From e0ad9e48e55d6f46293f2e043ebd527be802bcac Mon Sep 17 00:00:00 2001
From: Tim Vaughan <tgvaughan@gmail.com>
Date: Sat, 24 Aug 2019 00:07:25 +0200
Subject: [PATCH] Drops privs after starting listener.

---
 lambdamail.scm | 58 +++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 46 insertions(+), 12 deletions(-)

diff --git a/lambdamail.scm b/lambdamail.scm
index f1d8c46..d185d89 100644
--- a/lambdamail.scm
+++ b/lambdamail.scm
@@ -10,7 +10,9 @@
         (chicken file)
         (chicken time)
         (chicken time posix)
+        (chicken process)
         (chicken process-context)
+        (chicken process-context posix)
         (chicken condition)
         srfi-1 srfi-13 matchable)
 
@@ -18,7 +20,7 @@
 
 
 (define-record config
-  host port spool-dir)
+  host port spool-dir user group)
 (define-record message to from text helo)
 (define (make-empty-message) (make-message "" "" "" ""))
 
@@ -43,6 +45,14 @@
 ;;; Server initialization
 ;;
 
+(define (drop-privs config)
+  (let ((uid (config-user config))
+        (gid (config-group config)))
+    (if (not (null? gid)) ; Group first, since only root can switch groups.
+        (set! (current-group-id) gid))
+    (if (not (null? uid))
+        (set! (current-user-id) uid))))
+
 (define (run-server config)
   (set-buffering-mode! (current-output-port) #:line)
   (let ((listener (tcp-listen (config-port config) 10 "::")))
@@ -50,6 +60,7 @@
            " listening on port " (config-port config) " ...")
     (print "(Host name: " (config-host config)
            ", Spool dir: " (config-spool-dir config) ")")
+    (drop-privs config)
     (server-loop listener config)))
 
 
@@ -162,21 +173,44 @@
 ;;
 
 (define (print-usage progname)
-  (print "Usage: " progname " hostname [port [spooldir]]"))
+  (print "Usage:\n"
+         progname " -h/--help\n"
+         progname " [-u/--user UID] [-g/--group GID] hostname [[port [spooldir]]\n"
+         "\n"
+         "The -u and -g options can be used to set the UID and GID of the process\n"
+         "following the creation of the TCP port listener (which often requires root)."))
 
 (define (main)
   (let ((progname (pathname-file (car (argv))))
-        (args (cdr (argv)))
-        (config (make-config "" 25 "/var/spool/mail")))
-    (if (null? args)
+        (config (make-config "" 25 "/var/spool/mail" '() '())))
+    (if (null? (cdr (argv)))
         (print-usage progname)
-        (begin
-          (config-host-set! config (car args))
-          (unless (null? (cdr args))
-            (config-port-set! config (string->number (cadr args)))
-            (unless (null? (cddr args))
-              (config-spool-dir-set! (caddr args))))
-          (run-server config)))))
+        (let loop ((args (cdr (argv))))
+          (let ((this-arg (car args))
+                (rest-args (cdr args)))
+            (if (string-prefix? "-" this-arg)
+                (cond
+                 ((or (equal? this-arg "-u")
+                      (equal? this-arg "--user"))
+                  (config-user-set! config (string->number (car rest-args)))
+                  (loop (cdr rest-args)))
+                 ((or (equal? this-arg "-g")
+                      (equal? this-arg "--group"))
+                  (config-group-set! config (string->number (car rest-args)))
+                  (loop (cdr rest-args)))
+                 ((or (equal? this-arg "-h")
+                      (equal? this-arg "--help"))
+                  (print-usage progname))
+                 (else
+                  (print "Unknown option " this-arg "\n")
+                  (print-usage progname)))
+                (begin
+                  (config-host-set! config this-arg)
+                  (unless (null? rest-args)
+                    (config-port-set! config (string->number (car rest-args)))
+                    (unless (null? (cdr rest-args))
+                      (config-spool-dir-set! (cadr rest-args))))
+                  (run-server config))))))))
 
 (main)
 
-- 
2.20.1