk.c a regression was introduced in the
revisions base r37271, base r337317. Its about detecting the correct number of
sectors/blocks for a block device. The change was introduced because a certain
device (Mediasonic HD3-U2B PATA to USB enclosure) was not working. I tested
various SASTA disks, even a 10TB disk, and USB Sticks, all detecting the wrong
number of sectors instead. Reverting the change leaded to the correct detection
of the number of sectors and therefore I was able to boot again. I tested also
revision base r342785 of the file /head/stand/i386/libi386/biosdisk.c: The
detection of the number of sectors is implemented the right way again and has
improved of course.

I propose the following patch to fix the regression for FreeBSD 12

Index: stand/i386/libi386/biosdisk.c
===================================================================
--- stand/i386/libi386/biosdisk.c       (revision 343578)
+++ stand/i386/libi386/biosdisk.c       (working copy)
@@ -270,8 +270,8 @@

                total = (uint64_t)params.cylinders *
                    params.heads * params.sectors_per_track;
-               if (total > 0 && bd->bd_sectors > total)
-                       bd->bd_sectors = total;
+                if (bd->bd_sectors < total)
+                        bd->bd_sectors = total;

                ret = 1;
        }

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