[PATCH 5.12 335/384] kbuild: update config_data.gz only when the content of .config is changed

From: Greg Kroah-Hartman
Date: Mon May 10 2021 - 08:41:18 EST

From: Masahiro Yamada <masahiroy@xxxxxxxxxx>

commit 46b41d5dd8019b264717978c39c43313a524d033 upstream.

If the timestamp of the .config file is updated, config_data.gz is
regenerated, then vmlinux is re-linked. This occurs even if the content
of the .config has not changed at all.

This issue was mitigated by commit 67424f61f813 ("kconfig: do not write
.config if the content is the same"); Kconfig does not update the
.config when it ends up with the identical configuration.

The issue is remaining when the .config is created by *_defconfig with
some config fragment(s) applied on top.

This is typical for powerpc and mips, where several *_defconfig targets
are constructed by using merge_config.sh.

One workaround is to have the copy of the .config. The filechk rule
updates the copy, kernel/config_data, by checking the content instead
of the timestamp.

With this commit, the second run with the same configuration avoids
the needless rebuilds.

$ make ARCH=mips defconfig all
[ snip ]
$ make ARCH=mips defconfig all
*** Default configuration is based on target '32r2el_defconfig'
Using ./arch/mips/configs/generic_defconfig as base
Merging arch/mips/configs/generic/32r2.config
Merging arch/mips/configs/generic/el.config
Merging ./arch/mips/configs/generic/board-boston.config
Merging ./arch/mips/configs/generic/board-ni169445.config
Merging ./arch/mips/configs/generic/board-ocelot.config
Merging ./arch/mips/configs/generic/board-ranchu.config
Merging ./arch/mips/configs/generic/board-sead-3.config
Merging ./arch/mips/configs/generic/board-xilfpga.config
# configuration written to .config
SYNC include/config/auto.conf
CALL scripts/checksyscalls.sh
CALL scripts/atomic/check-atomics.sh
CHK include/generated/compile.h
CHK include/generated/autoksyms.h

Reported-by: Elliot Berman <eberman@xxxxxxxxxxxxxx>
Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
kernel/.gitignore | 1 +
kernel/Makefile | 9 +++++++--
2 files changed, 8 insertions(+), 2 deletions(-)

--- a/kernel/.gitignore
+++ b/kernel/.gitignore
@@ -1,4 +1,5 @@
# SPDX-License-Identifier: GPL-2.0-only
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -138,10 +138,15 @@ obj-$(CONFIG_SCF_TORTURE_TEST) += scftor

$(obj)/configs.o: $(obj)/config_data.gz

-targets += config_data.gz
-$(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE
+targets += config_data config_data.gz
+$(obj)/config_data.gz: $(obj)/config_data FORCE
$(call if_changed,gzip)

+filechk_cat = cat $<
+$(obj)/config_data: $(KCONFIG_CONFIG) FORCE
+ $(call filechk,cat)
$(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz

quiet_cmd_genikh = CHK $(obj)/kheaders_data.tar.xz