linux-kernel - [PATCH v3 3/9] kbuild: add stamp file for vmlinux BTF data

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Message-Id: <20250429-module-hashes-v3-3-00e9258def9e@weissschuh.net>
Date: Tue, 29 Apr 2025 15:04:30 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Masahiro Yamada <masahiroy@...nel.org>, 
 Nathan Chancellor <nathan@...nel.org>, 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>, Paul Moore <paul@...l-moore.com>, 
 James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, 
 Jonathan Corbet <corbet@....net>, Madhavan Srinivasan <maddy@...ux.ibm.com>, 
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
 Christophe Leroy <christophe.leroy@...roup.eu>, 
 Naveen N Rao <naveen@...nel.org>, Mimi Zohar <zohar@...ux.ibm.com>, 
 Roberto Sassu <roberto.sassu@...wei.com>, 
 Dmitry Kasatkin <dmitry.kasatkin@...il.com>, 
 Eric Snowberg <eric.snowberg@...cle.com>, 
 Nicolas Schier <nicolas.schier@...ux.dev>, 
 Nicolas Schier <nicolas.schier@...ux.dev>
Cc: Fabian Grünbichler <f.gruenbichler@...xmox.com>, 
 Arnout Engelen <arnout@...t.net>, Mattia Rizzolo <mattia@...reri.org>, 
 kpcyrd <kpcyrd@...hlinux.org>, Christian Heusel <christian@...sel.eu>, 
 Câju Mihai-Drosi <mcaju95@...il.com>, 
 linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-arch@...r.kernel.org, linux-modules@...r.kernel.org, 
 linux-security-module@...r.kernel.org, linux-doc@...r.kernel.org, 
 linuxppc-dev@...ts.ozlabs.org, linux-integrity@...r.kernel.org, 
 Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH v3 3/9] 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 51367c2bfc21ef9a8ebbc30670b1edd220b571a3..5f060787ce3fbcbcfdca0c95789d619e2a1c7b72 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -113,6 +113,7 @@ 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} ${1}
@@ -133,6 +134,11 @@ gen_btf()
 	fi
 	printf "${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}
 }

-- 
2.49.0