tmutil is broken by SIP in Mojave

A diskutil bug unceremoniously erased an entire hard drive of mine a few weeks back.  While I was able to successfully (AFAICT) restore the drive’s contents to it from various backups, the erasure gave the drive a new identity (UUID, specifically).  The next time Time Machine ran, it compounded the diskutil bug by also unceremoniously deleting all my old backups (bar one, the latest), because it didn’t recognise the new drive with identical contents to the old drive as being the same drive, and tried to back it up again, requiring way more space, causing all existing backups to be purged, etc.

Sigh.

It turns out there’s actually a nominally supported way to address exactly this scenario – tmutil associatedisk (kudos to Simon Heimlicher for documenting this).  From the man page:

     associatedisk [-a] mount_point snapshot_volume

             Bind a snapshot volume directory to the specified local disk, thereby reconfigur-

             ing the backup history. Requires root privileges.

             In Mac OS X, HFS+ volumes have a persistent UUID that is assigned when the file

             system is created. Time Machine uses this identifier to make an association

             between a source volume and a snapshot volume. Erasing the source volume creates

             a new file system on the disk, and the previous UUID is not retained. The new

             UUID causes the source volume -> snapshot volume association to be broken. If one

             were just erasing the volume and starting over, it would likely be of no real

             consequence, and the new UUID would not be a concern; when erasing a volume in

             order to clone another volume to it, recreating the association may be desired.

             A concrete example of when and how you would use associatedisk:

             After having problems with a volume, you decide to erase it and manually restore

             its contents from a Time Machine backup or copy of another nature. (I.e., not via

             Time Machine System Restore or Migration Assistant.) On your next incremental

             backup, the data will be copied anew, as though none of it had been backed up

             before. Technically, it is true that the data has not been backed up, given the

             new UUID. However, this is probably not what you want Time Machine to do. You

             would then use associatedisk to reconfigure the backup so it appears that this

             volume has been backed up previously:

             thermopylae:~ thoth$ sudo tmutil associatedisk [-a] “/Volumes/MyNewStuffDisk”

             “/Volumes/Chronoton/Backups.backupdb/thermopylae/Latest/MyStuff”

             The result of the above command would associate the snapshot volume MyStuff in

             the specified snapshot with the source volume MyNewStuffDisk. The snapshot volume

             would also be renamed to match. The -a option tells associatedisk to find all

             snapshot volumes in the same machine directory that match the identity of

             MyStuff, and then perform the association on all of them.

Perfect – and I particularly like the subtext of the prose, which seems to be a subtle acknowledgment that this is a thing that happens frequently, and that macOS’s default behaviour is stupid… “recreating the association may be desired”.  No shit.

Unfortunately, that command doesn’t work in Mojave.  I’m apparently not the first person to notice.

It appears the tightened security, and in particular expansion of SIP to cover many more parts of the system including Time Machine backups, are to blame.  Even granting tmutil Full Disk Access etc in the system security settings is of no use (contrary to the stated purpose of Full Disk Access).

So you have to disable SIP first – which requires a reboot, obnoxiously – and only then does tmutil work again.  You’ll want to enable SIP again once you’re done, most likely, as the protections it provides are useful – it appears tmutil nve eeds to be updated to account for the new protections.