grub-mkrescue hfsplus GPT partition is not mountable on Linux

[Andrei Borzenkov]

Current GRUB GIT, Ubuntu 14.04 (kernel 3.13.0-74-generic), USB stick (I
have no CD drive).

xorriso used under openSUSE Tumbleweed:

xorriso 1.4.0
ISO 9660 Rock Ridge filesystem manipulator and CD/DVD/BD burn program
Copyright (C) 2014, Thomas Schmitt <address@hidden>, libburnia project.
xorriso version   :  1.4.0
Version timestamp :  2015.05.17.112001
Build timestamp   :  -none-given-
libisofs   in use :  1.4.0  (min. 1.4.0)
libburn    in use :  1.4.0  (min. 1.4.0)
libburn OS adapter:  internal GNU/Linux SG_IO adapter sg-linux
libisoburn in use :  1.4.0  (min. 1.4.0)
Provided under GNU GPL version 2 or later.
There is NO WARRANTY, to the extent permitted by law.


Attempt to mount third GPR partition as HFS+ results in failure.
Additionally it tries to interpret the first (dummy) partition, which is
not fatal but annoying.

[24623.255932]  sdb: sdb1 sdb2 sdb3
[24623.256733] sd 5:0:0:0: [sdb] No Caching mode page found
[24623.256736] sd 5:0:0:0: [sdb] Assuming drive cache: write through
[24623.256738] sd 5:0:0:0: [sdb] Attached SCSI removable disk
[24623.483845] isofs_fill_super: bread failed, dev=sdb1, iso_blknum=71,
block=142
[24623.487605] hfsplus: invalid secondary volume header
[24623.487608] hfsplus: unable to find HFS+ superblock

xorriso report.

El Torito catalog  : 1674  1
El Torito cat path : /boot.catalog
El Torito images   :   N  Pltf  B   Emul  Ld_seg  Hdpt  Ldsiz         LBA
El Torito boot img :   1  BIOS  y   none  0x0000  0x00      4        7279
El Torito boot img :   2  UEFI  y   none  0x0000  0x00   5760          87
El Torito img path :   1  /boot/grub/i386-pc/eltorito.img
El Torito img opts :   1  boot-info-table grub2-boot-info
El Torito img path :   2  /efi.img
System area options: 0x00004201
System area summary: MBR protective-msdos-label grub2-mbr cyl-align-off
GPT APM
ISO image size/512 : 30292
Partition offset   : 0
MBR heads per cyl  : 64
MBR secs per head  : 32
MBR partition table:   N Status  Type        Start       Blocks
MBR partition      :   1   0x00  0xee            1        30291
GPT                :   N  Info
GPT disk GUID      :      9b29d8ed2179ff43a11959efd5fa085e
GPT entry array    :      20  176  separated
GPT lba range      :      64  30246  30291
GPT partition name :   1  4700610070003000
GPT partname local :   1  Gap0
GPT partition GUID :   1  9b29d8ed2179ff43a11a59efd5fa085e
GPT type GUID      :   1  a2a0d0ebe5b9334487c068b6b72699c7
GPT partition flags:   1  0x1000000000000001
GPT start and size :   1  64  284
GPT partition name :   2
450046004900200062006f006f007400200070006100720074006900740069006f006e00
GPT partname local :   2  EFI boot partition
GPT partition GUID :   2  9b29d8ed2179ff43a11b59efd5fa085e
GPT type GUID      :   2  28732ac11ff8d211ba4b00a0c93ec93b
GPT partition flags:   2  0x1000000000000001
GPT start and size :   2  348  5760
GPT partition path :   2  /efi.img
GPT partition name :   3  4700610070003100
GPT partname local :   3  Gap1
GPT partition GUID :   3  9b29d8ed2179ff43a11859efd5fa085e
GPT type GUID      :   3  a2a0d0ebe5b9334487c068b6b72699c7
GPT partition flags:   3  0x1000000000000001
GPT start and size :   3  6108  24136
APM                :   N  Info
APM block size     :      2048
APM gap fillers    :      2
APM partition name :   1  Gap0
APM partition type :   1  ISO9660_data
APM start and size :   1  16  1511
APM partition name :   2  HFSPLUS_Hybrid
APM partition type :   2  Apple_HFS
APM start and size :   2  1527  5884
APM partition name :   3  Gap1
APM partition type :   3  ISO9660_data
APM start and size :   3  7411  162

apple2 is actually smaller than gpt3, and Linux kernel checks secondary
header which is expected to be at the end of partition.

I am not sure what is intended here, but apparently Linux does not
support APM so being able to access HFS+ partition via GPT is a bonus.
In which case name "Gap1" is misleading :)

And is the very first partition really needed? It is not used by GRUB
anyway, and there is no requirement that each bit of media is covered.