[Buildroot] [PATCH v2] fs/tar: explicitly set extended header values to ensure binary reproducibility

[Atharva Lele]

Since we use --xattrs-include='*' to include all extended attributes,
tar creates a PAX formatted archive. The archive metadata captures atime
and ctime of files. To fix this, GNU recommends that we pass this added
argument to tar to create binary reproducible packages. Setting of mtime
is handled in fs/common.mk using touch on all files.

Diffoscope output pre-change: https://gitlab.com/snippets/1871111
Diffoscope output after change is blank i.e. binary reproducibile rootfs
is created.

GNU Recommendation: https://www.gnu.org/software/tar/manual/tar.html#SEC147

Signed-off-by: Atharva Lele <itsatharva at gmail.com>
---
Changes v1 -> v2:
  - Added comment about mtime handling
---
 fs/tar/tar.mk | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/tar/tar.mk b/fs/tar/tar.mk
index 4c6327ace8..f6d11baa84 100644
--- a/fs/tar/tar.mk
+++ b/fs/tar/tar.mk
@@ -8,6 +8,10 @@ TAR_OPTS := $(call qstrip,$(BR2_TARGET_ROOTFS_TAR_OPTIONS))
 
 ROOTFS_TAR_DEPENDENCIES = $(BR2_TAR_HOST_DEPENDENCY)
 
+ifeq ($(BR2_REPRODUCIBLE),y)
+TAR_OPTS += --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0,ctime:=0
+endif
+
 define ROOTFS_TAR_CMD
 	(cd $(TARGET_DIR); find -print0 | LC_ALL=C sort -z | \
 		tar $(TAR_OPTS) -cf $@ --null --xattrs-include='*' --no-recursion -T - --numeric-owner)
-- 
2.20.1