• mckusick's avatar
    Various new check-hash checks have been added to the UFS filesystem · 5ce3cee7
    mckusick authored
    over various major releases. Superblock check hashes were added for
    the 12 release and cylinder-group and inode check hashes will appear
    in the 13 release.
    
    When a disk with a UFS filesystem is writably mounted, the kernel
    clears the feature flags for anything that it does not support. For
    example, if a UFS disk from a 12-stable kernel is mounted on an
    11-stable system, the 11-stable kernel will clear the flag in the
    filesystem superblock that indicates that superblock check-hashs
    are being maintained. Thus if the disk is later moved back to a
    12-stable system, the 12-stable system will know to ignore its
    incorrect check-hash.
    
    If the only filesystem modification done on the earlier kernel is
    to run a utility such as growfs(8) that modifies the superblock but
    neither updates the check-hash nor clears the feature flag indicating
    that it does not support the check-hash, the disk will fail to mount
    if it is moved back to its original newer kernel.
    
    This patch moves the code that clears the filesystem feature flags
    from the mount code (ffs_mountfs()) to the code that reads the
    superblock (ffs_sbget()). As ffs_sbget() is used by the kernel mount
    code and is imported into libufs(3), all the filesystem utilities
    will now also clear these flags when they make modifications to the
    filesystem.
    
    As suggested by John Baldwin, fsck_ffs(8) has been changed to accept
    and repair bad superblock check-hashes rather than refusing to run.
    This change allows fsck to recover filesystems that have been impacted
    by utilities older than those created after this change and is a
    sensible thing to do in any event.
    
    Reported by:  John Baldwin (jhb@)
    MFC after:    2 weeks
    Sponsored by: Netflix
    5ce3cee7