mkinitcpio: Produce reproducible initramfs images

We achieve this by stripping timestamps from within the filesystem,
and by using a pipeline to strip inodes from the cpio archive.

It functions for at least the 'gzip', 'xz', 'bzip2', 'lz4' and 'cat'
compressors. The 'lzop' compressor embeds a runtime timestamp.

Motivation: https://reproducible-builds.org

Signed-off-by: Daniel Edgecumbe <git@esotericnonsense.com>

2 files changed, 29 insertions, 10 deletions

diff --git a/man/mkinitcpio.8.txt b/man/mkinitcpio.8.txt
index 931a167..6a9b59e 100644
--- a/man/mkinitcpio.8.txt
+++ b/man/mkinitcpio.8.txt
@@ -301,6 +301,22 @@ These are only the variables that the core of mkinitcpio honor. Additional
 hooks may look for other environment variables and should be documented by the
 help output for the hook.
 
+Reproducibility
+---------------
+mkinitcpio aims to create reproducible initramfs images by default.
+
+This means that two subsequent runs of mkinitcpio should produce two files
+that are identical at the binary level.
+
+Timestamps within the initramfs are set to the Unix epoch of 1970-01-01.
+
+Note that in order for the build to be fully reproducible, the compressor
+specified (e.g. gzip, xz) must also produce reproducible archives. At the time
+of writing, as an inexhaustive example, the lzop compressor is incapable of
+producing reproducible archives due to the insertion of a runtime timestamp.
+
+More information can be found at https://reproducible-builds.org.
+
 Files
 -----
 '/etc/mkinitcpio.conf'::
diff --git a/mkinitcpio b/mkinitcpio
index 5583d07..db7a61a 100755
--- a/mkinitcpio
+++ b/mkinitcpio
@@ -211,25 +211,28 @@ build_image() {
             ;;
     esac
 
-    cpio_opts=('-0' '-o' '-H' 'newc')
-    (( _optquiet )) && cpio_opts+=('--quiet')
-    if (( EUID != 0 )); then
-        warning 'Not building as root, ownership cannot be preserved'
-        cpio_opts+=('-R' '0:0')
-    fi
-
     pushd "$BUILDROOT" >/dev/null
-    find . -mindepth 1 -printf '%P\0' |
-            LANG=C bsdcpio "${cpio_opts[@]}" |
+
+    # Reproducibility: set all timestamps to 0
+    find . -mindepth 1 -execdir touch -hcd "@0" "{}" +
+
+    find . -mindepth 1 -printf '%P\0' | sort -z |
+            LANG=C bsdtar --null -cnf - -T - |
+            LANG=C bsdtar --uid 0 --gid 0 --null -cf - --format=newc @- |
             $compress "${COMPRESSION_OPTIONS[@]}" > "$out"
+
     pipesave=("${PIPESTATUS[@]}") # save immediately
     popd >/dev/null
 
     if (( pipesave[0] )); then
         errmsg="find reported an error"
     elif (( pipesave[1] )); then
-        errmsg="bsdcpio reported an error"
+        errmsg="sort reported an error"
     elif (( pipesave[2] )); then
+        errmsg="bsdtar (step 1) reported an error"
+    elif (( pipesave[3] )); then
+        errmsg="bsdtar (step 2) reported an error"
+    elif (( pipesave[4] )); then
         errmsg="$compress reported an error"
     fi