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

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Date:   Wed, 16 Feb 2022 17:37:13 -0700
From:   Nathan Chancellor <nathan@...nel.org>
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>,
        David Gow <davidgow@...gle.com>, linux-um@...ts.infradead.org,
        linux-kbuild@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kunit-dev@...glegroups.com, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] um: Allow builds with Clang

On Wed, Feb 16, 2022 at 04:28:43PM -0800, Kees Cook 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

Clang-friendly assembly?

> ("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>

I am not super familiar with UML but this seems reasonable.

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

One small nit below if you have to send a v2, not sure it is worth it
otherwise.

> ---
>  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

It might be nice to keep this in alphabetical order.

>  CLANG_TARGET_FLAGS		:= $(CLANG_TARGET_FLAGS_$(SRCARCH))
>  
>  ifeq ($(CROSS_COMPILE),)
> -- 
> 2.30.2
> 
>