linux-hardening - Re: [PATCH] um: Allow builds with Clang

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Date:   Thu, 17 Feb 2022 10:59:27 +0800
From:   David Gow <davidgow@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jeff Dike <jdike@...toit.com>, Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-um <linux-um@...ts.infradead.org>,
        linux-kbuild@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        KUnit Development <kunit-dev@...glegroups.com>,
        llvm@...ts.linux.dev,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        x86@...nel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] um: Allow builds with Clang

On Thu, Feb 17, 2022 at 8:28 AM Kees Cook <keescook@...omium.org> wrote:
>
> Add x86-64 target for Clang+um and update user-offsets.c to use
> Clang-friendly assembler, similar to the fix from commit cf0c3e68aa81
> ("kbuild: fix asm-offset generation to work with clang").
>
> This lets me run KUnit tests with Clang:
>
> $ ./tools/testing/kunit/kunit.py config --make_options LLVM=1
> ...
> $ ./tools/testing/kunit/kunit.py run --make_options LLVM=1
> ...
>
> Cc: Jeff Dike <jdike@...toit.com>
> Cc: Richard Weinberger <richard@....at>
> Cc: Anton Ivanov <anton.ivanov@...bridgegreys.com>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: David Gow <davidgow@...gle.com>
> Cc: linux-um@...ts.infradead.org
> Cc: linux-kbuild@...r.kernel.org
> Cc: linux-kselftest@...r.kernel.org
> Cc: kunit-dev@...glegroups.com
> Cc: llvm@...ts.linux.dev
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---

Thanks, this worked fine for me, with one small note:

I get the following warning with clang (13.0.1) under UML (but not
under x86_64):
clang: warning: argument unused during compilation:
'-mno-global-merge' [-Wunused-command-line-argument]

It's not really a problem unless -Werror is enabled, though, so this
is still definitely an improvement over clang not working at all.

With that caveat, this is:
Tested-by: David Gow <davidgow@...gle.com>

Cheers,
-- David

>  arch/x86/um/user-offsets.c | 4 ++--
>  scripts/Makefile.clang     | 1 +
>  2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/um/user-offsets.c b/arch/x86/um/user-offsets.c
> index bae61554abcc..d9071827b515 100644
> --- a/arch/x86/um/user-offsets.c
> +++ b/arch/x86/um/user-offsets.c
> @@ -10,10 +10,10 @@
>  #include <asm/types.h>
>
>  #define DEFINE(sym, val) \
> -       asm volatile("\n->" #sym " %0 " #val : : "i" (val))
> +       asm volatile("\n.ascii \"->" #sym " %0 " #val "\"": : "i" (val))
>
>  #define DEFINE_LONGS(sym, val) \
> -       asm volatile("\n->" #sym " %0 " #val : : "i" (val/sizeof(unsigned long)))
> +       asm volatile("\n.ascii \"->" #sym " %0 " #val "\"": : "i" (val/sizeof(unsigned long)))
>
>  void foo(void)
>  {
> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 51fc23e2e9e5..857b23de51c6 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -10,6 +10,7 @@ CLANG_TARGET_FLAGS_powerpc    := powerpc64le-linux-gnu
>  CLANG_TARGET_FLAGS_riscv       := riscv64-linux-gnu
>  CLANG_TARGET_FLAGS_s390                := s390x-linux-gnu
>  CLANG_TARGET_FLAGS_x86         := x86_64-linux-gnu
> +CLANG_TARGET_FLAGS_um          := x86_64-linux-gnu
>  CLANG_TARGET_FLAGS             := $(CLANG_TARGET_FLAGS_$(SRCARCH))
>
>  ifeq ($(CROSS_COMPILE),)
> --
> 2.30.2
>
> --
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