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, 31 insertions, 0 deletions

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";