linux-boot: Make /etc/mtab a symlink to /proc/self/mounts.

Fixes <http://bugs.gnu.org/19491>.

* gnu/build/linux-boot.scm (mount-root-file-system): Make /root/etc/mtab
  a symlink to /proc/self/mounts.
* gnu/build/file-systems.scm (mount-file-system): Don't update /etc/mtab.
* guix/build/syscalls.scm (mount, umount): Have #:update-mtab? default to
  #f.

gnu/build/file-systems.scm

@@ -287,13 +287,6 @@ (define* (mount-file-system spec #:key (root "/root"))
        (mount source mount-point type (mount-flags->bit-mask flags)
               (if options
                   (string->pointer options)
-                  %null-pointer))
-
-       ;; Update /etc/mtab.
-       (mkdir-p (string-append root "/etc"))
-       (let ((port (open-file (string-append root "/etc/mtab") "a")))
-         (format port "~a ~a ~a ~a 0 0~%"
-                 source mount-point type (or options "rw"))
-         (close-port port))))))
+                  %null-pointer))))))
 
 ;;; file-systems.scm ends here

gnu/build/linux-boot.scm

@@ -275,7 +275,10 @@ (define (mark-as-not-killable pid)
         (check-file-system root type)
         (mount root "/root" type)))
 
-  (copy-file "/proc/mounts" "/root/etc/mtab"))
+  ;; Make sure /root/etc/mtab is a symlink to /proc/self/mounts.
+  (when (file-exists? "/root/etc/mtab")
+    (delete-file "/root/etc/mtab"))
+  (symlink "/proc/self/mounts" "/root/etc/mtab"))
 
 (define (switch-root root)
   "Switch to ROOT as the root file system, in a way similar to what

guix/build/syscalls.scm

@@ -130,7 +130,7 @@ (define mount
   (let* ((ptr  (dynamic-func "mount" (dynamic-link)))
          (proc (pointer->procedure int ptr `(* * * ,unsigned-long *))))
     (lambda* (source target type #:optional (flags 0) options
-                     #:key (update-mtab? #t))
+                     #:key (update-mtab? #f))
       "Mount device SOURCE on TARGET as a file system TYPE.  Optionally, FLAGS
 may be a bitwise-or of the MS_* <sys/mount.h> constants, and OPTIONS may be a
 string.  When FLAGS contains MS_REMOUNT, SOURCE and TYPE are ignored.  When
@@ -159,7 +159,7 @@ (define umount
   (let* ((ptr  (dynamic-func "umount2" (dynamic-link)))
          (proc (pointer->procedure int ptr `(* ,int))))
     (lambda* (target #:optional (flags 0)
-                     #:key (update-mtab? #t))
+                     #:key (update-mtab? #f))
       "Unmount TARGET.  Optionally FLAGS may be one of the MNT_* or UMOUNT_*
 constants from <sys/mount.h>."
       (let ((ret (proc (string->pointer target) flags))