linux-hardening - Re: [PATCH v2] kbuild: clang: Support building UM with SUBARCH=i386

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Message-ID: <20250304162845.GA1479759@ax162>
Date: Tue, 4 Mar 2025 17:28:45 +0100
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Thomas Weißschuh <linux@...ssschuh.net>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>, llvm@...ts.linux.dev,
	linux-kbuild@...r.kernel.org, David Gow <davidgow@...gle.com>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] kbuild: clang: Support building UM with SUBARCH=i386

On Tue, Mar 04, 2025 at 08:21:29AM -0800, Kees Cook wrote:
> The UM builds distinguish i386 from x86_64 via SUBARCH, but we don't
> support building i386 directly with Clang. To make SUBARCH work for
> i386 UM, we need to explicitly test for it.
> 
> This lets me run i386 KUnit tests with Clang:
> 
> $ ./tools/testing/kunit/kunit.py run \
> 	--make_options LLVM=1 \
> 	--make_options SUBARCH=i386
> ...
> 
> Fixes: c7500c1b53bf ("um: Allow builds with Clang")
> Signed-off-by: Kees Cook <kees@...nel.org>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Thomas Weißschuh <linux@...ssschuh.net>
> Cc: Bill Wendling <morbo@...gle.com>
> Cc: Justin Stitt <justinstitt@...gle.com>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Nicolas Schier <nicolas@...sle.eu>
> Cc: llvm@...ts.linux.dev
> Cc: linux-kbuild@...r.kernel.org
> ---
>  scripts/Makefile.clang | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 2435efae67f5..b67636b28c35 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -12,6 +12,8 @@ CLANG_TARGET_FLAGS_riscv	:= riscv64-linux-gnu
>  CLANG_TARGET_FLAGS_s390		:= s390x-linux-gnu
>  CLANG_TARGET_FLAGS_sparc	:= sparc64-linux-gnu
>  CLANG_TARGET_FLAGS_x86		:= x86_64-linux-gnu
> +# This is only for i386 UM builds, which need the 32-bit target not -m32
> +CLANG_TARGET_FLAGS_i386		:= i386-linux-gnu
>  CLANG_TARGET_FLAGS_um		:= $(CLANG_TARGET_FLAGS_$(SUBARCH))
>  CLANG_TARGET_FLAGS		:= $(CLANG_TARGET_FLAGS_$(SRCARCH))
>  
> -- 
> 2.34.1
>