[OpenAFS-devel] aklog on MacOS X was Re: Service Ticket Questions

emoy@apple.com emoy at apple.com
Sat Mar 18 04:34:07 EST 2006

On Mar 17, 2006, at 7:33 PM, Jeffrey Altman wrote:

> I am cc'ing the openafs-devel at openafs.org mailing list because this is
> really an OpenAFS discussion.  krbdev at mit.edu is meant to be a mailing
> list focused on development of the MIT Kerberos reference  
> implementation.
> The fundamental issue being discussed here is whether the Kerberos.App
> display of the Kerberos Credential Cache contents can be used as an
> indication by end users that the AFS kernel module contains tokens for
> that user.  Hank is claiming that presence of an "afs/cellname at REALM"
> service ticket in the credential cache is an indicator that there are
> AFS tokens installed in the AFS kernel module.
> I believe that end users should be discouraged from checking the
> Kerberos credential cache to see if they have AFS tokens because doing
> so is fundamentally flawed.  There are many reasons why tokens  
> might be
> removed from the AFS kernel after their initial installation let alone
> reasons why tokens might not be able to be stored in the first place.
> Therefore, using the Kerberos credential cache as a replacement for  
> the
> "tokens" command or a GUI token display will only make the lives of  
> end
> users and those that support them more difficult.
> If there is a concern that the presence of the AFS service ticket will
> be misinterpreted as meaning that tokens are present then perhaps the
> thing to do is modify aklog and anything that derives from its code  
> base
> to not use the default credential cache and instead use a local memory
> cache.  We could make this the default behavior and allow the default
> credential cache be used when "-d" is specified on the command line to
> allow the presence of the service tickets to be used for debugging  
> purposes.

One other place that the credential cache and the AFS token get out  
of sync is when trying to renew the TGT.  Renewal of the TGT and AFS  
token is a whole subject on its own, but suffice it to say that  
running "kinit -R" will renew the TGT, but remove the AFS service  
ticket, while the AFS token remains unchanged.

> While not part of the same topic, Derrick Brashear spent time this  
> week
> attempting to prepare a KFM KLL plug-in for aklog that would work on
> Tiger and discovered that under Tiger we will not be able to provide
> such functionality.   We will work with Apple to try to make this  
> happen
> in a future release.  For those who are unaware, the KFM KLL plug-ins
> have been used in previous releases of MacOS X to allow the Kerberos
> initial ticket getting functionality to be extended such that  
> whenever a
> new Kerberos 5 Initial Ticket is obtained a new AFS token would be
> acquired at the same time.  Without this functionality it is not
> possible to provide a single sign-on experience for AFS on Tiger.

(Remember, I'm just a developer who happens to work at Apple; I don't  
speak for Apple.)

It turns out I was working this week on something similar, a PAM  
module for aklog.  This is still highly experimental, but I have  
modified aklog to be more reentrant and embeddible, by converting all  
the global variables into fields of a structure.  I have add an  
additional routine that allocates such a structure, which is then  
passed to the aklog() routine.  So now, the main() routine of the  
aklog program calls alloc_aklog_globals() to get a block of memory,  
and passes that to the aklog() routine with the arguments.

The PAM modules does the same thing.  There are some differences that  
are taken care of with a macro definition, like the fact that argv[0]  
is the name of the aklog program, but is the first real argument from  
the PAM modules.  Also, the aklog program exits on error; the PAM  
modules does a longjmp back to the aklog() routine to do an error  
return (because there is no exit, there might be some memory leaks or  
other cleanup that will need to be done).

Creating a loadable PAM module did present some problem.  The LWP  
code uses assembly language (process.s for ppc and process.i386.s for  
x86), but that code references a global variable in way that would  
require a writable text segment.  So I added the usual assembly goop  
to move the relocation info to a data segment.  (I had tried to  
replace LWP with the pthread code, but was running into a bunch of  
undefined symbol problems, so I fell back to LWP.  I did notice four  
places that allocate pthread mutexes but don't initialize with  
PTHREAD_MUTEX_INITIALIZER as per POSIX, and would have made a  
difference on Mac OS X, since when uninitialized, it would reside in  
common segment, which is not allowed for a bundle.  Initializing it  
moves it into a data segment.)

So I finally created the PAM module, which only does session open and  
close.  The open calls aklog to create the token.  The close does the  
equivalent of unlog, but I then commented that out, since I can  
imagine being logging into the GUI, when an ssh session comes in,  
renews the AFS token and then removes it on logout, leaving the GUI  
without a token.  Some mechanism for the last-one-out removing the  
token is needed.

So fine, I have a PAM module, but so what?  Well, I also wrote a  
loginwindow "LoginHook", that is called loginwindow_pam_helper.  It  
gets run when loginwindow finishes authenticating (presuming /etc/ 
authorization is set up to do Kerberos authentication).  It then does  
the usual PAM stuff to open a session and I set up the PAM config  
file to call my pam_aklog module.  In loginwindow_pam_helper, I use a  
kqueue to monitor when loginwindow terminates (usually when the user  
logs out, or if loginwindow dies) and then close the session.

Well, this actually all works!  I log in, and my AFS tokens are  
there.  I can even set up a .xlog file and get tokens for multiple  

Now I'm not saying this is the best approach.  My thinking was that a  
PAM module would be useful for other Unix platforms as well, but  
doesn't really fit in so nicely in Mac OS X.  The KFM might be a  
little cleaner, though Mac OS X specific.  But it might be possible  
to get loginwindow to do PAM directly, so my loginwindow_pam_helper  
wouldn't be needed anymore, and then the PAM module might fit in  
better (of course, I'm not on the loginwindow team, so I can't say  
they will make this change).

Mac OS X also has it's own way of doing things, like the  
Authorization Services framework.  Now I'm a Unix guy, so the  
Authorization Services framework is new to me.  But it might also be  
feasible to put in aklog as a plug-in.  Then loginwindow could get to  
aklog through Authorization Services, and since there is already a  
pam_securityserver.so module that calls into the Authorization  
Services framework, things like ssh might also be able to get to  
aklog.  Again this is Mac OS specific, but could be the cleanest way  
for both Mac OS X applications and command-line programs to get aklog  
access.  And since aklog is embeddable, we could build the aklog  
program, the aklog PAM module, the aklog Authorization plug-in and  
possibly even the KFM KLL plug-in, all using the same base code.

Anyways, Derrick, let me know if I can be of help with the KFM KLL  
plug-in (which is something else I don't know much about).

Edward Moy

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