Re: how to create early build deps?

[Nicolas Schier <nicolas@fjasle.eu>]

Hi Kees,

On Wed, Apr 16, 2025 at 07:55:02PM -0700, Kees Cook wrote:
> Hi!
> 
> I am stumped...
> 
> I swear there was a time when changing the gcc-plugins would trigger a
> full rebuild of all kernel sources. I tried to bisect where that stopped
> happening, but it went back far enough that things stopped building with
> my compiler at all. ;)
> 
> Anyway, I need this also for the Clang randstruct seed file -- if it
> changes, we need to rebuild everything. I thought this worked back in
> v5.19 when I moved the randstruct seed generation into scripts/basic[1],
> but regenerating it doesn't trigger a rebuild (with v5.19 nor current
> Linus nor linux-next):
> 
> $ make O=clang-all LLVM=1 allmodconfig kernel/seccomp.o
> make[1]: Entering directory '/srv/code/clang-all'
> ...
>   GENSEED scripts/basic/randstruct.seed
> ...
>   CC      kernel/seccomp.o
> make[1]: Leaving directory '/srv/code/clang-all'
> 
> $ rm clang-all/scripts/basic/randstruct.seed
> 
> $ make O=clang-all LLVM=1 kernel/seccomp.o
> make[1]: Entering directory '/srv/code/clang-all'
>   GEN     Makefile
>   GENSEED scripts/basic/randstruct.seed
>   DESCEND objtool
>   CALL    ../scripts/checksyscalls.sh
>   INSTALL libsubcmd_headers
> make[1]: Leaving directory '/srv/code/clang-all'
> 
> kernel/seccomp.o doesn't get rebuilt :(
> 
> By what mechanism can I convince kbuild to rebuild everything if
> randstruct.seed (or the gcc-plugins) are changed?

I think the easiest way to achieve a full rebuild due to changes in
scripts/basic/randstruct.seed would be to let it show up in
.$(target).cmd.  Limited testing with a very hacky thing like

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 8c311b997e24..1aceedfe0791 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -206,4 +206,5 @@ if_changed_dep = $(if $(if-changed-cond),$(cmd_and_fixdep),@:)
 cmd_and_fixdep =                                                             \
        $(cmd);                                                              \
        $(objtree)/scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).cmd;\
+       sed -i -re 's,../include/linux/compiler.h,& $(objtree)/scripts/basic/randstruct.seed,' $(dot-target).cmd ;\
        rm -f $(depfile)

looks promising to me.  But as far as I can see, fixdep does not have
similar hard-coded dependencies included, yet.  Thus I am unsure if
fixdep is really the this we want to touch for randstruct.seed.

Kind regards,
Nicolas