Time Machine – Permissions Restoring: Finder vs ‘cp’ Copy

findermacososx lionpermissionstime-machine

Note: this question was starting to sprawl so I rewrote it.

I have a folder that I'm trying to restore from a Time Machine backup. Using cp -R works fine, but certain folders cannot be restored with either the Time Machine UI or Finder.

Other users have reported similar errors and the cp -R workaround was suggested (e.g. Restoring from Time Machine – Permissions Error). But I wanted to understand:

  1. Why cp -R works when the Finder and the Time Machine UI do not.
  2. Whether I could prevent the errors by changing file permissions before the backup.

There do indeed seem to be some permissions that Finder works with and some that it does not. I've narrowed the errors down to folders with the user ben (that's me) and the group wheel.

Here's a simplified reproduction. I have four folders with the owner/group combinations I've seen so far:

ben ~/Desktop/test $ ls -lea
total 16
drwxr-xr-x   7 ben   staff   238 27 Nov 14:31 .
drwx------+ 17 ben   staff   578 27 Nov 14:29 ..
 0: group:everyone deny delete
-rw-r--r--@  1 ben   staff  6148 27 Nov 14:31 .DS_Store
drwxr-xr-x   3 ben   staff   102 27 Nov 14:30 ben-staff
drwxr-xr-x   3 ben   wheel   102 27 Nov 14:30 ben-wheel
drwxr-xr-x   3 root  admin   102 27 Nov 14:31 root-admin
drwxr-xr-x   3 root  wheel   102 27 Nov 14:31 root-wheel

Each contains a single file called file with the same owner/group:

ben ~/Desktop/test $ cd ben-staff
ben ~/Desktop/test/ben-staff $ ls -lea
total 0
drwxr-xr-x  3 ben  staff  102 27 Nov 14:30 .
drwxr-xr-x  7 ben  staff  238 27 Nov 14:31 ..
-rw-r--r--  1 ben  staff    0 27 Nov 14:30 file

In the backup, they look like this:

ben /Volumes/Deimos/Backups.backupdb/Ben’s MacBook Air/Latest/Macintosh HD/Users/ben/Desktop/test $ ls -leA
total 16
-rw-r--r--@ 1 ben   staff  6148 27 Nov 14:34 .DS_Store
 0: group:everyone deny write,delete,append,writeattr,writeextattr,chown
drwxr-xr-x@ 3 ben   staff   102 27 Nov 14:51 ben-staff
 0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown
drwxr-xr-x@ 3 ben   wheel   102 27 Nov 14:51 ben-wheel
 0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown
drwxr-xr-x@ 3 root  admin   102 27 Nov 14:52 root-admin
 0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown
drwxr-xr-x@ 3 root  wheel   102 27 Nov 14:52 root-wheel
 0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown

Of these, ben-staff can be restored with Finder without errors. root-wheel and root-admin ask for my password and then restore without errors. But ben-wheel does not prompt for my password and gives the error:

The operation can’t be completed because you don’t have permission to access “file”.

Interestingly, I can restore the file from this folder by dragging it directly to my local drive (instead of dragging its parent folder), but when I do so its permissions are changed to ben/staff.

Here are the permissions after the restore for the three folders that worked correctly, and the file from ben-wheel that was changed to ben/staff.

ben ~/Desktop/test-restore $ ls -leA
total 16
-rw-r--r--@ 1 ben   staff  6148 27 Nov 14:46 .DS_Store
drwxr-xr-x  3 ben   staff   102 27 Nov 14:30 ben-staff
-rw-r--r--  1 ben   staff     0 27 Nov 14:30 file
drwxr-xr-x  3 root  admin   102 27 Nov 14:31 root-admin
drwxr-xr-x  3 root  wheel   102 27 Nov 14:31 root-wheel

Can anyone explain this behaviour? Why do Finder and the Time Machine UI break with the ben / wheel permissions? And why does cp -R work (even without sudo)?

Best Answer

The issue here is that Finder implements special handling of files restored from Time Machine to preserve all their permissions, which failed for files owned by the current user's account, but not a group he's a member of.

Usually, when copying files using cp, their attributes aren't retained. This can be changed using the -p option.

-p Cause cp to preserve the following attributes of each source file in the copy: modification time, access time, file flags, file mode, user ID, and group ID, as allowed by permissions. Access Control Lists (ACLs) and Extended Attributes (EAs), including resource forks, will also be preserved.

In both cases, you either copy all or none or these metadata. cp is clever enough to restore them only after all contained files have been processed ([source, see near If -p is in effect, set all the attributes).

If you don't have root permissions, ownership is not retained. The reason for this is that only root can make files owned by users not him and by groups he's not a member of.

To make Time Machine backups viewable but otherwise immutable in Finder, they are protected by Access Control Lists denying all users all modification permissions.

0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown

Since other attributes (other ACLs, extended attributes, file dates and permissions) should be retained when restoring from backup, special handling of these folders is required in Finder. It must remove a specific ACL entry, but retain everything else.

Additionally, Apple apparently wants Finder to retain all ownership information when copying files and directories from backup. This includes the group membership.

If your account is not the owner of the directories in question, Finder requests an administrator password and hands the copying off to its elevated privileges helper program Locum. This does not happen when you are the owner of a file in the backup set.

Now, one of the following cases happens:

  • You are not the owner of the file: Finder asks for your password, Locum restores all permissions just as in the backup.
  • You are the owner and a member of the file's group: Finder copies the file over and restores the group.
  • You are the owner but not a member of the file's group: Finder copies the file over and fails to restore its group.

It's like it's trying to chown username:groupname the file, and prints an error if it fails — which it does if you're neither root (sudo) nor username and a member of groupname.

It behaves slightly differently if you're not copying folders, but files: While file ownership is retained, group membership is discarded if you're not a member of the group. This is what you saw when only copying a single file.

The obvious solutions to this problem:

  • Prevent file ownerships that cause restoration to fail (i.e. owned by you and a group you're not a member of — most of the time, this isn't useful anyway)
  • Make yourself a member of that particular group at least temporarily. Unfortunately I wasn't able to do this using dscl from the command line in a way that had an effect without logout or restart. Another side effect is that with wheel, you might run into problems with permissions depending on your system configuration.
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