[Bug 210661] geom_part does not recognize MBR partition type !12 (0Ch) used by BeagleBone Black

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210661

            Bug ID: 210661
           Summary: geom_part does not recognize MBR partition type !12
                    (0Ch) used by BeagleBone Black
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: emaste at freebsd.org

Using the most recent snapshot sd card image on a BeagleBone Black, gpart does
not recognize the FAT-formatted boot partition type:

root at beaglebone:~ # gpart show
=>      63  61896641  mmcsd0  MBR  (30G)
        63      4095       1  !12  [active]  (2.0M)
      4158  61886402       2  freebsd  (30G)
  61890560      6144          - free -  (3.0M)

=>       0  61886402  mmcsd0s2  BSD  (30G)
         0        66            - free -  (33K)
        66  61886336         1  freebsd-ufs  (30G)

=>      1  3751935  mmcsd1  MBR  (1.8G)
        1       62          - free -  (31K)
       63   144522       1  !12  [active]  (71M)
   144585  3598560       2  linux-data  (1.7G)
  3743145     8791          - free -  (4.3M)

=>      1  3751935  diskid/DISK-56856FEC  MBR  (1.8G)
        1       62                        - free -  (31K)
       63   144522                     1  !12  [active]  (71M)
   144585  3598560                     2  linux-data  (1.7G)
  3743145     8791                        - free -  (4.3M)

For comparison Linux fdisk reports this as:

root at beaglebone:~# fdisk -l /dev/mmcblk0  

Disk /dev/mmcblk0: 1920 MB, 1920991232 bytes
255 heads, 63 sectors/track, 233 cylinders, total 3751936 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

        Device Boot      Start         End      Blocks   Id  System
/dev/mmcblk0p1   *          63      144584       72261    c  W95 FAT32 (LBA)
/dev/mmcblk0p2          144585     3743144     1799280   83  Linux

Wikipedia concurs that this is FAT32 with LBA. Despite this, the filesystem
contained in the stock eMMC on the BBB is actually FAT12:

/dev/mmcsd0s1: DOS/MBR boot sector, code offset 0x3c+2, OEM-ID "BSD4.4  ", root
entries 512, sectors 4095 (volumes <=32 MB) , sectors/FAT 12, sectors/track 63,
heads 255, serial number 0x5de21501, label: "MSDOSBOOT  ", FAT (12 bit),
followed by FAT

For comparison the filesystem on the snapshot SD card image is:

root at beaglebone:~ # file -s /dev/mmcsd1s1
/dev/mmcsd1s1: DOS/MBR boot sector, code offset 0x58+2, OEM-ID "mkdosfs", Media
descriptor 0xf8, sectors/track 16, heads 4, sectors 144522 (volumes > 32 MB) ,
FAT (32 bit), sectors/FAT 1112, serial number 0x51470be5, label: "BEAGLEBONE "

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