[PATCH v2 2/2] x86/build: Fix stack alignment for CLang

Matthias Kaehlcke
2017-08-15T01:30:45Z

Commit:

d77698df39a5 ("x86/build: Specify stack alignment for clang")

intended to use the same stack alignment for clang as with gcc.

The two compilers use different options to configure the stack alignment
(gcc: -mpreferred-stack-boundary=n, clang: -mstack-alignment=n).

The above commit assumes that the clang option uses the same parameter
type as gcc, i.e. that the alignment is specified as 2^n. However clang
interprets the value of this option literally to use an alignment of n,
in consequence the stack remains misaligned.

Change the values used with -mstack-alignment to be the actual alignment
instead of a power of two.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
---
Changes in v2:
- use the reworked [__]cc-option instead of cc-option-3, the macro now
also evaluates the alternative option

arch/x86/Makefile | 16 +++++-----------
1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 1e902f926be3..c7b25ba2d2fa 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -11,14 +11,6 @@ else
KBUILD_DEFCONFIG := $(ARCH)_defconfig
endif

-# For gcc stack alignment is specified with -mpreferred-stack-boundary,
-# clang has the option -mstack-alignment for that purpose.
-ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)
- cc_stack_align_opt := -mpreferred-stack-boundary
-else ifneq ($(call cc-option, -mstack-alignment=4),)
- cc_stack_align_opt := -mstack-alignment
-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.
#
@@ -36,7 +28,9 @@ REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os -D__KERNEL__ \

REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -ffreestanding)
REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -fno-stack-protector)
-REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align_opt)=2)
+REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS),\
+ -mpreferred-stack-boundary=2,-mstack-alignment=4)
+
export REALMODE_CFLAGS

# BITS is used as extension for files which are available in a 32 bit
@@ -76,7 +70,7 @@ ifeq ($(CONFIG_X86_32),y)
# 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)
+ KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2,-mstack-alignment=4)

# 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:
@@ -115,7 +109,7 @@ else
# 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)
+ KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3,-mstack-alignment=8)

# Use -mskip-rax-setup if supported.
KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
--
2.14.0.434.g98096fd7a8-goog