cocoa - Is it possible to replace the Mac login screen?

Question

Is it possible to replace the Mac OS X login window, /System/Library/CoreServices/loginwindow.app, with a custom login window application? (See my rational for doing so.)

I'm afraid my Cocoa programming skills are rudimentary. I do find it interesting that, when I run probe CGSession (which is a undocumented utility that performs fast user switching) to see what functions it uses, by doing

nm -mg /System/Library/CoreServices/Menu Extras/User.menu/Contents/Resources/CGSession

that one of the linked function is:

(undefined [lazy bound]) external _CGSCreateLoginSession (from ApplicationServices)

I haven't found documentation on the ApplicationServices framework. I suspect I'm getting deep into Service Provider Interfaces instead of Application Programmer Interfaces.

I did find this really interesting snippet: (google cache) (direct link to down page; it appears the site is undergoing re-organization) from an application that claims to switch to the login window even if fast user switching is disabled.

#include "CGSInternal.h"

int main (int argc, const char * argv[]) {
    // switch to the login window
    CGSCreateLoginSession(NULL);

    return 0;
}

I take CG to mean CoreGraphics, and don't understand what that has to do with logging in (except with perhaps putting a login dialog up over the current user's work).

Even if it is not possible to achieve a replacement for the login window, I'd be interested to know what can be done along these lines (by people who don't work for Apple).

Answer 1

The login window application is defined as part of the launchd configuration in /System/Library/LaunchDaemons/com.apple.loginwindow.plist.

In theory you can replace the login window with your own but I don't know what you have to do in the new app - I think the login window does a bit more then authentication and setting up the user session -> amongst others, it looks like its doing some launchd related chores.

I've compiled an application that calls CGSCreateLoginSession and once you run it it transitions to the login window via the rotating cube. I imagine this is why it requires CoreGraphics - it's just a graphics function that calls logout at the end.

You could try an InputManager and see it the login window loads the code -> if it does, you could then alter the loginwindow NIB (LoginWindowUI.nib) and add some buttons to display a new window with the user browser. Once the student chooses a picture of him/herself you could autofill the username and password fields in the loginwindow.

Node this is all theory, and it looks very fragile and unsafe.

Good luck.

Later edit

Please note this is very unsafe so use with care - I did hose my system a couple of times when trying out this stuff

Here's a proof-of-concept implementation that injects code in the loginwindow.

#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <strings.h>
#include <syslog.h>

#import <Cocoa/Cocoa.h>

#include <execinfo.h>

@interface LLApp:NSApplication
@end
@implementation LLApp
- (void)run
{
    syslog(LOG_ERR, "LLApp being run");
    [super run];
}
@end

void my_openlog(const char *ident, int logopt, int facility);

typedef struct interpose_s 
{
        void * new_func;
        void * orig_func;
} interpose_t;

int MyNSApplicationMain(int argc,  const char ** argv);


static const interpose_t interposers[] __attribute__ ((section("__DATA, __interpose")))     = {
{ (void *) my_openlog, (void *) openlog },
};

void my_openlog(const char *ident, int logopt, int facility)
{
        openlog(ident, logopt, facility);

    if(!strcmp(ident, "/System/Library/CoreServices/loginwindow.app/Contents/MacOS/loginwindow"))
    {

        [LLApp poseAsClass:[NSApplication class]];
    }
    else
    {
        syslog(LOG_ERR, "Ignoring unknown indent: >%s<", ident);
    }
    return;
}

The code compiles with:

gcc -Wall -dynamiclib -undefined dynamic_lookup -o ~/Desktop/libinterposers.dylib testin.m -framework Cocoa

Code loading is based on interposing so the launchd definition of loginwindow has to contain an additional entry (to enable interposing in the dynamic linker), i.e.:

<key>EnvironmentVariables</key>
<dict>    
    <key>DYLD_INSERT_LIBRARIES</key>
    <string>path_to/Desktop/libinterposers.dylib</string>
</dict>