system: Allow Linux-libre to find our 'modprobe' command.

Fixes <http://bugs.gnu.org/18525>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/build/activation.scm (activate-modprobe): New procedure.
* gnu/system.scm (modprobe-wrapper): New procedure.
  (operating-system-activation-script): Use both.

gnu/build/activation.scm

@@ -27,6 +27,7 @@ (define-module (gnu build activation)
             activate-etc
             activate-setuid-programs
             activate-/bin/sh
+            activate-modprobe
             activate-current-system))
 
 ;;; Commentary:
@@ -252,6 +253,12 @@ (define (activate-/bin/sh shell)
   (symlink shell "/bin/sh.new")
   (rename-file "/bin/sh.new" "/bin/sh"))
 
+(define (activate-modprobe modprobe)
+  "Tell the kernel to use MODPROBE to load modules."
+  (call-with-output-file "/proc/sys/kernel/modprobe"
+    (lambda (port)
+      (display modprobe port))))
+
 (define %current-system
   ;; The system that is current (a symlink.)  This is not necessarily the same
   ;; as the system we booted (aka. /run/booted-system) because we can re-build

gnu/system.scm

@@ -477,6 +477,20 @@ (define (user-account->gexp account)
       #$(user-account-password account)
       #$(user-account-system? account)))
 
+(define (modprobe-wrapper)
+  "Return a wrapper for the 'modprobe' command that knows where modules live.
+
+This wrapper is typically invoked by the Linux kernel ('call_modprobe', in
+kernel/kmod.c), a situation where the 'LINUX_MODULE_DIRECTORY' environment
+variable is not set---hence the need for this wrapper."
+  (let ((modprobe "/run/current-system/profile/bin/modprobe"))
+    (gexp->script "modprobe"
+                  #~(begin
+                      (setenv "LINUX_MODULE_DIRECTORY"
+                              "/run/booted-system/kernel/lib/modules")
+                      (apply execl #$modprobe
+                             (cons #$modprobe (cdr (command-line))))))))
+
 (define (operating-system-activation-script os)
   "Return the activation script for OS---i.e., the code that \"activates\" the
 stateful part of OS, including user accounts and groups, special directories,
@@ -498,6 +512,7 @@ (define (service-activations services)
                        (etc      (operating-system-etc-directory os))
                        (modules  (imported-modules %modules))
                        (compiled (compiled-modules %modules))
+                       (modprobe (modprobe-wrapper))
                        (accounts (operating-system-accounts os)))
     (define setuid-progs
       (operating-system-setuid-programs os))
@@ -540,6 +555,9 @@ (define group-specs
                     ;; Activate setuid programs.
                     (activate-setuid-programs (list #$@setuid-progs))
 
+                    ;; Tell the kernel to use our 'modprobe' command.
+                    (activate-modprobe #$modprobe)
+
                     ;; Run the services' activation snippets.
                     ;; TODO: Use 'load-compiled'.
                     (for-each primitive-load '#$actions)