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, 4 insertions, 13 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