Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754004AbdFMXKJ (ORCPT ); Tue, 13 Jun 2017 19:10:09 -0400 Received: from mail-pf0-f176.google.com ([209.85.192.176]:33914 "EHLO mail-pf0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753957AbdFMXKH (ORCPT ); Tue, 13 Jun 2017 19:10:07 -0400 From: Matthias Kaehlcke To: Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , "H . J . Lu" , David Woodhouse , Masahiro Yamada , Michal Marek Cc: x86@kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, Michael Davidson , Greg Hackmann , Nick Desaulniers , Stephen Hines , Kees Cook , Arnd Bergmann , Bernhard.Rosenkranzer@linaro.org, Peter Foley , Behan Webster , Douglas Anderson , Matthias Kaehlcke Subject: [PATCH v2 4/4] x86/build: Specify stack alignment for clang Date: Tue, 13 Jun 2017 16:08:54 -0700 Message-Id: <20170613230854.112282-5-mka@chromium.org> X-Mailer: git-send-email 2.13.1.518.g3df882009-goog In-Reply-To: <20170613230854.112282-1-mka@chromium.org> References: <20170613230854.112282-1-mka@chromium.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3517 Lines: 80 For gcc stack alignment is configured with -mpreferred-stack-boundary=N, clang has the option -mstack-alignment=N for that purpose. Use the same alignment as with gcc. If the alignment is not specified clang assumes an alignment of 16 bytes, as required by the standard ABI. However as mentioned in d9b0cde91c60 ("x86-64, gcc: Use -mpreferred-stack-boundary=3 if supported") the standard kernel entry on x86-64 leaves the stack on an 8-byte boundary, as a consequence clang will keep the stack misaligned. Signed-off-by: Matthias Kaehlcke --- Changes in v2: - Changed option name to cc_stack_align_opt - Updated comments about stack alignment arch/x86/Makefile | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 6d3ae4b20a5b..9c3d13999dd3 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -11,6 +11,14 @@ else KBUILD_DEFCONFIG := $(ARCH)_defconfig endif +# Handle different option names for specifying stack alignment with gcc and +# clang. +ifeq ($(cc-name),clang) + cc_stack_align_opt := -mstack-alignment +else + cc_stack_align_opt := -mpreferred-stack-boundary +endif + # How to compile the 16-bit code. Note we always compile for -march=i386; # that way we can complain to the user if the CPU is insufficient. # @@ -28,7 +36,7 @@ REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os -D__KERNEL__ \ REALMODE_CFLAGS += $(call cc-option-raw, $(CC), $(REALMODE_CFLAGS), -ffreestanding) REALMODE_CFLAGS += $(call cc-option-raw, $(CC), $(REALMODE_CFLAGS), -fno-stack-protector) -REALMODE_CFLAGS += $(call cc-option-raw, $(CC), $(REALMODE_CFLAGS), -mpreferred-stack-boundary=2) +REALMODE_CFLAGS += $(call cc-option-raw, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align_opt)=2) export REALMODE_CFLAGS # BITS is used as extension for files which are available in a 32 bit @@ -65,8 +73,10 @@ ifeq ($(CONFIG_X86_32),y) # with nonstandard options KBUILD_CFLAGS += -fno-pic - # prevent gcc from keeping the stack 16 byte aligned - KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2) + # Align the stack to the register width instead of using the default + # alignment of 16 bytes. This reduces stack usage and the number of + # alignment instructions. + KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align_opt)=2) # Disable unit-at-a-time mode on pre-gcc-4.0 compilers, it makes gcc use # a lot more stack due to the lack of sharing of stacklots: @@ -98,8 +108,14 @@ else KBUILD_CFLAGS += $(call cc-option,-mno-80387) KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387) - # Use -mpreferred-stack-boundary=3 if supported. - KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3) + # By default gcc and clang use a stack alignment of 16 bytes for x86. + # However the standard kernel entry on x86-64 leaves the stack on an + # 8-byte boundary. If the compiler isn't informed about the actual + # alignment it will generate extra alignment instructions for the + # default alignment which keep the stack *mis*aligned. + # Furthermore an alignment to the register width reduces stack usage + # and the number of alignment instructions. + KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align_opt)=3) # Use -mskip-rax-setup if supported. KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup) -- 2.13.1.518.g3df882009-goog