[GPT] Get secondary header address from primary if otherwise valid - #306
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The UEFI spec states that to validate a GPT we should check the sig, CRC, self-referential LBA, partition entry array CRC, and for the primary GPT, the GPT at the alternateLBA. Here we were instead checking the GPT at the end of the disk, which may not be the same as the alternateLBA if the disk was grown by a non-partition-aware tool (e.g. in a RAID array). By checking the indicated alternateLBA, we accept more valid GPT-based disks, where before we may have been looking at uninitialized data.
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The UEFI spec states that to validate a GPT we should check the sig, CRC, self-referential LBA, partition entry array CRC, and for the primary GPT, the GPT at the alternateLBA. Here we were instead checking the GPT at the end of the disk, which may not be the same as the alternateLBA if the disk was grown by a non-partition-aware tool (e.g. in a RAID array). By checking the indicated alternateLBA, we accept more valid GPT-based disks, where before we may have been looking at uninitialized data.