Weirdness of Mac OS X mounting Linux Samba share

We have the common situation where our file servers are running Linux and the clients with their different operating systems are mounting the existing shares via a given protocol. A possible candidate for this is the cross-platform protocol samba. This works well with Unix and Windows clients, but as all too often, poses problems for clients running Mac OS X. One of the samba settings that turns out to be crucial is whether unix extensions are turned on or off.

unix extensions off -> Mac OS can't see or change file permissions

The common lore is that unix extensions should be turned off for Mac OS X clients. As a downside, the Mac user cannot correctly see or change the file permissions on the smb share. To illustrate this we consider a folder with three empty files having different permissions and run ls -l on the Linux file server and in the smb mount on a Mac client.

Linux

ls -l
   -r-xr--r-- 1 user staff 0 2011-10-26 09:48 executable.txt
   -r--r--r-- 1 user staff 0 2011-10-26 09:48 readonly.txt
   -rw-r--r-- 1 user staff 0 2011-10-26 10:04 writable.txt

Mac

ls -l
   -rwx------ 1 user staff 0 Oct 26 09:48 executable.txt
   -rwx------ 1 user staff 0 Oct 26 09:48 readonly.txt
   -rwx------ 1 user staff 0 Oct 26 10:04 writable.txt

The Linux displays the correct file permissions, whereas the Mac shows identical and wrong permissions for all files. If you create a text file with a script on the mac, it will be executable even though it doesn't have the executable bit on a Linux client. Moreover the Mac user cannot change the permissions, neither with chmod, nor with the Finder. One can issue chmod on the Mac without error message, but the command has no effect on the permissions of the file on the server. It seems to get simply ignored.

When unix extensions are switched on on the smb server, the Mac can correctly display and change file permissions on the command line. Unfortunately this comes at the price of no longer being able to copy certain symbolic links to the smb share. The following scenario illustrates the problem.

Let's create, locally on the Mac, a folder containing a file b.txt and a symlink a.txt to that file.

mkdir TestFolder
cd TestFolder
touch b.txt
ln -s b.txt a.txt
ls -l
   lrwxrwxrwx 1 user staff 5 Oct 26 10:33 a.txt -> b.txt
   -rw-r--r-- 1 user staff 0 Oct 26 10:33 b.txt

Then, within the Finder, drag'n'drop that folder to the mounted smb share with enabled unix extensions. The copying fails with the error message

     The operation can't be completed because one or more required items can't be found. (Error code -43)

A similar error message is produced when copying the folder with rsync -av. After some more digging we realized that the alphabetical order matters. If the symlink is called c.txt instead of a.txt, Finder is able to copy the folder. This indicates that the Finder copies in alphabetical order. When the symlink precedes the actual file it points to, the smb server seems to complain, that it cannot link to a non-existing file, yielding the above error message in Mac OS X.

Conclusion

From the above we must conclude that there is no possible choice for the unix extensions setting in samba that does not lead to issues with smb shares on Mac OS X.

Further reading