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

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Message-ID: <202505021555.A74E678976@keescook>
Date: Fri, 2 May 2025 15:57:13 -0700
From: Kees Cook <kees@...nel.org>
To: Nathan Chancellor <nathan@...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

On Fri, May 02, 2025 at 09:12:09AM -0700, Nathan Chancellor wrote:
> 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.

Thanks! Yeah, I have been poking at it for most of today. 😭

I think I have a viable solution, that I actually end up liking a bit
better, which I just sent out:
https://lore.kernel.org/lkml/20250502224512.it.706-kees@kernel.org/

-- 
Kees Cook