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kbuild: build init/built-in.a just once

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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>
This commit is contained in:
Masahiro Yamada
2022-08-28 11:39:53 +09:00
parent 561daaacb4
commit 2df8220cc5
12 changed files with 120 additions and 131 deletions
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17
scripts/link-vmlinux.sh
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@@ -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