linux-kernel - Re: [PATCH 2/3] randstruct: Force full rebuild when seed changes

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Message-ID: <20250502161209.GA2850065@ax162>
Date: Fri, 2 May 2025 09:12:09 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Petr Pavlu <petr.pavlu@...e.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kbuild@...r.kernel.org, Justin Stitt <justinstitt@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	kasan-dev@...glegroups.com
Subject: Re: [PATCH 2/3] randstruct: Force full rebuild when seed changes

Hi Kees,

On Thu, May 01, 2025 at 12:48:17PM -0700, Kees Cook wrote:
> While the randstruct GCC plugin was being rebuilt if the randstruct
> seed changed, Clangs build did not notice the change. Include the hash
> header directly so that it becomes a universal build dependency and full
> rebuilds will happen if it changes.
> 
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nicolas Schier <nicolas.schier@...ux.dev>
> Cc: Petr Pavlu <petr.pavlu@...e.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: <linux-kbuild@...r.kernel.org>
> ---
>  include/linux/vermagic.h    |  1 -
>  scripts/Makefile.randstruct |  3 ++-
>  scripts/basic/Makefile      | 11 ++++++-----
>  3 files changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/vermagic.h b/include/linux/vermagic.h
> index 939ceabcaf06..335c360d4f9b 100644
> --- a/include/linux/vermagic.h
> +++ b/include/linux/vermagic.h
> @@ -33,7 +33,6 @@
>  #define MODULE_VERMAGIC_MODVERSIONS ""
>  #endif
>  #ifdef RANDSTRUCT
> -#include <generated/randstruct_hash.h>
>  #define MODULE_RANDSTRUCT "RANDSTRUCT_" RANDSTRUCT_HASHED_SEED
>  #else
>  #define MODULE_RANDSTRUCT
> diff --git a/scripts/Makefile.randstruct b/scripts/Makefile.randstruct
> index 24e283e89893..ab87219c6149 100644
> --- a/scripts/Makefile.randstruct
> +++ b/scripts/Makefile.randstruct
> @@ -12,6 +12,7 @@ randstruct-cflags-y	\
>  	+= -frandomize-layout-seed-file=$(objtree)/scripts/basic/randstruct.seed
>  endif
>  
> -export RANDSTRUCT_CFLAGS := $(randstruct-cflags-y)
> +export RANDSTRUCT_CFLAGS := $(randstruct-cflags-y) \
> +			    -include $(objtree)/scripts/basic/randstruct_hash.h

As the kernel test robot points out (on a report that you weren't
included on for some reason...), this breaks the build in several
places on next-20250502.

https://lore.kernel.org/202505021409.yC9C70lH-lkp@intel.com/

  $ make -skj"$(nproc)" ARCH=arm LLVM=1 clean allmodconfig arch/arm/vdso/vgettimeofday.o
  clang: error: cannot specify -o when generating multiple output files

There are places in the kernel that filter out RANDSTRUCT_CFLAGS and
this appears to cause other '-include' flags to be filtered out as well,
such as the one in the efistub that includes hidden.h.

>  KBUILD_CFLAGS	+= $(RANDSTRUCT_CFLAGS)
> diff --git a/scripts/basic/Makefile b/scripts/basic/Makefile
> index dd289a6725ac..31637ce4dc5c 100644
> --- a/scripts/basic/Makefile
> +++ b/scripts/basic/Makefile
> @@ -8,9 +8,10 @@ hostprogs-always-y	+= fixdep
>  # before running a Clang kernel build.
>  gen-randstruct-seed	:= $(srctree)/scripts/gen-randstruct-seed.sh
>  quiet_cmd_create_randstruct_seed = GENSEED $@
> -cmd_create_randstruct_seed = \
> -	$(CONFIG_SHELL) $(gen-randstruct-seed) \
> -		$@ $(objtree)/include/generated/randstruct_hash.h
> -$(obj)/randstruct.seed: $(gen-randstruct-seed) FORCE
> +      cmd_create_randstruct_seed = $(CONFIG_SHELL) $(gen-randstruct-seed) \
> +		$(obj)/randstruct.seed $(obj)/randstruct_hash.h
> +
> +$(obj)/randstruct_hash.h $(obj)/randstruct.seed: $(gen-randstruct-seed) FORCE
>  	$(call if_changed,create_randstruct_seed)
> -always-$(CONFIG_RANDSTRUCT) += randstruct.seed
> +
> +always-$(CONFIG_RANDSTRUCT) += randstruct.seed randstruct_hash.h
> -- 
> 2.34.1
>