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>

1 files changed, 13 insertions, 23 deletions

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);