[OE-core] [PATCH] kernel.bbclass: improve reproducibility

[Juro Bystricky]

Kernel and kernel modules contain hard coded paths referencing the host
build system. This is usually because the source code contains __FILE__
at some place. This prevents binary reproducibility. However, some compilers
allow remapping of the __FILE__ value. If we detect the compiler is capable
of doing this, we replace the source path $(S) part of __FILE__ by a string "/kernel-source".
For example:

​/​data/​master/​build-​repro/​tmp/​work-​shared/​qemux86/​kernel-​source/​drivers/​media/​v4l2-​core/​videobuf2-​core.​c

will be replaced by a reproducible value:

/kernel-source/drivers/media/v4l2-core/videobuf2-core.c.

Signed-off-by: Juro Bystricky <juro.bystricky at intel.com>
---
 meta/classes/kernel.bbclass | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/meta/classes/kernel.bbclass b/meta/classes/kernel.bbclass
index 8954b28..0a2cd3d 100644
--- a/meta/classes/kernel.bbclass
+++ b/meta/classes/kernel.bbclass
@@ -249,6 +249,18 @@ python do_devshell_prepend () {
 
 addtask bundle_initramfs after do_install before do_deploy
 
+get_cc_option () {
+		# Check if KERNEL_CC supports the option "file-prefix-map".
+		# This option allows us to build images with __FILE__ values that do not
+		# contain the host build path.
+		cc_option_supported=`${KERNEL_CC} -Q --help=joined | grep ffile-prefix-map`
+		cc_extra=""
+		if [ $cc_option_supported = "-ffile-prefix-map=<old=new>" ]; then
+			cc_extra=-ffile-prefix-map=${S}=/kernel-source/
+		fi
+		echo $cc_extra
+}
+
 kernel_do_compile() {
 	unset CFLAGS CPPFLAGS CXXFLAGS LDFLAGS MACHINE
 	# The $use_alternate_initrd is only set from
@@ -264,8 +276,9 @@ kernel_do_compile() {
 		copy_initramfs
 		use_alternate_initrd=CONFIG_INITRAMFS_SOURCE=${B}/usr/${INITRAMFS_IMAGE_NAME}.cpio
 	fi
+	cc_extra=$(get_cc_option)
 	for typeformake in ${KERNEL_IMAGETYPE_FOR_MAKE} ; do
-		oe_runmake ${typeformake} CC="${KERNEL_CC}" LD="${KERNEL_LD}" ${KERNEL_EXTRA_ARGS} $use_alternate_initrd
+		oe_runmake ${typeformake} CC="${KERNEL_CC} $cc_extra " LD="${KERNEL_LD}" ${KERNEL_EXTRA_ARGS} $use_alternate_initrd
 		for type in ${KERNEL_IMAGETYPES} ; do
 			if test "${typeformake}.gz" = "${type}"; then
 				mkdir -p "${KERNEL_OUTPUT_DIR}"
@@ -279,7 +292,8 @@ kernel_do_compile() {
 do_compile_kernelmodules() {
 	unset CFLAGS CPPFLAGS CXXFLAGS LDFLAGS MACHINE
 	if (grep -q -i -e '^CONFIG_MODULES=y$' ${B}/.config); then
-		oe_runmake -C ${B} ${PARALLEL_MAKE} modules CC="${KERNEL_CC}" LD="${KERNEL_LD}" ${KERNEL_EXTRA_ARGS}
+		cc_extra=$(get_cc_option)
+		oe_runmake -C ${B} ${PARALLEL_MAKE} modules CC="${KERNEL_CC} $cc_extra " LD="${KERNEL_LD}" ${KERNEL_EXTRA_ARGS}
 
 		# Module.symvers gets updated during the 
 		# building of the kernel modules. We need to
-- 
2.7.4