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>

5 files changed, 98 insertions, 36 deletions

diff --git a/init/.gitignore b/init/.gitignore
new file mode 100644
index 000000000000..cbbe270cec00
--- /dev/null
+++ b/init/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/utsversion-tmp.h
diff --git a/init/Makefile b/init/Makefile
index d82623d7fc8e..ba90eb817185 100644
--- a/init/Makefile
+++ b/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
diff --git a/init/build-version b/init/build-version
new file mode 100755
index 000000000000..537d45815083
--- /dev/null
+++ b/init/build-version
@@ -0,0 +1,10 @@
+#!/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}
diff --git a/init/version-timestamp.c b/init/version-timestamp.c
new file mode 100644
index 000000000000..179e93bae539
--- /dev/null
+++ b/init/version-timestamp.c
@@ -0,0 +1,31 @@
+// 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";
diff --git a/init/version.c b/init/version.c
index 3391c4051bf3..01d4ab05f0ba 100644
--- a/init/version.c
+++ b/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);