linux-kernel - [PATCH RFC 1/2] kbuild: add stamp file for vmlinux BTF data

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Message-Id: <20241225-module-hashes-v1-1-d710ce7a3fd1@weissschuh.net>
Date: Wed, 25 Dec 2024 23:51:59 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Masahiro Yamada <masahiroy@...nel.org>, 
 Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
 Arnd Bergmann <arnd@...db.de>, Luis Chamberlain <mcgrof@...nel.org>, 
 Petr Pavlu <petr.pavlu@...e.com>, Sami Tolvanen <samitolvanen@...gle.com>, 
 Daniel Gomez <da.gomez@...sung.com>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-arch@...r.kernel.org, linux-modules@...r.kernel.org, 
 Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH RFC 1/2] kbuild: add stamp file for vmlinux BTF data

The upcoming module hashes functionality will build the modules in
between the generation of the BTF data and the final link of vmlinux.
Having a dependency from the modules on vmlinux would make this
impossible as it would mean having a cyclic dependency.
Break this cyclic dependency by introducing a new target.

Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
---
 scripts/Makefile.modfinal | 4 ++--
 scripts/link-vmlinux.sh   | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 542ba462ed3ec9607e0df10e26613a4c7ac318e8..5d01b553ec9a4565c8e5a6edd05665c409003bc1 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -52,8 +52,8 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check),      \
 	printf '%s\n' 'savedcmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
 
 # Re-generate module BTFs if either module's .ko or vmlinux changed
-%.ko: %.o %.mod.o .module-common.o $(objtree)/scripts/module.lds $(and $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),$(objtree)/vmlinux) FORCE
-	+$(call if_changed_except,ld_ko_o,$(objtree)/vmlinux)
+%.ko: %.o %.mod.o .module-common.o $(objtree)/scripts/module.lds $(and $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),$(objtree)/.tmp_vmlinux_btf.stamp) FORCE
+	+$(call if_changed_except,ld_ko_o,$(objtree)/.tmp_vmlinux_btf.stamp)
 ifdef CONFIG_DEBUG_INFO_BTF_MODULES
 	+$(if $(newer-prereqs),$(call cmd,btf_ko))
 endif
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index d853ddb3b28c1238ec9079ebbbe77df26980a0a1..803c8d6f35a7f29fb68b29afa8546f4dde0bd4cb 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -112,6 +112,7 @@ vmlinux_link()
 gen_btf()
 {
 	local btf_data=${1}.btf.o
+	local btf_stamp=.tmp_vmlinux_btf.stamp
 
 	info BTF "${btf_data}"
 	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}
@@ -132,6 +133,11 @@ gen_btf()
 	fi
 	printf "${et_rel}" | dd of="${btf_data}" conv=notrunc bs=1 seek=16 status=none
 
+	info STAMP $btf_stamp
+	if ! cmp --silent $btf_data $btf_stamp; then
+		cp $btf_data $btf_stamp
+	fi
+
 	btf_vmlinux_bin_o=${btf_data}
 }
 

-- 
2.47.1