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

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

[Petr Pavlu]

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

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

[Petr Pavlu]

On 4/2/26 4:17 PM, Petr Pavlu wrote:
> 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.

I noticed that sorting id+flags in BTF_SET8 by resolve_btfids doesn't
seem to be idempotent, so this requires additional work.

-- Petr

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

[Petr Pavlu]

On 4/7/26 1:30 PM, Petr Pavlu wrote:
> On 4/2/26 4:17 PM, Petr Pavlu wrote:
>> 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.
> 
> I noticed that sorting id+flags in BTF_SET8 by resolve_btfids doesn't
> seem to be idempotent, so this requires additional work.

It is possible to make sorting id+flags in BTF_SET8 by resolve_btfids
idempotent. One approach would be to also update the offsets (st_value)
of the __BTF_ID__* symbols so that they reflect the result after
sorting.

However, I don't think this is worth doing. Since this logic would be
relevant in specific cases when vmlinux changes and only the BTF data
needs to be regenerated, it would have limited usage and testing.
Importantly, always relinking the modules adds only about 6% to the
rebuild time of the modules target on my system when vmlinux is touched.
The work required for BTF and Makefile processing dominates this target.
When developing the kernel locally, it's common to use a custom config
with a limited amount of modules. In such a case, avoiding the relinking
of modules saves very little.

I plan to instead send a patch to replace the current condition that
invokes ld_ko_o from if_changed_except to if_changed, and remove the
if_changed_except logic.

-- Petr