Re: [PATCH 3/3] kbuild: move scripts/mod target out of single-build

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] kbuild: unordered modpost_prepare and stack_protector_prepare
  mutates CFLAGS
--

commit 08ffc13380249741520c2ea434688d701b5d0e9b
Author: Vincent Mailhol <mailhol@kernel.org>

kbuild: move scripts/mod target out of single-build

This commit moves the scripts/mod build out of prepare0 into a new
modpost_prepare target depended on by modules_prepare and modpost.

> diff --git a/Makefile b/Makefile
> index 2c1289a44c560..93236f310f080 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1405,7 +1405,7 @@ scripts: scripts_basic scripts_dtc
>  # archprepare is used in arch Makefiles and when processed asm symlink,
>  # version.h and scripts_basic is processed / created.
>  
> -PHONY += prepare archprepare
> +PHONY += prepare archprepare modpost_prepare
>  
>  archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
>  	asm-generic $(version_h) include/generated/utsrelease.h \
> @@ -1414,6 +1414,8 @@ archprepare: outputmakefile archheaders archscripts scripts include/config/kerne
>  
>  prepare0: archprepare
>  	$(Q)$(MAKE) $(build)=. prepare $(if $(single-build),,prepare_checks)
> +
> +modpost_prepare: archprepare
>  	$(Q)$(MAKE) $(build)=scripts/mod

[Severity: Low]
Should modpost_prepare be ordered relative to stack_protector_prepare (or
depend on prepare)?

On arm, arm64, powerpc, and riscv, prepare depends on
stack_protector_prepare, which depends on prepare0 and mutates the
exported KBUILD_CFLAGS variable at recipe execution time:

arch/arm64/Makefile:
prepare: stack_protector_prepare
stack_protector_prepare: prepare0
	$(eval KBUILD_CFLAGS += -mstack-protector-guard=sysreg		  \
				-mstack-protector-guard-reg=sp_el0	  \
				-mstack-protector-guard-offset=$(shell	  \
			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
				$(objtree)/include/generated/asm-offsets.h))

Previously, scripts/mod was built inside prepare0, so it always ran before
stack_protector_prepare modified KBUILD_CFLAGS.  With modpost_prepare
only depending on archprepare, a parallel build runs modpost_prepare
concurrently with prepare0 before stack_protector_prepare updates
KBUILD_CFLAGS, whereas a serial build or make modules_prepare runs
prepare first and passes the updated KBUILD_CFLAGS to scripts/mod.

Because scripts/mod/empty.o is compiled with KBUILD_CFLAGS, switching
between parallel and serial or single-module builds changes its recorded
command line, rebuilding empty.o, regenerating elfconfig.h (which uses
if_changed), and forcing a full rebuild of modpost.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111-reduce-single-build-dependencies-v1-0-7196ca986a85@kernel.org?part=3