linux-hardening - Re: [RFC PATCH 04/11] kbuild: don't execute .ko recipe in --dry-run mode

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Message-ID: <ZyaUwuYIGOhZ58lS@fjasle.eu>
Date: Sat, 2 Nov 2024 22:08:18 +0100
From: Nicolas Schier <nicolas@...sle.eu>
To: Vegard Nossum <vegard.nossum@...cle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, linux-kbuild@...r.kernel.org,
 Nathan Chancellor <nathan@...nel.org>,
 Michael Ellerman <mpe@...erman.id.au>,
 Morten Linderud <morten@...derud.pw>,
 Haelwenn Monnier <contact@...odan.eu>, Jann Horn <jannh@...gle.com>,
 Kees Cook <kees@...nel.org>,
 James Bottomley <James.Bottomley@...senpartnership.com>,
 Theodore Ts'o <tytso@....edu>, linux-hardening@...r.kernel.org,
 Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>
Subject: Re: [RFC PATCH 04/11] kbuild: don't execute .ko recipe in --dry-run
 mode

On Mon, Aug 19, 2024 at 06:03:01PM +0200 Vegard Nossum wrote:
> Prefixing a line in a make recipe with + makes that command execute even
> in --dry-run mode. We don't need that here; remove it.

In case somebody else also wants to know why make behaves that way:
https://www.gnu.org/software/make/manual/make.html#MAKE-Variable

> 
> Fixes: 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF is enabled and pahole supports it")
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
> ---
>  scripts/Makefile.modfinal | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> index 696888f0a0bde..2679304f158ad 100644
> --- a/scripts/Makefile.modfinal
> +++ b/scripts/Makefile.modfinal
> @@ -60,9 +60,9 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check),      \
>  
>  # Re-generate module BTFs if either module's .ko or vmlinux changed
>  %.ko: %.o %.mod.o scripts/module.lds $(and $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),vmlinux) FORCE
> -	+$(call if_changed_except,ld_ko_o,vmlinux)
> +	$(call if_changed_except,ld_ko_o,vmlinux)
>  ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> -	+$(if $(newer-prereqs),$(call cmd,btf_ko))
> +	$(if $(newer-prereqs),$(call cmd,btf_ko))

I assumed that some ld might evaluate the jobserver variables (MAKEFLAGS), but
following the discussion in [1], this does not seem to be relevant.

Can you at a note to the commit message, that there is no need to run $(LD) or
$(PAHOLE) in parallel here?

[1]: https://lore.kernel.org/all/20220616104541.16289-1-jslaby@suse.cz/#t