services: slim: Add pam-gnupg support.

* gnu/system/pam.scm (unix-pam-service): Add account and session PAM entries
for pam-gnupg.  Don't pass "#f" to "allow-root?" argument, because "lambda*"
already does this by default.
* doc/guix.texi (X Window): Document this.
* gnu/services/xorg.scm (<slim-configuration>)[gnupg?]: New record field.
(slim-pam-service): Pass "#:gnupg?" argument to "unix-pam-service".

doc/guix.texi

@@ -18065,6 +18065,14 @@ Data type representing the configuration of @code{slim-service-type}.
 @item @code{allow-empty-passwords?} (default: @code{#t})
 Whether to allow logins with empty passwords.
 
+@item @code{gnupg?} (default: @code{#f})
+If enabled, @code{pam-gnupg} will attempt to automatically unlock the
+user's GPG keys with the login password via @code{gpg-agent}.  The
+keygrips of all keys to be unlocked should be written to
+@file{~/.pam-gnupg}, and can be queried with @code{gpg -K
+--with-keygrip}.  Presetting passphrases must be enabled by adding
+@code{allow-preset-passphrase} in @file{~/.gnupg/gpg-agent.conf}.
+
 @item @code{auto-login?} (default: @code{#f})
 @itemx @code{default-user} (default: @code{""})
 When @code{auto-login?} is false, SLiM presents a log-in screen.

gnu/services/xorg.scm

@@ -9,6 +9,7 @@
 ;;; Copyright © 2020 Jakub Kądziołka <kuba@kadziolka.net>
 ;;; Copyright © 2020 Alex Griffin <a@ajgrf.com>
 ;;; Copyright © 2021 Brice Waegeneire <brice@waegenei.re>
+;;; Copyright © 2021 Oleg Pykhalov <go.wigust@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -543,6 +544,8 @@ (define-record-type* <slim-configuration>
         (default slim))
   (allow-empty-passwords? slim-configuration-allow-empty-passwords?
                           (default #t))
+  (gnupg? slim-configuration-gnupg?
+          (default #f))
   (auto-login? slim-configuration-auto-login?
                (default #f))
   (default-user slim-configuration-default-user
@@ -572,7 +575,9 @@ (define (slim-pam-service config)
          "slim"
          #:login-uid? #t
          #:allow-empty-passwords?
-         (slim-configuration-allow-empty-passwords? config))))
+         (slim-configuration-allow-empty-passwords? config)
+         #:gnupg?
+         (slim-configuration-gnupg? config))))
 
 (define (slim-shepherd-service config)
   (let* ((xinitrc (xinitrc #:fallback-session

gnu/system/pam.scm

@@ -27,6 +27,7 @@ (define-module (gnu system pam)
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
   #:use-module ((guix utils) #:select (%current-system))
+  #:use-module (gnu packages linux)
   #:export (pam-service
             pam-service-name
             pam-service-account
@@ -207,14 +208,16 @@ (module "pam_unix.so")))
         (env  (pam-entry ; to honor /etc/environment.
                (control "required")
                (module "pam_env.so"))))
-    (lambda* (name #:key allow-empty-passwords? (allow-root? #f) motd
-                   login-uid?)
+    (lambda* (name #:key allow-empty-passwords? allow-root? motd
+              login-uid? gnupg?)
       "Return a standard Unix-style PAM service for NAME.  When
 ALLOW-EMPTY-PASSWORDS? is true, allow empty passwords.  When ALLOW-ROOT? is
 true, allow root to run the command without authentication.  When MOTD is
 true, it should be a file-like object used as the message-of-the-day.
 When LOGIN-UID? is true, require the 'pam_loginuid' module; that module sets
-/proc/self/loginuid, which the libc 'getlogin' function relies on."
+/proc/self/loginuid, which the libc 'getlogin' function relies on.  When
+GNUPG? is true, require the 'pam_gnupg.so' module; that module hands over
+the login password to 'gpg-agent'."
       ;; See <http://www.linux-pam.org/Linux-PAM-html/sag-configuration-example.html>.
       (pam-service
        (name name)
@@ -229,7 +232,12 @@ (module "pam_rootok.so")))
                                 (control "required")
                                 (module "pam_unix.so")
                                 (arguments '("nullok")))
-                               unix))))
+                               unix))
+                     (if gnupg?
+                         (list (pam-entry
+                                (control "required")
+                                (module (file-append pam-gnupg "/lib/security/pam_gnupg.so"))))
+                         '())))
        (password (list (pam-entry
                         (control "required")
                         (module "pam_unix.so")
@@ -247,6 +255,11 @@ (module "pam_motd.so")
                                (control "required")
                                (module "pam_loginuid.so")))
                         '())
+                  ,@(if gnupg?
+                        (list (pam-entry
+                               (control "required")
+                               (module (file-append pam-gnupg "/lib/security/pam_gnupg.so"))))
+                        '())
                   ,env ,unix))))))
 
 (define (rootok-pam-service command)