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.
I personally think this reads easier. Also has the side effect of
directly comparing the primary and backup tables instead of presuming
they are equal if the crc32 matches.
This ensures all code modifying GPT data include the same sanity check
that repair does. If revalidation fails the status flags are left in the
appropriate state.
The header was being relocated without checking the new location is
actually safe. If the BIOS thinks the disk is smaller than the OS then
repair may relocate the header into allocated space, failing the final
validation check. So only move it if the disk has grown.
Additionally, if the backup is valid then we can assume its current
location is good enough and leave it as-is.
Use the new status function which checks *_HEADER_VALID and
*_ENTRIES_VALID bits together. It doesn't make sense for the header and
entries bits to mismatch so don't allow for it.
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.
GPT_BOTH_VALID is 4 bits so simple a boolean check is not sufficient.
This broken condition allowed gptprio to trust bogus disk locations in
headers that were marked invalid causing arbitrary disk corruption.
GRUB assumes that no disk is ever larger than 1EiB and rejects
reads/writes to such locations. Unfortunately this is not conveyed in
the usual way with the special GRUB_DISK_SIZE_UNKNOWN value.
The first hint of something practical, a command that can restore any of
the GPT structures from the alternate location. New test case must run
under QEMU because the loopback device used by the other unit tests does
not support writing.
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.
