partitions/efi: detect hybrid MBRs

One of the biggest problems with GPT is compatibility with older, non-GPT
systems.  The problem is addressed by creating hybrid mbrs, an extension,
or variant, of the traditional protective mbr.  This contains, apart from
the 0xEE partition, up three additional primary partitions that point to
the same space marked by up to three GPT partitions.  The result is that
legacy OSs can see the three required MBR partitions and at the same time
ignore the GPT-aware partitions that protect the GPT structures.

While hybrid MBRs are hacks, workarounds and simply not part of the GPT
standard, they do exist and we have no way around them.  For instance, by
default, OSX creates a hybrid scheme when using multi-OS booting.

In order for Linux to properly discover protective MBRs, it must be made
aware of devices that have hybrid MBRs.  No functionality is changed by
this patch, just a debug message informing the user of the MBR scheme that
is being used.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Reviewed-by: Karel Zak <kzak@redhat.com>
Acked-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/block/partitions/efi.h b/block/partitions/efi.h
index e645ecb35bf3..7fef625c04de 100644
--- a/block/partitions/efi.h
+++ b/block/partitions/efi.h
@@ -37,6 +37,9 @@
 #define EFI_PMBR_OSTYPE_EFI 0xEF
 #define EFI_PMBR_OSTYPE_EFI_GPT 0xEE
 
+#define GPT_MBR_PROTECTIVE  1
+#define GPT_MBR_HYBRID      2
+
 #define GPT_HEADER_SIGNATURE 0x5452415020494645ULL
 #define GPT_HEADER_REVISION_V1 0x00010000
 #define GPT_PRIMARY_PARTITION_TABLE_LBA 1