Make initrd and the kernel builds repeatable.

pkgs/build-support/kernel/cpio-clean.pl

@@ -0,0 +1,20 @@
+use strict;
+
+# Make inode number, link info and mtime consistent in order to get a consistent hash.
+#
+# Author: Alexander Kjeldaas <ak@formalprivacy.com>
+
+use Archive::Cpio;
+
+my $cpio = Archive::Cpio->new;
+my $IN = \*STDIN;
+my $ino = 1;
+$cpio->read_with_handler($IN, sub {
+        my ($e) = @_;
+        $e->{inode} = $ino;
+        $ino++;
+        $e->{nlink} = 1;
+        $e->{mtime} = 1;
+	$cpio->write_one(\*STDOUT, $e);
+    });
+$cpio->write_trailer(\*STDOUT);

pkgs/build-support/kernel/make-initrd.nix

@@ -12,10 +12,10 @@
 # `contents = {object = ...; symlink = /init;}' is a typical
 # argument.
 
-{stdenv, perl, cpio, contents, ubootChooser, compressor}:
+{stdenv, perl, perlArchiveCpio, cpio, contents, ubootChooser, compressor}:
 
 let
-  inputsFun = ubootName : [perl cpio]
+  inputsFun = ubootName : [perl cpio perlArchiveCpio ]
     ++ stdenv.lib.optional (ubootName != null) [ (ubootChooser ubootName) ];
   makeUInitrdFun = ubootName : (ubootName != null);
 in
@@ -35,6 +35,7 @@ stdenv.mkDerivation {
   exportReferencesGraph =
     map (x: [("closure-" + baseNameOf x.symlink) x.object]) contents;
   pathsFromGraph = ./paths-from-graph.pl;
+  cpioClean = ./cpio-clean.pl;
 
   crossAttrs = {
     nativeBuildInputs = inputsFun stdenv.cross.platform.uboot;

pkgs/build-support/kernel/make-initrd.sh

@@ -36,7 +36,7 @@ storePaths=$(perl $pathsFromGraph closure-*)
 
 # Put the closure in a gzipped cpio archive.
 mkdir -p $out
-(cd root && find * -print0 | cpio -o -H newc --null | $compressor > $out/initrd)
+(cd root && find * -print0 | cpio -o -H newc --null | perl $cpioClean | $compressor > $out/initrd)
 
 if [ -n "$makeUInitrd" ]; then
     mv $out/initrd $out/initrd.gz