Date:   Thu, 2 Dec 2021 09:56:31 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Kefeng Wang <wangkefeng.wang@huawei.com>
Cc:     Marco Elver <elver@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, llvm@lists.linux.dev
Subject: Re: [PATCH v3] arm64: Enable KCSAN
Message-ID: <Yaj6vxHk0wFQb8E3@archlinux-ax161>
References: <20211202143331.15259-1-wangkefeng.wang@huawei.com>
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In-Reply-To: <20211202143331.15259-1-wangkefeng.wang@huawei.com>
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On Thu, Dec 02, 2021 at 10:33:31PM +0800, Kefeng Wang wrote:
> This patch enables KCSAN for arm64, with updates to build rules
> to not use KCSAN for several incompatible compilation units.
> 
> Resent GCC version(at least GCC10) made outline-atomics as the
> default option(unlike Clang), which will cause linker errors
> for kernel/kcsan/core.o. Disables the out-of-line atomics by
> no-outline-atomics to fix the linker errors.
> 
> Meanwhile, as Mark said[1], there is a specific issue on arm64
> about ARM64_BTI with Clang 11 if KCSAN enabled, which is fixed
> by Clang 12, add CLANG_VERSION check. And also some latent issues

I do not think this addition is necessary because we make BTI depend on
clang 12 or newer after commit 8cdd23c23c3d ("arm64: Restrict
ARM64_BTI_KERNEL to clang 12.0.0 and newer") upstream. I figured it was
better to go after the root cause rather than add a bunch of version
checks like this to the various sanitizers.

> are need to be fixed which isn't just a KCSAN problem, we make
> the KCSAN depends on EXPERT for now.
> 
> Tested selftest and kcsan_test(built with GCC11 and Clang 13),
> and all passed.
> 
> [1] https://lkml.org/lkml/2021/12/1/354
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> Tested on Qemu with clang 13 / gcc 11, based on 5.16-rc3.
> 
> [    0.221518] kcsan: enabled early
> [    0.222422] kcsan: strict mode configured
> ...
> [    5.839223] kcsan: selftest: 3/3 tests passed
> ...
> [  517.895102] # kcsan: pass:24 fail:0 skip:0 total:24
> [  517.896393] # Totals: pass:168 fail:0 skip:0 total:168
> [  517.897502] ok 1 - kcsan
> 
> v3:
> - add EXPERT and CLANG_VERSION depends suggested by Mark Rutland
> v2:
> - tested on GCC11 and disable outline-atomics for kernel/kcsan/core.c
>   suggested by Marco Elver
> 
>  arch/arm64/Kconfig               | 1 +
>  arch/arm64/kernel/vdso/Makefile  | 1 +
>  arch/arm64/kvm/hyp/nvhe/Makefile | 1 +
>  kernel/kcsan/Makefile            | 1 +
>  4 files changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 4ff73299f8a9..3a7b17742cde 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -150,6 +150,7 @@ config ARM64
>  	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
>  	select HAVE_ARCH_KASAN_SW_TAGS if HAVE_ARCH_KASAN
>  	select HAVE_ARCH_KASAN_HW_TAGS if (HAVE_ARCH_KASAN && ARM64_MTE)
> +	select HAVE_ARCH_KCSAN if EXPERT && (CC_IS_GCC || CLANG_VERSION >= 120000)
>  	select HAVE_ARCH_KFENCE
>  	select HAVE_ARCH_KGDB
>  	select HAVE_ARCH_MMAP_RND_BITS
> diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
> index 700767dfd221..60813497a381 100644
> --- a/arch/arm64/kernel/vdso/Makefile
> +++ b/arch/arm64/kernel/vdso/Makefile
> @@ -32,6 +32,7 @@ ccflags-y += -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
>  CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) -Os $(CC_FLAGS_SCS) $(GCC_PLUGINS_CFLAGS) \
>  				$(CC_FLAGS_LTO)
>  KASAN_SANITIZE			:= n
> +KCSAN_SANITIZE			:= n
>  UBSAN_SANITIZE			:= n
>  OBJECT_FILES_NON_STANDARD	:= y
>  KCOV_INSTRUMENT			:= n
> diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
> index c3c11974fa3b..24b2c2425b38 100644
> --- a/arch/arm64/kvm/hyp/nvhe/Makefile
> +++ b/arch/arm64/kvm/hyp/nvhe/Makefile
> @@ -89,6 +89,7 @@ KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_CFI)
>  # cause crashes. Just disable it.
>  GCOV_PROFILE	:= n
>  KASAN_SANITIZE	:= n
> +KCSAN_SANITIZE	:= n
>  UBSAN_SANITIZE	:= n
>  KCOV_INSTRUMENT	:= n
>  
> diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
> index c2bb07f5bcc7..e893b0e1d62a 100644
> --- a/kernel/kcsan/Makefile
> +++ b/kernel/kcsan/Makefile
> @@ -8,6 +8,7 @@ CFLAGS_REMOVE_debugfs.o = $(CC_FLAGS_FTRACE)
>  CFLAGS_REMOVE_report.o = $(CC_FLAGS_FTRACE)
>  
>  CFLAGS_core.o := $(call cc-option,-fno-conserve-stack) \
> +	$(call cc-option,-mno-outline-atomics) \
>  	-fno-stack-protector -DDISABLE_BRANCH_PROFILING
>  
>  obj-y := core.o debugfs.o report.o
> -- 
> 2.26.2
> 
> 
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