[PATCH v4 06/17] kbuild: add stamp file for vmlinux BTF data

[Thomas Weißschuh]

On 2026-03-10 14:36:06-0700, Eric Biggers wrote:
> On Tue, Jan 13, 2026 at 01:28:50PM +0100, Thomas Weißschuh wrote:
> > 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 at weissschuh.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 149e12ff5700..adfef1e002a9 100644
> > --- a/scripts/Makefile.modfinal
> > +++ b/scripts/Makefile.modfinal
> > @@ -56,8 +56,8 @@ 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 4ab44c73da4d..8c98f8645a5c 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -111,6 +111,7 @@ 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} ${1}
> > @@ -131,6 +132,11 @@ gen_btf()
> >  	fi
> >  	printf "${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

This patch will be gone from the next revision of the series.
Making use of the recently introduced vmlinux.unstripped,
as suggested by Petr, removes any modifications to link-vmlinux.sh
and the issue that this patch tried to address.

> A "stamp file" is traditionally an empty file that is written when some
> build step has completed.  The above code is instead copying the entire
> .tmp_vmlinux1.btf.o file (megabytes in size) to .tmp_vmlinux_btf.stamp.

The goal here was not to only have a reference timestamp, but
specifically the reference file contents.

Note: The duplicated vmlinux.unstripped in its current form is by far
larger than .tmp_vmlinux1.btf.o.

> So, it's not clear to me why the stamp file is needed at all, versus
> depending directly on .tmp_vmlinux1.btf.o.
>
> I guess 'make' doesn't know about the dependencies of
> .tmp_vmlinux1.btf.o.  But the same is true of the stamp file, right?  So
> either way, how would 'make' know to finish rebuilding the file before
> starting to execute the "Re-generate module BTFs" rule?

The problem was not the ordering, this is handled within link-vmlinux.sh.
IIRC originally without this patch even no-op rebuilds would end up
rebuilding the modules. Using .tmp_vmlinux1.btf.o may have worked too.
But in v4, the patch "kbuild: generate module BTF based on
vmlinux.unstripped" was added, which also solves this problem.

> Also, passing the long option '--silent' to 'cmp' creates a dependency
> on the GNU implementation of 'cmp', which isn't documented as a kernel
> build dependency.  Probably better to use the short option '-s'.

Ack.

> Also, the stamp file isn't being deleted by 'make clean'.  It looks like
> it would need to be added to cleanup() in link-vmlinux.sh.

Ack.


Thomas