This document describes Guile-PAM version 0.0.1, a way to write, configure, and maintain your system’s Pluggable Authentication Module’s (PAM) in GNU Guile.
Please don’t mind an occasional Guile warning. Guile-PAM works great!
The GNU Autoconf macro GUILE_FLAGS
requires two files from Gnulib that for some reason are not provided automatically. The
missing files can be added via your distribution’s build system. Here
is how we do it in GNU Guix:
(lambda* (#:key inputs #:allow-other-keys)
(let ((build-aux (dirname (search-input-file inputs "/src/gnulib/build-aux/config.rpath"))))
(mkdir-p "build-aux")
(copy-recursively build-aux "build-aux"))
(let ((m4 (dirname (search-input-file inputs "/src/gnulib/m4/lib-link.m4"))))
(mkdir-p "m4")
(copy-recursively m4 "m4")))
This software is in alpha stage. All interfaces are subject to change.
pam_guile.so
How did you like configuring Linux-PAM? Did you find all the modules you were looking for? Do you feel confident that it works well in all situations?
That experience could be a matter of the past. Now you can write PAM modules in GNU Guile, a dialect of Scheme.
This software can call Guile scripts via a specially crafted PAM shared object. You can even replace the entire PAM stack with logic written in Guile.
Linux-PAM was revolutionary when it came out thirty years ago. By using
shared library modules in an ingenious way, security-relevant
applications like su or login automatically followed
consistent authentication policies. When a system administrator changed
those policies, the executables did not have to be recompiled.
Guile-PAM hopes to build on that legacy by making authentication even easier for system administrators.
To use this software, please install the PAM shared object ‘pam_guile.so’ from the folder ‘lib/c’ into your PAM setup. It calls GNU Guile via the mechanism described in the Tortoise Tutorial.
You can use this software in three ways:
required
calls to ‘pam_guile.so’ and handle both recipe selection and recipe
logic in GNU Guile.
To handle all services in Guile-PAM, please replace all services in ‘/etc/pam.d’ with something like this:
auth required pam_guile.so /etc/pam-environment.nul /etc/pam.scm account required pam_guile.so /etc/pam-environment.nul /etc/pam.scm session required pam_guile.so /etc/pam-environment.nul /etc/pam.scm passed required pam_guile.so /etc/pam-environment.nul /etc/pam.scm
Here is a Guile-PAM module that says hello:
(lambda (action handle flags options) ;; do for any action (format #t "Hello, this is your friendly PAM module.~%") 'PAM_IGNORE)
The action parameter will be one of the six symbols:
The handle is an opaque variable known as the PAM handle. It gives the module access to a variety of PAM-internal data, such as the username, the PAM service (which is often the name of the application), and the conversation function.
Please have a look at Linux-PAM’s Module Writers Manual for a list.
The flags are an integer bitmask.
The options are a list of the strings that followed the module in the service definition.
Guile-PAM combines those parameters in an anonymous procedure.
The same anonymous procedures appear everywhere in Guile-PAM, so it made sense to give them a name. We call them Pamdas, which is a portmanteau of PAM and Lambda.
Pamdas always look like this:
(lambda (action handle flags options) ...)
Pamdas are not byte-compiled when they are installed. There is no point because they carry no module data. The compiled versions cannot be located via the module system.
Pamdas are found via their paths in the file system.
Let’s try to do something a little bit more useful than saying hello. Linux-PAM ships a shared object called ‘pam_limits.so’ that adjusts system limits. You can use it to increase the number of open files like this:
session required pam_limits.so
The module reads ‘/etc/security/limits.conf’ which could contain those lines:
* soft nofile 100000 * hard nofile 100000
You can do the same thing in Guile-PAM. We ship a module called ‘set-resource-limits.scm’ in the ‘modules/’ folder. Here is how you use it:
(lambda (action handle flags options)
(let* ((pamda (load "modules/set-resource-limits.scm")))
(pamda action handle flags '((nofile 100000 100000)))))
If you find all that too complicated, you can also call setrlimit
yourself:
(lambda (action handle flags options)
(if (eq? 'pam_sm_open_session action)
(setrlimit 'nofile 100000 100000))
'PAM_SUCCESS)
Finally, you can continue to use the Linux-PAM shared object:
(use-modules (pam legacy module)) (lambda (action handle flags options) (call-legacy-module "pam_limits.so" action handle flags))
It can be fun to see Guile-PAM at work:
(lambda (action handle flags options)
(case action
;; authentication management
((pam_sm_authenticate)
(format #t "In a working module, we would now identify you.~%"))
((pam_sm_setcred)
(format #t "In a working module, we would now help you manage additional credentials.~%"))
;; account management
((pam_sm_acct_mgmt)
(format #t "In a working module, we would now confirm your access rights.~%"))
;; password management
((pam_sm_chauthtok)
(format #t "In a working module, we would now change your password.~%"))
;; session management
((pam_sm_open_session)
(format #t "In a working module, we would now open a session for you.~%"))
((pam_sm_close_session)
(format #t "In a working module, we would now close your session.~%"))
(else
(format #t "In a working module, we would not know what to do about action '~s'.~%"
action)))
'PAM_SUCCESS)
Ideally, we would use a PAM conversation function instead of printing to
(current-output-port) via format but the author has yet to
figure out how to use callbacks with Nyacc.
This software comes with a complete reimplementation of the Linux-PAM stack in GNU Guile.
The Guile version of PAM stacks looks like this:
(use-modules (pam stack))
(lambda (action handle flags options)
(stack action handle flags
(list
(gate optional (load "welcome.scm"))
(gate required (load "do-something.scm")))))
Each gate procedure is like a line in a Linux-PAM configuration
file. It expects a pamda. Here are the signatures:
Define a gate around pamda using the plan. Pass options to the module, if present.
Returns a specialized lambda that is only useful when used with
stack.
The lambda will return PAM_IGNORE if #:only-actions
is given but action does not match, or if #:only-services is
given but a different service is active. The selectors make it easier
to convert existing systems to Guile-PAM.
The stack procedure evaluates a list of gates in order:
Traverse the list of gates, passing each the action, the PAM handle handle, and flags. Processing may cease depending on the evaluation at each gate.
Returns a symbolic PAM status code like PAM_SUCCESS or #f
if none was acquired.
Forwards exceptions from code below.
We’ll learn how to write the plan in a little while. In many cases, it’s just the plain variable ‘required’ or ‘optional’.
You can also stack Linux-PAM’s shared objects:
(use-modules (pam stack))
(lambda (action handle flags options)
(stack action handle flags
(list
(gate required
(lambda (action handle flags options)
(call-legacy-module "pam_motd.so") action handle flags
#:options
(list (string-append "motd=" message-file))
#:implements
'(pam_sm_open_session))))))
Linux-PAM’s shared objects often do not behave well for actions they do
not implement. The keyword parameter #:implements makes sure we
return PAM_IGNORE for actions that aren’t implemented.
Otherwise the stack may terminate when it shouldn’t.
There are some convenience procedures if you are bothered by the repetitive nature of the ‘lambdas’. The code from the previous section can be simplified to:
(use-modules (pam legacy modules)
(pam stack))
(stack-pamda
(list
(gate required
(legacy-pamda "pam_motd.so"
#:options
(list (string-append "motd=" message-file))
#:implements
'(pam_sm_open_session)))))
One procedure is in (pam stack).
Returns a pamda that evaluates gates in the order given.
Returns a symbolic PAM status code, or #f if none was acquired.
stack.
And the other is in (pam legacy module).
Returns a pamda that calls the Linux-PAM shared object located at path.
Returns a symbolic PAM status code.
Additionally, returns PAM_IGNORE if action does not match
any symbol listed in #:implements. That is necessary because many
Linux-PAM shared objects return the status PAM_SERVICE_ERR unless
they were called with an action they expected. That could
otherwise cause the stack to fail when it should not.
Raises an exception if the shared object or the ELF symbol requested therein cannot be located.
Like Linux-PAM stacks, Guile-PAM offers the keywords required,
optional, sufficient, and requisite.
They are variables so you can use their plain names.
For a custom plan like ‘success=ok new_authtok_reqd=ok ignore=ignore default=bad’ we have to delve a little bit into how stacks work.
By the way, that string is Linux-PAM’s
definition of the required plan.
The stack can take one of three actions on a return status: It can
Here are the rules for the custom plan above:
(list (cons 'PAM_SUCCESS (acquire 1)) (cons 'PAM_NEW_AUTHTOK_REQD (acquire 1)) (cons 'PAM_IGNORE (discard 1)))
For this plan, the two return statuses indicating success are
PAM_SUCCESS and PAM_NEW_AUTHTOK_REQD. The rules also
honor the module’s request to be ignored when its return status is
PAM_IGNORE. The stack will discard the result and move on.
The number one means to advance to the next gate. It is also possible to skip over modules by using numbers greater than one.
The default stack action for the custom plan above is:
(lock 1)
That’s because for the required plan all other statuses are
failures. They lock the stack in a way from which it cannot recover.
Together the rules and the default form what we call a plan.
Here is the full set of predefined plans:
;; Copyright © 2022-2024 Felix Lechner
;;
;; This program is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
(define (make-sufficient-rules step)
(list
(cons 'PAM_SUCCESS (acquire step))
(cons 'PAM_NEW_AUTHTOK_REQD (acquire step))))
(define (make-necessary-rules step)
(cons (cons 'PAM_IGNORE (discard step))
(make-sufficient-rules step)))
(define-public required
(let* ((rules (make-necessary-rules 1))
(default (lock 1)))
(make-plan rules default)))
(define-public requisite
(let* ((rules (make-necessary-rules 1))
(default (lock 0)))
(make-plan rules default)))
(define-public sufficient
(let* ((rules (make-sufficient-rules 0))
(default (discard 1)))
(make-plan rules default)))
(define-public optional
(let* ((rules (make-sufficient-rules 1))
(default (discard 1)))
(make-plan rules default)))
In the Guile module (pam legacy stack) is a procedure
legacy-plan->modern-plan that translates Linux-PAM strings like
‘success=ok new_authtok_reqd=ok ignore=ignore default=bad’ to a
plan you can use with Guile-PAM.
Returns a plan for use with Guile-PAM that is equivalent to the Linux-PAM string plan.
Raises an error if plan does not conform to Linux-PAM’s specifications.
Guile-PAM is different from Linux-PAM in several ways:
This software is still being tested. Linux-PAM is mature and trusted.
PAM_IGNORE.Linux-PAM skips actions that belong together when the other returned
PAM_IGNORE. It groups the six actions into four categories
called auth, account, session, and passwd.
You already know them from the Linux-PAM configuration files. For the
two categories that each bundle two actions, Linux-PAM will not call the
second action if the first one returned PAM_IGNORE. The behavior
is documented in the
‘pam_sm_setcred(3)’ manual page that comes with Linux-PAM.
The implementation in (pam stack) is different. It evaluates
each module for each action.
When legacy instruction sets like ‘success=1 new_authtok_reqd=1 ignore=ignore default=bad’ use explicit skip counts, an unlocked Guile-PAM stack will always acquire the result. In Linux-PAM, the side effect depends on the action as described here.
You’ll notice quickly that in Guile-PAM everything is a pamda. Any user of Guile-PAM is a module writer. This section addresses a more in-depth interaction with the PAM subsystem.
As a small caution, please remember that your code probably runs inside
a program that is setuid to the ‘root’ user. The Bash
developers like to say that you have super cow powers.
GNU Guile’s superb foreign function interface allows programs to call code written in the ‘C’ language seamlessly. It works great!
Guile-PAM uses the FFI Helper from Nyacc (Not Yet Another Compiler Compiler) to provide a Scheme interface to Linux-PAM’s ‘libpam’ library.
The FFI helper automatically creates Guile bindings from C header files. No manual maintenance is needed, but they can be a bit cumbersome to use.
In addition, Guile-PAM provides some higher-level procedures in
(pam) that avoid the complex distractions of FFI.
For example, you can call get-username to get the name of the
user seeking authentication, or call get-service to get the name
of the PAM service, which is usually the name of the calling
application.
Get the name of the user seeking authentication from handle.
Returns #f if the name is not available.
Get the name of the PAM service from handle.
Returns #f if the service is not available.
There is also value->symbol, which translates numerical PAM
return values to their symbolic names.
Get the symbolic PAM status for the numerical PAM return code
number. For example, the number ‘0’ stands for the status
PAM_SUCCESS.
The full list is available in the (pam) module.
Returns #f if the number has no symbol associated wit it..
In the module (pam legacy module) you can find a procedure that
will call a Linux-PAM shared object. It has the following signature:
Call Linux-PAM shared object located at path. Pass the symbol action, the PAM handle handle, an integer bitmask flags, and options, which is a list of strings.
Returns a symbolic PAM status code.
Additionally, returns PAM_IGNORE if action is not listed in
#:implements. That is necessary because many Linux-PAM shared
objects return the status PAM_SERVICE_ERR unless they were called
with an action they expected. That could otherwise cause the
stack to fail when it should not.
Raises an exception if the shared object or the ELF symbol requested therein cannot be located.
Please have a look at Linux-PAM’s System Administrator’s Guide for a list of available shared objects.
The term service is used in many contexts. In Linux-PAM, it refers to a recipe how to authenticate a user. Such recipes are often named after the programs that call them.
The author would love to find a new word for PAM services. Perhaps the word recipe will find some acceptance.
There are some interesting effects when writing PAM modules in an interpreted language. An update of Linux-PAM shared objects can lead to library ABI conflicts and stability issues. There seem to be fewer such issues when a live system is updated with new pamdas.
The author has not yet figured out how to use ‘C’ callbacks with Nyacc. That needs to be resolved in order for PAM conversation functions to work. They are needed for interactive authentication modules in Guile, such as ‘pam_unix.so’.
Primary concerns are unauthorized access and the loss of secrets. For both, the use of an interpreted language should be an improvement.
It’s probably fair to say that Modules written in Guile-PAM are easier to read than the ‘C’ code in Linux-PAM. If you share a module, recipients are therefore more likely to look at it. Over time the code will receive more views, however cursory they may be. By comparison, it’s not clear how many folks look at the Linux-PAM or the OpenPAM source codes.
Attacks via injection are an additional concern. GNU Guile
separates well between strings and code but code and data are often
mixed when quoting. Quoted S-Expressions are probably harmless. Please
do not use Guile’s read procedure, however, especially not when
dealing with unknown inputs.
Tainted environments are another concern. It is imperative that we use the modules we expect. There should be no risk of substitution.
The search path for GNU Guile modules can be modified via the
environment variables GUILE_LOAD_PATH and
GUILE_LOAD_COMPILED_PATH. The ‘pam_guile.so’ shared object
tries to isolate those environment variables from the application.
The shared object also restores that part of the environment before
returning to the application. Therefore some environment variables like
LANG may not end up persisting after being set by
‘pam_env.so’.
As an additional precaution, pamdas should be loaded via absolute paths.
Guile-PAM modules performed at adequate speeds in practical tests.
Speed, it should be noted, is not always an advantage in authentication. It is common to slow down processing for the repeated entry of secrets so that an attacker will find it harder to exhaust the possible search space.
Please build delays into your modules.
Here are examples of complex things that are easier in Guile-PAM:
Here is a module that does something truly useful and unique. Called
‘user-session-with-piped-secret.scm’, it stores the token received
in the pam_sm_authenticate stage. The module then retrieves the
token in the pam_sm_open_session stage and pipes it to a user
pamda.
The user pamda, which is shown below, reads the password and mounts a Gocryptfs folder as ‘/home/lechner’ during the login process. The mount operation happens when the user logs in.
In the Linux-PAM universe there is something called ‘pam_mount.so’ but it mounts volumes as the ‘root’ user. which does not work with FUSE folders using standard settings or with kerberized NFSv4.
This use case was the author’s motivation for writing Guile-PAM.
;; Copyright © 2022-2024 Felix Lechner
;;
;; This program is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;;
;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;;
;; You should have received a copy of the GNU General Public License
;; along with this program. If not, see <https://www.gnu.org/licenses/>.
(use-modules (ice-9 format)
(ice-9 popen)
(pam stack)
(srfi srfi-1)
(rnrs io ports))
(define (feed-string str executable . args)
(let* ((port (apply open-pipe* OPEN_WRITE executable args)))
(format port "~a~%" str)
(status:exit-val (close-pipe port))))
(define (already-mounted? folder)
(let* ((all-mounts (call-with-input-file "/proc/mounts"
(lambda (port)
(get-string-all port))))
(lines (string-split all-mounts #\newline)))
(any (lambda (line)
(if (string-null? line)
#f
(let* ((pieces (string-split line #\space))
(mount-point (cadr pieces)))
(string=? folder mount-point))))
lines)))
(define (mount-home-gocryptfs action handle flags options)
(let* ((cleartext-folder "/home/lechner"))
(case action
((pam_sm_open_session)
;; ideally, we would count sessions instead
(if (already-mounted? cleartext-folder)
'PAM_SUCCESS
(let* ((token (car options)))
(if token
(let* ((exit-val (feed-string token
"/run/current-system/profile/bin/gocryptfs"
"../home.gocryptfs"
cleartext-folder)))
(if (eqv? 0 exit-val)
'PAM_SUCCESS
'PAM_SESSION_ERR))
'PAM_AUTHINFO_UNAVAIL))))
(else
'PAM_SUCCESS))))
(lambda (action handle flags options)
(stack action handle flags
(list
(gate required mount-home-gocryptfs
#:options options))))
You can experiment with your code on a live system by installing a new
recipe that is not used by any application. Or you can hijack an
existing recipe that is rarely used. Just add ‘pam_guile.so’ with
the optional rule set like this:
auth optional pam_guile.so /etc/pam-environment.nul /etc/pam.scm
You may find the tiny pamtester program helpful.
For anyone using GNU Guix that ad-hoc method is not available. In Guix configuration files are read-only. They reside in something called the store, which is read-only.
Another solution was therefore needed:
Guile-PAM comes with a small program called load that allows you
to test your modules before installing them on your system. You can
find it in a folder called ‘exec’.
The load program has several command-line options.
The only required option is --module-path or -m. It
designates the shared object to load. The path should point to your
location for ‘pam_guile.so’.
That’s usually all you need. On the command line you would list the path to your Guile-PAM module as well as any options to that module.
The other options are:
--action or -a picks one of the six PAM entry points. The
default is pam_sm_authenticate.
--user or -u sets the PAM user. That is the user seeking
authentication. The default is pamtester.
--service or -s sets the PAM service. That is the
particular recipe, or PAM stack, being run. The default is
‘login’.
--flags or -f sets the integer bitmask that passed to each
PAM module. That parameter is rarely used. The default value is the
integer zero.
--auto-compile or -a sets the environment variable
GUILE_AUTO_COMPILE. You can find out more about in the
GNU Guile manual. The default is the string ‘0’.
--locale or -l selects the locale for the pamdas. The
locale helps to make sure that literal strings are processed correctly.
The default is ‘en_US.utf8’.
In order to use the load program, you also have to amend or set a
couple of environment variables:
GUILE_LOAD_PATH helps Guile to find modules like (ffi pam)
or (pam stack)
GUILE_EXTENSIONS_PATH helps Guile locate ‘libpam.so’ and any
other shared ELF libraries your Guile modules load dynamically. On most
systems, the library path for ld.so is available in
LIBRARY_PATH or in LD_LIBRARY_PATH.
Putting everything together, your invocation of the ‘load’ program inside the code repo might look something like that:
$ GUILE_EXTENSIONS_PATH="$LIBRARY_PATH" GUILE_LOAD_PATH="./scm:$GUILE_LOAD_PATH" exec/load -m ../libpam-guile-c/.libs/pam_guile.so /absolute/path/to/your/pamda.scm
Happy hacking!
By experimenting with this software, you could get locked out of your computer. Please make sure you know how to recover before you proceed.
It may be helpful to keep a ‘root’ window open on the host being modified.
Other tools might include a good live boot system on a USB stick from your operating system vendor. Please make sure your equipment can boot from USB.
The author has not needed any boot media for Guile-PAM because GNU Guix offers roll-backs.
For desperate cases, please remember the kernel option ‘init=/bin/bash’. I have not needed it for Guile-PAM and hope you don’t, either.
pam_guile.soPlease install the shared object ‘pam_guile.so’ in your Linux-PAM recipes like any other Linux-PAM shared object.
The first command-line argument in the Linux-PAM configuration file is the absolute path to the runtime environment for the shared object. The second argument is the absolute path to the Scheme pamda you wish to run.
The runtime environment for pamdas is partially isolated from the calling application. The shared object adds several environment variables before calling pamdas. Here are the most important:
LANG, for the locale reading files from disk;
GUILE_AUTO_COMPILE, which prevents Guile’s automatic compilation
and is always set to ‘"0"’;
GUILE_LOAD_PATH, which points to the Guile modules that are
available to the pamda;
GUILE_LOAD_COMPILED_PATH, which points to compiled versions of
those modules; and
GUILE_EXTENSIONS_PATH, which FFI needs to locate ‘libpam’ and
potentially other shared libraries.
The full list is available in (pam environment).
To load the values when the shared object is called, ‘pam_guile.so’ receives as its first parameter the path to a file that contains the desired environment.
The path to the environment file must be absolute.
The file contains strings with variable assigments that look the way you
would enter them into a shell, except the strings are terminated by
‘\0’ characters. That is also the format expected by the
putenv(3) library call in Glibc.
Usually, the environment file will look something like this, except each line is terminated by a ‘\0’ character:
LANG=C.utf8 GUILE_AUTO_COMPILE=0 GUILE_LOAD_PATH=/usr/share/guile/site/3.0 GUILE_LOAD_COMPILED_PATH=/usr/lib/guile/3.0/site-ccache GUILE_EXTENSIONS_PATH=/usr/lib
Before the shared object returns to the application, it restores those variables to what they were before.
Pamdas can and do change enviroment variables that persist in the calling process. (They may not be mentioned in the environment file.) Without that, modules like ‘pam_env.so’ would not work.
In the module (pam environment) is a procedure that could help
you write the null-terminated environment file:
Writes to port an environment file that is suitable for Guile-PAM.
Guile-PAM provokes a Guile warning the author has been unable to turn off:
Guile-PAM’s functionality, however, does not seem to be affected.
This section will help you migrate your system to using Guile-PAM on top of Linux-PAM.
In (pam legacy configuration) you can find the procedure
configuration-file->gates, which will read a Linux-PAM service
file and convert in on the fly for use with Guile-PAM’s stack
implementation.
Returns a list of Guile-PAM gates equivalent to the Linux-PAM configuration file file found in folder folder..
Raises an error if the configuration file does not conform to Linux-PAM’s specifications.
GNU Guix is an entire operating system configured in GNU Guile. If you use it, you can try the patch series that converts your system to Guile-PAM.
Below are the relevant parts of the author’s system configuration in hope of getting you started:
(define welcome-pamda-file
(scheme-file
"welcome-pamda-file"
#~(begin
(lambda (action handle flags options)
(case action
;; authentication management
((pam_sm_authenticate)
(format #t "In a working module, we would now identify you.~%"))
((pam_sm_setcred)
(format #t "In a working module, we would now help you manage additional credentials.~%"))
;; account management
((pam_sm_acct_mgmt)
(format #t "In a working module, we would now confirm your access rights.~%"))
;; password management
((pam_sm_chauthtok)
(format #t "In a working module, we would now change your password.~%"))
;; session management
((pam_sm_open_session)
(format #t "In a working module, we would now open a session for you.~%"))
((pam_sm_close_session)
(format #t "In a working module, we would now close your session.~%"))
(else
(format #t "In a working module, we would not know what to do about action '~s'.~%"
action)))
'PAM_SUCCESS))))
(define motd
(plain-file
"motd"
"This is your message of the day."))
(define my-pamda-file
(scheme-file
"my-pamda-file"
#~(begin
(use-modules ((pam) #:prefix guile-pam:)
(pam legacy module)
(pam stack))
(lambda (action handle flags options)
;; for gocryptfs
(if (eq? 'pam_sm_open_session action)
(setrlimit 'nofile 100000 100000))
(stack action handle flags
(list
(gate optional (load #$welcome-pamda-file))
(let* ((username (guile-pam:get-username handle))
(hostname (gethostname))
(file (string-append "/acct/"
username
"/"
hostname
"/away/pam.scm")))
(gate required (load #$(file-append guile-pam
"/share/modules/user-session-with-piped-secret.scm"))
#:options (list file)
#:only-services '("login"
"greetd"
"su"
"slim"
"gdm-password"
"sddm")))
(gate optional
(lambda (action handle flags options)
(call-legacy-module #$(file-append linux-pam "/lib/security/pam_motd.so")
action handle flags
#:options
(list (string-append "motd=" #$motd))
#:implements
'(pam_sm_open_session))))))))))
This is the ‘service’ stanza for the ‘operating-system’ definition:
(service guile-pam-module-service-type
(guile-pam-module-configuration
(rules "optional")
(module my-pamda-file)
(services '("login"
"greetd"
"su"
"slim"
"gdm-password"
"sddm"))))
Guile-PAM aims to be fully compatible with applications that rely on Linux-PAM.
Please see Linux-PAM’s Application Developer’s Guide on how to rely on Guile-PAM in your application.
The purpose of this project is to simplify authentication schemes. The PAM stack is one such method. Are there others?
One day, Guile-PAM hopes to offer Scheme implementations for all of Linux-PAM’s legacy shared objects.
The purpose would be to bring more transparency to the authentication tasks all of us depend upon.
This section is a placeholder for your creative ways to drop PAM stacks altogether.
This software should work with OpenPAM if you configure it with the
option --with-openpam.
You will also need to provide a pkg-config file, unless upstream accepted the author’s merge request to ship one. You can find the proposed patch here.
While Linux lacks the BSD system calls ‘pledge(2)’ and ‘unveil(2)’, OpenBSD scripts may eventually become usable on Linux systems when using this software. That would lead to greater convergence and consistency among free-software systems.
Please report any defects as soon as you see them. Please do not worry if you don’t have all the information. Better safe than sorry!
If you have a heart for nature and for your fellow creatures, please think twice before you associate the problems in your software with those mostly harmless creatures. (Okay, maybe not mosquitoes.)
The idea of squashing bugs at a party never appealed to the author. Following some nudging by a friend, this manual calls software defects what they are, namely defects.
Please remind the author when his posture here is inconsistent with his conduct in case he, too, continues to use the word ‘bugs’.
Development for Guile-PAM is generously hosted on Codeberg.org. Please feel free to use all enabled features, including the filing of issues.
The hope of this project is to help you write your own PAM modules. Please share your work to avoid the duplication of efforts.
Please offer to include your pamdas in this distribution. The sole requirement, aside from having general interest, is that you release your code under the same license.
For help with Guile, please ask in ‘#scheme’ or ‘#guile’ on Libera.chat.
There is also ‘#guile-pam’ but it’s very lonely there.
Guile-PAM is based on GNU Guile, which was designed to be a flexible and versatile configuration language for programs written in other languages.
The author was assisted generously by folks in the ‘#scheme’ IRC channel on Libera.chat, especially by the users ‘Zipheir’ and ‘wasamasa’.
Many people in the ‘#guile’ IRC channel on Libera.chat also helped.
The foreign-function interface for Guile-PAM was generated automatically
by the amazing ‘FFI helper’ tool that comes bundled with
nyacc. Matt Wette, the
upstream author, graciously helped the author in moments of great
despair.
Finally, homage must be paid to the pioneering developers of Linux-PAM. They revolutionized the way privileged applications ensure that they are being used in an authorized fashion. Without those developers, this project would not exist.
Thanks also to anybody not mentioned who helped with this collaborative effort by reporting defects, by providing artwork, or by making suggestions!
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