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

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Message-ID: <CABVgOSnEaTVUfZB6KACFJE4mM=oSWnUtcrf7ETHFq8h_k=mFTw@mail.gmail.com>
Date: Thu, 6 Mar 2025 16:59:53 +0800
From: David Gow <davidgow@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, 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, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] kbuild: clang: Support building UM with SUBARCH=i386

On Wed, 5 Mar 2025 at 00:21, Kees Cook <kees@...nel.org> 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>
> ---

Thanks for this -- I'd been using `--kconfig_add CONFIG_64BIT=n` as a
way to build i386 UML kernels, which seems to work even without this
patch -- but this is definitely an improvement.

It may be worth noting that CONFIG_FORTIFY_SOURCE is incompatible with
clang + i386, which seems to affect UML as well. (That might actually
not be required, as I think UML doesn't use any of the strange
-mregparm calling convention stuff which appears to be what breaks
FORTIFY_SOURCE here.)

Regardless,

Tested-by: David Gow <davidgow@...gle.com>

Cheers,
-- David


> 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
>

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