Desktop Manager

What is DesktopManager?

DesktopManager is a utility for Macintoshes which allows you to have multiple workspaces. This means that you do not have to have all your applications on the desktop at once. Instead you can have a desktop for internet applications, a desktop for word processing, etc. Some of your windows--say your mail program--can be on all desktops so you can always access it. Hotkeys, and the desktop pagers, allow you to move between desktops.

DesktopManager's History

DesktopManager was originally written (by Rich Wareham) for Mac OS X.3, before the Intel Mac came to be, and relies upon some code called mach_inject which was very PPC-specific. Fortunately, Bertran Guihéneuf came to the rescue and wrote a Universal version of mach_inject. Bertran Guihéneuf then incorporated his update into DesktopManager and created a Universal version of DesktopManager. So now those of us with Intel Macs can use DesktopManager.

There is just one problem. The purpose of mach_inject is to inject code into a running process. For DesktopManager, the idea is to alter the behaviour of the dock. But while this is good for DesktopManager, it is disconcerting to think that any program could conceivably alter the behaviour of a running program. Isn't this a security risk? At least for Intel Macs, it apparently is. With Mac OS X.4.4, Apple disabled the ability for programs on Intel Macs to "mach inject". However, the ability is restored for users in the procmod group. By default, only root--who is allowed to do anything anyway--is a member of the procmod group. This obviously makes it difficult then for DesktopManager to work.

This problem was also solved by Bertran Guihéneuf, who wrote a framework called InjectionControl. The InjectionControl API consists of two functions which, when combined, test whether the current user is a member of the procmod group, and if not, then with the confirmation of (and an administrator password from) the user, adds the current user to the procmod group. Then DesktopManager can run.

My Contribution

It seems to me--but keep in mind that Bertran Guihéneuf headed the team which developed iCal, and I only know enough to be dangerous--that allowing every program a user runs to allow mach_inject just so that DesktopManager can run is not only overkill but negates the presumably important Mac OS X.4.4 security update which introduced the procmod group in the first place. A simpler, and less drastic solution is to allow DesktopManager (and other trusted programs) and only DesktopManager (and other trusted programs) to run as procmod. This can be done by using the setgid bit of the applications' file permissions after setting the group ownership of the application.

Unfortunately, this does not immediately work with the Universal version of DesktopManager, for the simple reason that the Universal DesktopManager now includes InjectionControl code which checks if the user, and not the process is in the procmod group. So even though setting the group and setgid bit of DesktopManager should have worked, DesktopManager didn't know it and popped up a dialog box. I solved this by

• Adding the function

        InjectionControlError isInjectionEnabledForCurrentProcess(int* enabled)
      

  to Bertran Guihéneuf's functions

        InjectionControlError setInjectionEnabledForCurrentUser(int enabled)
        InjectionControlError isInjectionEnabledForCurrentUser(int* enabled)
      

• Changing the call in DesktopManager from isInjectionEnabledForCurrentUser to isInjectionEnabledForCurrentProcess. (isInjectionEnabledForCurrentProcess first checks if the process is running under the procmod group, and, if not, returns the result of isInjectionEnabledForCurrentUser.)
• Used chmod and chgrp to get DesktopManager to run in the procmod group. (Details can be found with the installation instructions, and the installer does this automatically for installations onto Intel Macs.)

This could be overkill--in principle, by using setgid, InjectionControl is unnecessary--but this allows InjectionControl to be used as a fallback. In particular, InjectionControl is still necessary if the installation disk is mounted with the nosuid flag.

Desktop Manager