[PATCH] kbuild/btf: Avoid relinking modules when only vmlinux changes

[Petr Pavlu <petr.pavlu@suse.com>]

Commit 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF is enabled
and pahole supports it") in 2020 introduced CONFIG_DEBUG_INFO_BTF_MODULES
to enable generation of split BTF for kernel modules. This change required
the %.ko Makefile rule to additionally depend on vmlinux, which is used as
a base for deduplication. The regular ld_ko_o command executed by the rule
was then modified to be skipped if only vmlinux changes. This was done by
introducing a new if_changed_except command and updating the original call
to '+$(call if_changed_except,ld_ko_o,vmlinux)'.

Later, commit 214c0eea43b2 ("kbuild: add $(objtree)/ prefix to some
in-kernel build artifacts") in 2024 updated the rule's reference to vmlinux
from 'vmlinux' to '$(objtree)/vmlinux'. This accidentally broke the
previous logic to skip relinking modules if only vmlinux changes. The issue
is that '$(objtree)' is typically '.' and GNU Make normalizes the resulting
prerequisite './vmlinux' to just 'vmlinux', while the exclusion logic
retains the raw './vmlinux'. As a result, if_changed_except doesn't
correctly filter out vmlinux. Consequently, with
CONFIG_DEBUG_INFO_BTF_MODULES=y, modules are relinked even if only vmlinux
changes.

Additionally, commit 522397d05e7d ("resolve_btfids: Change in-place update
with raw binary output") in 2025 reworked the method for patching BTF data
into the resulting modules by using 'objcopy --add-section'. This command
fails if a section already exists.

Fix the unnecessary relinking issue by also excluding the normalized form
'vmlinux' when invoking ld_ko_o. Adjust embed_btf_data() to first use the
--remove-section option to remove the patched BTF section if it is already
present.

Fixes: 214c0eea43b2 ("kbuild: add $(objtree)/ prefix to some in-kernel build artifacts")
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
---
 scripts/Makefile.modfinal | 14 ++++++++++++--
 scripts/gen-btf.sh        |  6 ++++--
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index adcbcde16a07..2981745a172f 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -54,9 +54,19 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check),      \
 	$(cmd);                                                              \
 	printf '%s\n' 'savedcmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
 
-# Re-generate module BTFs if either module's .ko or vmlinux changed
+# Build final module objects.
+#
+# The *.ko files are usually independent of vmlinux. However, special handling
+# is required for module BTFs, which need to be regenerated if either the
+# module's .ko file or vmlinux has changed, as vmlinux is used as a base for
+# deduplication. Consequently, vmlinux is included in the rule prerequisites.
+#
+# The regular ld_ko_o call is conditional and is skipped if only vmlinux has
+# changed. The exclusion pattern used in the if_changed_except call contains
+# '$(objtree)/vmlinux' to match the exact prerequisite and plain 'vmlinux' to
+# cover the case when make normalizes './vmlinux' to 'vmlinux'.
 %.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,$(objtree)/vmlinux)
+	+$(call if_changed_except,ld_ko_o,$(objtree)/vmlinux vmlinux)
 ifdef CONFIG_DEBUG_INFO_BTF_MODULES
 	+$(if $(newer-prereqs),$(call cmd,btf_ko))
 endif
diff --git a/scripts/gen-btf.sh b/scripts/gen-btf.sh
index 8ca96eb10a69..6d4e629c79ca 100755
--- a/scripts/gen-btf.sh
+++ b/scripts/gen-btf.sh
@@ -104,12 +104,14 @@ gen_btf_o()
 
 embed_btf_data()
 {
-	${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF ${ELF_FILE}
+	${OBJCOPY} --remove-section .BTF \
+		--add-section .BTF=${ELF_FILE}.BTF ${ELF_FILE}
 
 	# a module might not have a .BTF_ids or .BTF.base section
 	btf_base="${ELF_FILE}.BTF.base"
 	if [ -f "${btf_base}" ]; then
-		${OBJCOPY} --add-section .BTF.base=${btf_base} ${ELF_FILE}
+		${OBJCOPY} --remove-section .BTF.base \
+			--add-section .BTF.base=${btf_base} ${ELF_FILE}
 	fi
 	btf_ids="${ELF_FILE}.BTF_ids"
 	if [ -f "${btf_ids}" ]; then

base-commit: 7aaa8047eafd0bd628065b15757d9b48c5f9c07d
-- 
2.53.0