Re: [PATCH] kbuild/btf: Remove broken module relinking exclusion

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

On 4/10/26 3:13 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.
> 
> It is possible to fix this Makefile issue. However, having the %.ko rule
> update the resulting file in place without starting from the original
> inputs is rather fragile. The logic is harder to debug if something breaks
> during a subsequent .ko update because the old input is lost due to the
> overwrite. Additionally, it requires that the BTF processing is idempotent.
> For example, sorting id+flags BTF_SET8 pairs in .BTF_ids by resolve_btfids
> currently doesn't have this property.
> 
> One option is to split the %.ko target into two rules: the first for
> partial linking and the second one for generating the BTF data. However,
> this approach runs into an issue with requiring additional intermediate
> files, which increases the size of the build directory. On my system, when
> using a large distribution config with ~5500 modules, the size of the build
> directory with debuginfo enabled is already ~25 GB, with .ko files
> occupying ~8 GB. Duplicating these .ko files doesn't seem practical.
> 
> Measuring the speed of the %.ko processing shows that the link step is
> actually relatively fast. It takes about 20% of the overall rule time,
> while the BTF processing accounts for 80%. Moreover, skipping the link part
> becomes relevant only during local development. In such cases, developers
> typically use configs that enable a limited number of modules, so having
> the %.ko rule slightly slower doesn't significantly impact the total
> rebuild time. This is supported by the fact that no one has complained
> about this optimization being broken for the past two years.
> 
> Therefore, remove the logic that prevents module relinking when only
> vmlinux changes and simplify Makefile.modfinal.
> 
> Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
> ---
> My previous attempt to fix this logic can be found at
> https://lore.kernel.org/linux-modules/20260402141911.1577711-1-petr.pavlu@suse.com/
> ---
>  scripts/Makefile.modfinal | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> index adcbcde16a07..01a37ec872b9 100644
> --- a/scripts/Makefile.modfinal
> +++ b/scripts/Makefile.modfinal
> @@ -46,17 +46,9 @@ quiet_cmd_btf_ko = BTF [M] $@
>  		$(CONFIG_SHELL) $(srctree)/scripts/gen-btf.sh --btf_base $(objtree)/vmlinux $@; \
>  	fi;
>  
> -# Same as newer-prereqs, but allows to exclude specified extra dependencies
> -newer_prereqs_except = $(filter-out $(PHONY) $(1),$?)
> -
> -# Same as if_changed, but allows to exclude specified extra dependencies
> -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
>  %.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,ld_ko_o)
>  ifdef CONFIG_DEBUG_INFO_BTF_MODULES
>  	+$(if $(newer-prereqs),$(call cmd,btf_ko))
>  endif
> 
> base-commit: 591cd656a1bf5ea94a222af5ef2ee76df029c1d2

The Sashiko review system makes the following observation [1]:

> Will this result in missing BTF data if the module is rebuilt solely due
> to a command-line change?
> 
> Since Kbuild's if_changed executes when either a prerequisite is newer or
> the command line has changed, a command-line-only change (such as modifying
> LDFLAGS_MODULE) will trigger ld_ko_o. This overwrites the .ko file with a
> freshly linked object that lacks a BTF section.
> 
> However, because no prerequisite files actually changed, Make's $? variable
> is empty, meaning newer-prereqs is also empty. The subsequent check
> $(if $(newer-prereqs),$(call cmd,btf_ko)) would then evaluate to false and
> skip generating the BTF data entirely.
> 
> Could these commands be unified under a single custom Kbuild rule
> (e.g., rule_ld_ko_o) and invoked together using $(call if_changed_rule,ld_ko_o)
> to ensure both linking and BTF generation occur atomically?

I'm aware of this issue. However, it is a separate problem that has
already been present. I'd like to address the exclusion rule first.

[1] https://sashiko.dev/#/patchset/20260410131343.2519532-1-petr.pavlu%40suse.com

-- Petr