The firmware and the OS may disagree on the disk configuration and size.
Although such a setup should be avoided users are unlikely to know about
the problem, assuming everything behaves like the OS. Tolerate this as
best we can and trust the reported on-disk location over the firmware
when looking for the backup GPT. If the location is inaccessible report
the error as best we can and move on.
Portions of the code attempted to handle the fact that GPT entries on
disk may be larger than the currently defined struct while others
assumed the data could be indexed by the struct size directly. This
never came up because no utility uses a size larger than 128 bytes but
for the sake of safety we need to do this by the spec.
In order to do anything with partition GUIDs they need to be stored in a
proper structure like the partition type GUIDs. Additionally add an
initializer macro to simplify defining both GUID types.
The header location fields refer to 'this header' and 'alternate header'
respectively, not 'primary header' and 'backup header'. The previous
field names are backwards for the backup header.
This module is a new implementation for reading GUID Partition Tables
which is much stricter than the existing part_gpt module and exports GPT
data directly instead of the generic grub_partition structure. It will
be the basis for modules that need to read/write/update GPT data.
The current code does nothing more than read and verify the table.
