By-passing default permissions when mounting HFS+ volumes in linux

Question

I have a dual booting macbook pro with Snow Leopard and Kubuntu 11.10, and want to read (don't care about write) my home Mac home directory when I'm running Kubuntu.

I can mount it without any problems, but my user on Kubuntu on can't see the files on the HFS+ owned by the mac user, because of different uid (502 on Mac, 1000 on Kubuntu).

Looking at kernel docs about HFS+ I read that:

When mounting an HFSPlus filesystem, the following options are accepted:
[CUT]
    uid=n, gid=n
        Specifies the user/group that owns all files on the filesystem
        that have uninitialized permissions structures.
        Default:  user/group id of the mounting process.

So I tried using these options:

$ sudo mount -t hfsplus -o uid=1000,gid=1000 /dev/sda2 /mnt/Mac

But they seem doing nothing: I still see the same permissions when I look around using ls -l. I may be missing something, any clue?

I know that I can change my user id on Ubuntu to match it with Mac Os X, but I'd prefer avoiding it if possible.

Answer 1

In the end, I created a linux user with the same UID of my mac os x user, but it can't browse every directory in my home on mac hfs+ volume because a lot of files were owned by mac user "unknown", UID 99 (see http://googlemac.blogspot.com/2007/03/user-99-unknown.html).

It seems that they did so to let you mount and read your volume when you connect it to a different computer. When a regular user look at those files owned by UID 99, he sees them as he is their owner. Quite strange. Only root sees them as they are.

So I rebooted in Mac Os X, logged in with a different user with administrative privileges and used chown -R 502:20 /Users/gerlos/* to change the owner of every file in my home. Now I can read everything without any problem.

Remarks:

• default kubuntu gui tool to create new users on Kubuntu 11.10 can't create users with UID less than 1000. Use adduser on the terminal instead.
• you can know your user UID using the "id" command on the terminal.
• on mac os x, you need to be root to see the real owner of the files. So expect different results if you type "ls -n /Users/gerlos" and "sudo ls -n /Users/gerlos".