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xfs: compile time offset checks for common v4/v5 metadata
The v5 superblock format added various metadata fields (such as crc, metadata lsn, owner uuid, etc.) to v4 metadata headers or created new v5 headers for blocks where no such headers existed on v4. Where v4 headers did exist, the v5 structures are careful to place v4 metadata at the original location. For example, the magic value is expected to be at the same location in certain blocks to facilitate version detection. While failure of this invariant is likely to cause severe and obvious problems at runtime, we can detect this condition at compile time via the more recently added on-disk format check infrastructure. Since there is no runtime cost, add some offset checks that start with v5 structure definitions, traverse down to the first bit of common metadata with v4 and ensure that common metadata is at the expected offset. Note that we don't care about blocks which had no v4 header because there is no common metadata in those cases. No functional changes. Signed-off-by:Brian Foster <bfoster@redhat.com> Reviewed-by: