[PATCH bpf-next v4 5/8] kbuild: Sync kconfig when PAHOLE_VERSION changes

[Ihor Solodrai <ihor.solodrai@linux.dev>]

This patch implements kconfig re-sync when the pahole version changes
between builds, similar to how it happens for compiler version change
via CC_VERSION_TEXT.

Define PAHOLE_VERSION in the top-level Makefile and export it for
config builds. Set CONFIG_PAHOLE_VERSION default to the exported
variable.

Kconfig records the PAHOLE_VERSION value in
include/config/auto.conf.cmd [1].

The Makefile includes auto.conf.cmd, so if PAHOLE_VERSION changes
between builds, make detects a dependency change and triggers
syncconfig to update the kconfig [2].

For external module builds, add a warning message in the prepare
target, similar to the existing compiler version mismatch warning.

Note that if pahole is not installed or available, PAHOLE_VERSION is
set to 0 by pahole-version.sh, so the (un)installation of pahole is
treated as a version change.

See previous discussions for context [3].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/kconfig/preprocess.c?h=v6.18#n91
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile?h=v6.18#n815
[3] https://lore.kernel.org/bpf/8f946abf-dd88-4fac-8bb4-84fcd8d81cf0@oracle.com/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 Makefile     | 9 ++++++++-
 init/Kconfig | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index e404e4767944..9b90a2a2218e 100644
--- a/Makefile
+++ b/Makefile
@@ -713,6 +713,7 @@ endif
 # upgrade.
 CC_VERSION_TEXT = $(subst $(pound),,$(shell LC_ALL=C $(CC) --version 2>/dev/null | head -n 1))
 RUSTC_VERSION_TEXT = $(subst $(pound),,$(shell $(RUSTC) --version 2>/dev/null))
+PAHOLE_VERSION = $(shell $(srctree)/scripts/pahole-version.sh $(PAHOLE))
 
 ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
 include $(srctree)/scripts/Makefile.clang
@@ -733,7 +734,7 @@ ifdef config-build
 # KBUILD_DEFCONFIG may point out an alternative default configuration
 # used for 'make defconfig'
 include $(srctree)/arch/$(SRCARCH)/Makefile
-export KBUILD_DEFCONFIG KBUILD_KCONFIG CC_VERSION_TEXT RUSTC_VERSION_TEXT
+export KBUILD_DEFCONFIG KBUILD_KCONFIG CC_VERSION_TEXT RUSTC_VERSION_TEXT PAHOLE_VERSION
 
 config: outputmakefile scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
@@ -1921,12 +1922,18 @@ clean: private rm-files := Module.symvers modules.nsdeps compile_commands.json
 PHONY += prepare
 # now expand this into a simple variable to reduce the cost of shell evaluations
 prepare: CC_VERSION_TEXT := $(CC_VERSION_TEXT)
+prepare: PAHOLE_VERSION := $(PAHOLE_VERSION)
 prepare:
 	@if [ "$(CC_VERSION_TEXT)" != "$(CONFIG_CC_VERSION_TEXT)" ]; then \
 		echo >&2 "warning: the compiler differs from the one used to build the kernel"; \
 		echo >&2 "  The kernel was built by: $(CONFIG_CC_VERSION_TEXT)"; \
 		echo >&2 "  You are using:           $(CC_VERSION_TEXT)"; \
 	fi
+	@if [ "$(PAHOLE_VERSION)" != "$(CONFIG_PAHOLE_VERSION)" ]; then \
+		echo >&2 "warning: pahole version differs from the one used to build the kernel"; \
+		echo >&2 "  The kernel was built with: $(CONFIG_PAHOLE_VERSION)"; \
+		echo >&2 "  You are using:             $(PAHOLE_VERSION)"; \
+	fi
 
 PHONY += help
 help:
diff --git a/init/Kconfig b/init/Kconfig
index fa79feb8fe57..317f3c0b13ad 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -171,7 +171,7 @@ config RUSTC_HAS_FILE_AS_C_STR
 
 config PAHOLE_VERSION
 	int
-	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
+	default "$(PAHOLE_VERSION)"
 
 config CONSTRUCTORS
 	bool
-- 
2.52.0