make-initrd: fix reproducibility problems

cpio includes the number of directory hard links in archives it creates.
Some filesystems, like btrfs, do not count directory hard links the same
way as more common filesystems like ext4 or tmpfs, so archives built
when /tmp is on such a filesystem do not reproduce. This patch replaces
cpio with bsdtar, which does not have this issue. The specific
invocation is from this page:
https://reproducible-builds.org/docs/archives/

nixos/modules/system/boot/stage-1.nix

@@ -400,7 +400,7 @@ let
         ${lib.optionalString (config.boot.initrd.secrets == {})
             "exit 0"}
 
-        export PATH=${pkgs.coreutils}/bin:${pkgs.cpio}/bin:${pkgs.gzip}/bin:${pkgs.findutils}/bin
+        export PATH=${pkgs.coreutils}/bin:${pkgs.libarchive}/bin:${pkgs.gzip}/bin:${pkgs.findutils}/bin
 
         function cleanup {
           if [ -n "$tmp" -a -d "$tmp" ]; then
@@ -420,7 +420,7 @@ let
           ) config.boot.initrd.secrets)
          }
 
-        (cd "$tmp" && find . -print0 | sort -z | cpio --quiet -o -H newc -R +0:+0 --reproducible --null) | \
+        (cd "$tmp" && find . -print0 | sort -z | bsdtar --uid 0 --gid 0 -cnf - -T - | bsdtar --null -cf - --format=newc @-) | \
           ${compressorExe} ${lib.escapeShellArgs initialRamdisk.compressorArgs} >> "$1"
       '';
 

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

@@ -18,7 +18,7 @@ let
   # compression type and filename extension.
   compressorName = fullCommand: builtins.elemAt (builtins.match "([^ ]*/)?([^ ]+).*" fullCommand) 1;
 in
-{ stdenvNoCC, perl, cpio, ubootTools, lib, pkgsBuildHost
+{ stdenvNoCC, perl, libarchive, ubootTools, lib, pkgsBuildHost
 # Name of the derivation (not of the resulting file!)
 , name ? "initrd"
 
@@ -82,7 +82,7 @@ in stdenvNoCC.mkDerivation rec {
 
   builder = ./make-initrd.sh;
 
-  nativeBuildInputs = [ perl cpio ]
+  nativeBuildInputs = [ perl libarchive ]
     ++ lib.optional makeUInitrd ubootTools;
 
   compress = "${_compressorExecutable} ${lib.escapeShellArgs _compressorArgsReal}";

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

@@ -40,7 +40,7 @@ for PREP in $prepend; do
   cat $PREP >> $out/initrd
 done
 (cd root && find * .[^.*] -exec touch -h -d '@1' '{}' +)
-(cd root && find * .[^.*] -print0 | sort -z | cpio -o -H newc -R +0:+0 --reproducible --null | eval -- $compress >> "$out/initrd")
+(cd root && find * .[^.*] -print0 | sort -z | bsdtar --uid 0 --gid 0 -cnf - -T - | bsdtar --null -cf - --format=newc @- | eval -- $compress >> "$out/initrd")
 
 if [ -n "$makeUInitrd" ]; then
     mkimage -A "$uInitrdArch" -O linux -T ramdisk -C "$uInitrdCompression" -d "$out/initrd" $out/initrd.img