kbuild: build init/built-in.a just once

Kbuild builds init/built-in.a twice; first during the ordinary
directory descending, second from scripts/link-vmlinux.sh.

We do this because UTS_VERSION contains the build version and the
timestamp. We cannot update it during the normal directory traversal
since we do not yet know if we need to update vmlinux. UTS_VERSION is
temporarily calculated, but omitted from the update check. Otherwise,
vmlinux would be rebuilt every time.

When Kbuild results in running link-vmlinux.sh, it increments the
version number in the .version file and takes the timestamp at that
time to really fix UTS_VERSION.

However, updating the same file twice is a footgun. To avoid nasty
timestamp issues, all build artifacts that depend on init/built-in.a
are atomically generated in link-vmlinux.sh, where some of them do not
need rebuilding.

To fix this issue, this commit changes as follows:

[1] Split UTS_VERSION out to include/generated/utsversion.h from
    include/generated/compile.h

    include/generated/utsversion.h is generated just before the
    vmlinux link. It is generated under include/generated/ because
    some decompressors (s390, x86) use UTS_VERSION.

[2] Split init_uts_ns and linux_banner out to init/version-timestamp.c
    from init/version.c

    init_uts_ns and linux_banner contain UTS_VERSION. During the ordinary
    directory descending, they are compiled with __weak and used to
    determine if vmlinux needs relinking. Just before the vmlinux link,
    they are compiled without __weak to embed the real version and
    timestamp.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

2 files changed, 15 insertions, 91 deletions

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index eecc1863e556..8d982574145a 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -75,6 +75,8 @@ vmlinux_link()
 		objs="${objs} .vmlinux.export.o"
 	fi
 
+	objs="${objs} init/version-timestamp.o"
+
 	if [ "${SRCARCH}" = "um" ]; then
 		wl=-Wl,
 		ld="${CC}"
@@ -213,19 +215,6 @@ if [ "$1" = "clean" ]; then
 	exit 0
 fi
 
-# Update version
-info GEN .version
-if [ -r .version ]; then
-	VERSION=$(expr 0$(cat .version) + 1)
-	echo $VERSION > .version
-else
-	rm -f .version
-	echo 1 > .version
-fi;
-
-# final build of init/
-${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init need-builtin=1
-
 #link vmlinux.o
 ${MAKE} -f "${srctree}/scripts/Makefile.vmlinux_o"
 
@@ -260,6 +249,8 @@ if is_enabled CONFIG_MODULES; then
 	${MAKE} -f "${srctree}/scripts/Makefile.vmlinux" .vmlinux.export.o
 fi
 
+${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init init/version-timestamp.o
+
 btf_vmlinux_bin_o=""
 if is_enabled CONFIG_DEBUG_INFO_BTF; then
 	btf_vmlinux_bin_o=.btf.vmlinux.bin.o
diff --git a/scripts/mkcompile_h b/scripts/mkcompile_h
index ca40a5258c87..f1a820d49e53 100755
--- a/scripts/mkcompile_h
+++ b/scripts/mkcompile_h
@@ -1,14 +1,9 @@
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
-TARGET=$1
-ARCH=$2
-SMP=$3
-PREEMPT=$4
-PREEMPT_DYNAMIC=$5
-PREEMPT_RT=$6
-CC_VERSION="$7"
-LD=$8
+UTS_MACHINE=$1
+CC_VERSION="$2"
+LD=$3
 
 # Do not expand names
 set -f
@@ -17,17 +12,6 @@ set -f
 LC_ALL=C
 export LC_ALL
 
-if [ -z "$KBUILD_BUILD_VERSION" ]; then
-	VERSION=$(cat .version 2>/dev/null || echo 1)
-else
-	VERSION=$KBUILD_BUILD_VERSION
-fi
-
-if [ -z "$KBUILD_BUILD_TIMESTAMP" ]; then
-	TIMESTAMP=`date`
-else
-	TIMESTAMP=$KBUILD_BUILD_TIMESTAMP
-fi
 if test -z "$KBUILD_BUILD_USER"; then
 	LINUX_COMPILE_BY=$(whoami | sed 's/\\/\\\\/')
 else
@@ -39,63 +23,12 @@ else
 	LINUX_COMPILE_HOST=$KBUILD_BUILD_HOST
 fi
 
-UTS_VERSION="#$VERSION"
-CONFIG_FLAGS=""
-if [ -n "$SMP" ] ; then CONFIG_FLAGS="SMP"; fi
-
-if [ -n "$PREEMPT_RT" ] ; then
-	CONFIG_FLAGS="$CONFIG_FLAGS PREEMPT_RT"
-elif [ -n "$PREEMPT_DYNAMIC" ] ; then
-	CONFIG_FLAGS="$CONFIG_FLAGS PREEMPT_DYNAMIC"
-elif [ -n "$PREEMPT" ] ; then
-	CONFIG_FLAGS="$CONFIG_FLAGS PREEMPT"
-fi
-
-# Truncate to maximum length
-UTS_LEN=64
-UTS_VERSION="$(echo $UTS_VERSION $CONFIG_FLAGS $TIMESTAMP | cut -b -$UTS_LEN)"
-
-# Generate a temporary compile.h
-
-{ echo /\* This file is auto generated, version $VERSION \*/
-  if [ -n "$CONFIG_FLAGS" ] ; then echo "/* $CONFIG_FLAGS */"; fi
+LD_VERSION=$($LD -v | head -n1 | sed 's/(compatible with [^)]*)//' \
+	      | sed 's/[[:space:]]*$//')
 
-  echo \#define UTS_MACHINE \"$ARCH\"
-
-  echo \#define UTS_VERSION \"$UTS_VERSION\"
-
-  printf '#define LINUX_COMPILE_BY "%s"\n' "$LINUX_COMPILE_BY"
-  echo \#define LINUX_COMPILE_HOST \"$LINUX_COMPILE_HOST\"
-
-  LD_VERSION=$($LD -v | head -n1 | sed 's/(compatible with [^)]*)//' \
-		      | sed 's/[[:space:]]*$//')
-  printf '#define LINUX_COMPILER "%s"\n' "$CC_VERSION, $LD_VERSION"
-} > .tmpcompile
-
-# Only replace the real compile.h if the new one is different,
-# in order to preserve the timestamp and avoid unnecessary
-# recompilations.
-# We don't consider the file changed if only the date/time changed,
-# unless KBUILD_BUILD_TIMESTAMP was explicitly set (e.g. for
-# reproducible builds with that value referring to a commit timestamp).
-# A kernel config change will increase the generation number, thus
-# causing compile.h to be updated (including date/time) due to the
-# changed comment in the
-# first line.
-
-if [ -z "$KBUILD_BUILD_TIMESTAMP" ]; then
-   IGNORE_PATTERN="UTS_VERSION"
-else
-   IGNORE_PATTERN="NOT_A_PATTERN_TO_BE_MATCHED"
-fi
-
-if [ -r $TARGET ] && \
-      grep -v $IGNORE_PATTERN $TARGET > .tmpver.1 && \
-      grep -v $IGNORE_PATTERN .tmpcompile > .tmpver.2 && \
-      cmp -s .tmpver.1 .tmpver.2; then
-   rm -f .tmpcompile
-else
-   echo "  UPD     $TARGET"
-   mv -f .tmpcompile $TARGET
-fi
-rm -f .tmpver.1 .tmpver.2
+cat <<EOF
+#define UTS_MACHINE		"${UTS_MACHINE}"
+#define LINUX_COMPILE_BY	"${LINUX_COMPILE_BY}"
+#define LINUX_COMPILE_HOST	"${LINUX_COMPILE_HOST}"
+#define LINUX_COMPILER		"${CC_VERSION}, ${LD_VERSION}"
+EOF