kernel.bbclass: improve reproducibility

Several tweaks to improve reproducibility:

1. If BUILD_REPRODUCIBLE_BINARIES == 1, set KBUILD_BUILD_TIMESTAMP
to a reproducible value. This is either a non-zero SOURCE_DATE_EPOCH, or the
value obtained from top entry of GIT repo, or (if there is no GIT repo)
fallback to REPRODUCIBLE_TIMESTAMP_ROOTFS as the last resort.
Also export KCONFIG_NOTIMESTAMP=1.

2. When compressing vmlinux.gz, use gzip "-n" option

3. 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/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.

(From OE-Core rev: 012a70da7ae0617740cd0cf807d01c3cd912c823)

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 36 insertions, 3 deletions

diff --git a/meta/classes/kernel.bbclass b/meta/classes/kernel.bbclass
index ce2cab65ae..2a765547ac 100644
--- a/meta/classes/kernel.bbclass
+++ b/meta/classes/kernel.bbclass
@@ -255,8 +255,39 @@ 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 $cc_extra
+}
+
 kernel_do_compile() {
         unset CFLAGS CPPFLAGS CXXFLAGS LDFLAGS MACHINE
+        if [ "$BUILD_REPRODUCIBLE_BINARIES" = "1" ]; then
+                # kernel sources do not use do_unpack, so SOURCE_DATE_EPOCH may not
+                # be set....
+                if [ "$SOURCE_DATE_EPOCH" = "0" ]; then
+                        olddir=`pwd`
+                        cd ${S}
+                        SOURCE_DATE_EPOCH=`git log  -1 --pretty=%ct`
+                        # git repo not guaranteed, so fall back to REPRODUCIBLE_TIMESTAMP_ROOTFS
+                        if [ $? -ne 0 ]; then
+                                SOURCE_DATE_EPOCH=${REPRODUCIBLE_TIMESTAMP_ROOTFS}
+                        fi
+                        cd $olddir
+                fi
+
+                ts=`LC_ALL=C date -d @$SOURCE_DATE_EPOCH`
+                export KBUILD_BUILD_TIMESTAMP="$ts"
+                export KCONFIG_NOTIMESTAMP=1
+                bbnote "KBUILD_BUILD_TIMESTAMP: $ts"
+        fi
         # The $use_alternate_initrd is only set from
         # do_bundle_initramfs() This variable is specifically for the
         # case where we are making a second pass at the kernel
@@ -270,20 +301,22 @@ kernel_do_compile() {
                 copy_initramfs
                 use_alternate_initrd=CONFIG_INITRAMFS_SOURCE=${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} $use_alternate_initrd
         done
         # vmlinux.gz is not built by kernel
         if (echo "${KERNEL_IMAGETYPES}" | grep -wq "vmlinux\.gz"); then
                 mkdir -p "${KERNEL_OUTPUT_DIR}"
-                gzip -9c < ${B}/vmlinux > "${KERNEL_OUTPUT_DIR}/vmlinux.gz"
+                gzip -9cn < ${B}/vmlinux > "${KERNEL_OUTPUT_DIR}/vmlinux.gz"
         fi
 }
 
 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