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>

arch/powerpc/boot/wrapper

@@ -433,7 +433,7 @@ fi
 # Extract kernel version information, some platforms want to include
 # it in the image header
 version=`${CROSS}strings "$kernel" | grep '^Linux version [-0-9.]' | \
-    cut -d' ' -f3`
+    head -n1 | cut -d' ' -f3`
 if [ -n "$version" ]; then
     uboot_version="-n Linux-$version"
 fi

arch/s390/boot/version.c

 // SPDX-License-Identifier: GPL-2.0
+#include <generated/utsversion.h>
 #include <generated/utsrelease.h>
 #include <generated/compile.h>
 #include "boot.h"

arch/x86/boot/compressed/kaslr.c

@@ -29,6 +29,7 @@
 #include <linux/uts.h>
 #include <linux/utsname.h>
 #include <linux/ctype.h>
+#include <generated/utsversion.h>
 #include <generated/utsrelease.h>
 
 #define _SETUP

arch/x86/boot/version.c

@@ -11,6 +11,7 @@
  */
 
 #include "boot.h"
+#include <generated/utsversion.h>
 #include <generated/utsrelease.h>
 #include <generated/compile.h>
 

init/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
+/utsversion-tmp.h

init/Makefile

@@ -19,20 +19,49 @@ mounts-y			:= do_mounts.o
 mounts-$(CONFIG_BLK_DEV_RAM)	+= do_mounts_rd.o
 mounts-$(CONFIG_BLK_DEV_INITRD)	+= do_mounts_initrd.o
 
-# dependencies on generated files need to be listed explicitly
-$(obj)/version.o: include/generated/compile.h
+#
+# UTS_VERSION
+#
+
+smp-flag-$(CONFIG_SMP)			:= SMP
+preempt-flag-$(CONFIG_PREEMPT_BUILD)	:= PREEMPT
+preempt-flag-$(CONFIG_PREEMPT_DYNAMIC)	:= PREEMPT_DYNAMIC
+preempt-flag-$(CONFIG_PREEMPT_RT)	:= PREEMPT_RT
+
+build-version = $(or $(KBUILD_BUILD_VERSION), $(build-version-auto))
+build-timestamp = $(or $(KBUILD_BUILD_TIMESTAMP), $(build-timestamp-auto))
+
+# Maximum length of UTS_VERSION is 64 chars
+filechk_uts_version = \
+	utsver=$$(echo '$(pound)'"$(build-version)" $(smp-flag-y) $(preempt-flag-y) "$(build-timestamp)" | cut -b -64); \
+	echo '$(pound)'define UTS_VERSION \""$${utsver}"\"
+
+#
+# Build version.c with temporary UTS_VERSION
+#
 
-# compile.h changes depending on hostname, generation number, etc,
-# so we regenerate it always.
-# mkcompile_h will make sure to only update the
-# actual file if its content has changed.
+$(obj)/utsversion-tmp.h: FORCE
+	$(call filechk,uts_version)
 
-quiet_cmd_compile.h = CHK     $@
-      cmd_compile.h = \
-	$(CONFIG_SHELL) $(srctree)/scripts/mkcompile_h $@	\
-	"$(UTS_MACHINE)" "$(CONFIG_SMP)" "$(CONFIG_PREEMPT_BUILD)"	\
-	"$(CONFIG_PREEMPT_DYNAMIC)" "$(CONFIG_PREEMPT_RT)" \
-	"$(CONFIG_CC_VERSION_TEXT)" "$(LD)"
+clean-files += utsversion-tmp.h
+
+$(obj)/version.o: include/generated/compile.h $(obj)/utsversion-tmp.h
+CFLAGS_version.o := -include $(obj)/utsversion-tmp.h
+
+filechk_compile.h = $(srctree)/scripts/mkcompile_h \
+	"$(UTS_MACHINE)" "$(CONFIG_CC_VERSION_TEXT)" "$(LD)"
 
 include/generated/compile.h: FORCE
-	$(call cmd,compile.h)
+	$(call filechk,compile.h)
+
+#
+# Build version-timestamp.c with final UTS_VERSION
+#
+
+include/generated/utsversion.h: build-version-auto = $(shell $(srctree)/$(src)/build-version)
+include/generated/utsversion.h: build-timestamp-auto = $(shell LC_ALL=C date)
+include/generated/utsversion.h: FORCE
+	$(call filechk,uts_version)
+
+$(obj)/version-timestamp.o: include/generated/utsversion.h
+CFLAGS_version-timestamp.o := -include include/generated/utsversion.h

init/build-version

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-only
+
+prev_ver=$(cat .version 2>/dev/null) &&
+ver=$(expr ${prev_ver} + 1 2>/dev/null) ||
+ver=1
+
+echo ${ver} > .version
+
+echo ${ver}

init/version-timestamp.c

+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <generated/compile.h>
+#include <generated/utsrelease.h>
+#include <linux/version.h>
+#include <linux/proc_ns.h>
+#include <linux/refcount.h>
+#include <linux/uts.h>
+#include <linux/utsname.h>
+
+struct uts_namespace init_uts_ns = {
+	.ns.count = REFCOUNT_INIT(2),
+	.name = {
+		.sysname	= UTS_SYSNAME,
+		.nodename	= UTS_NODENAME,
+		.release	= UTS_RELEASE,
+		.version	= UTS_VERSION,
+		.machine	= UTS_MACHINE,
+		.domainname	= UTS_DOMAINNAME,
+	},
+	.user_ns = &init_user_ns,
+	.ns.inum = PROC_UTS_INIT_INO,
+#ifdef CONFIG_UTS_NS
+	.ns.ops = &utsns_operations,
+#endif
+};
+
+/* FIXED STRINGS! Don't touch! */
+const char linux_banner[] =
+	"Linux version " UTS_RELEASE " (" LINUX_COMPILE_BY "@"
+	LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION "\n";

init/version.c

@@ -18,24 +18,6 @@
 #include <generated/utsrelease.h>
 #include <linux/proc_ns.h>
 
-struct uts_namespace init_uts_ns = {
-	.ns.count = REFCOUNT_INIT(2),
-	.name = {
-		.sysname	= UTS_SYSNAME,
-		.nodename	= UTS_NODENAME,
-		.release	= UTS_RELEASE,
-		.version	= UTS_VERSION,
-		.machine	= UTS_MACHINE,
-		.domainname	= UTS_DOMAINNAME,
-	},
-	.user_ns = &init_user_ns,
-	.ns.inum = PROC_UTS_INIT_INO,
-#ifdef CONFIG_UTS_NS
-	.ns.ops = &utsns_operations,
-#endif
-};
-EXPORT_SYMBOL_GPL(init_uts_ns);
-
 static int __init early_hostname(char *arg)
 {
 	size_t bufsize = sizeof(init_uts_ns.name.nodename);
@@ -51,11 +33,6 @@ static int __init early_hostname(char *arg)
 }
 early_param("hostname", early_hostname);
 
-/* FIXED STRINGS! Don't touch! */
-const char linux_banner[] =
-	"Linux version " UTS_RELEASE " (" LINUX_COMPILE_BY "@"
-	LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION "\n";
-
 const char linux_proc_banner[] =
 	"%s version %s"
 	" (" LINUX_COMPILE_BY "@" LINUX_COMPILE_HOST ")"
@@ -63,3 +40,16 @@ const char linux_proc_banner[] =
 
 BUILD_SALT;
 BUILD_LTO_INFO;
+
+/*
+ * init_uts_ns and linux_banner contain the build version and timestamp,
+ * which are really fixed at the very last step of build process.
+ * They are compiled with __weak first, and without __weak later.
+ */
+
+struct uts_namespace init_uts_ns __weak;
+const char linux_banner[] __weak;
+
+#include "version-timestamp.c"
+
+EXPORT_SYMBOL_GPL(init_uts_ns);

kernel/gen_kheaders.sh

@@ -31,8 +31,8 @@ if [ "$building_out_of_srctree" ]; then
 fi
 all_dirs="$all_dirs $dir_list"
 
-# include/generated/compile.h is ignored because it is touched even when none
-# of the source files changed.
+# include/generated/utsversion.h is ignored because it is generated after this
+# script is executed. (utsversion.h is unneeded for kheaders)
 #
 # When Kconfig regenerates include/generated/autoconf.h, its timestamp is
 # updated, but the contents might be still the same. When any CONFIG option is
@@ -42,7 +42,7 @@ all_dirs="$all_dirs $dir_list"
 #
 # Ignore them for md5 calculation to avoid pointless regeneration.
 headers_md5="$(find $all_dirs -name "*.h"			|
-		grep -v "include/generated/compile.h"	|
+		grep -v "include/generated/utsversion.h"	|
 		grep -v "include/generated/autoconf.h"	|
 		xargs ls -l | md5sum | cut -d ' ' -f1)"
 

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

scripts/mkcompile_h

 #!/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