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

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Date:   Thu, 17 Feb 2022 10:41:37 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        David Gow <davidgow@...gle.com>, linux-um@...ts.infradead.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, kunit-dev@...glegroups.com,
        llvm@...ts.linux.dev,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] um: Allow builds with Clang

On Thu, Feb 17, 2022 at 01:54:58PM +0900, Masahiro Yamada wrote:
> On Thu, Feb 17, 2022 at 9:28 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>
> > ---
> >  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
> 
> 
> Does this work for the i386 host?
> 
> UML supports i386 and x86_64 as the host architecture as of now,
> but this always compiles UML for x86_64?

I think the current code will work because arch/x86/Makefile.um includes
-m32 for CONFIG_X86_32, which will implicitly change x86_64-linux-gnu
into a 32-bit target triple:

$ echo | clang --target=x86_64-linux-gnu -x c -c -o test.o -

$ file test.o
test.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

$ echo | clang --target=x86_64-linux-gnu -m32 -x c -c -o test.o -

$ file test.o
test.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped

In fact, we rely on this for ARCH=i386 LLVM=1 right now, as it uses
x86_64-linux-gnu for the target flag.

While UML only supports x86, maybe it is worth using SUBARCH instead of
hardcoding the triple? No strong opinion around that though.

diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 51fc23e2e9e5..87285b76adb2 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		:= $(CLANG_TARGET_FLAGS_$(SUBARCH))
 CLANG_TARGET_FLAGS		:= $(CLANG_TARGET_FLAGS_$(SRCARCH))
 
 ifeq ($(CROSS_COMPILE),)

> >  CLANG_TARGET_FLAGS             := $(CLANG_TARGET_FLAGS_$(SRCARCH))
> >
> >  ifeq ($(CROSS_COMPILE),)
> > --
> > 2.30.2
> >
> 
> 
> -- 
> Best Regards
> Masahiro Yamada