Commit 8f918697 authored by Matthias Kaehlcke's avatar Matthias Kaehlcke Committed by Ingo Molnar
Browse files

x86/build: Fix stack alignment for CLang 



Commit:

  d77698df ("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.

cc-option isn't used here with the typical pattern of KBUILD_CFLAGS +=
$(call cc-option ...). The reason is that older gcc versions don't
support the -mpreferred-stack-boundary option, since cc-option doesn't
verify whether the alternative option is valid it would incorrectly
select the clang option -mstack-alignment..

Signed-off-by: default avatarMatthias Kaehlcke <mka@chromium.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bernhard.Rosenkranzer@linaro.org
Cc: Greg Hackmann <ghackmann@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michael Davidson <md@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephen Hines <srhines@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dianders@chromium.org
Link: http://lkml.kernel.org/r/20170817004740.170588-1-mka@chromium.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent e20f7e5e

arch/x86/Makefile

+8 −6
Original line number Diff line number Diff line
@@ -14,9 +14,11 @@ 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

      cc_stack_align4 := -mpreferred-stack-boundary=2

      cc_stack_align8 := -mpreferred-stack-boundary=3

else ifneq ($(call cc-option, -mstack-alignment=16),)

      cc_stack_align4 := -mstack-alignment=4

      cc_stack_align8 := -mstack-alignment=8

endif



# How to compile the 16-bit code.  Note we always compile for -march=i386;
@@ -36,7 +38,7 @@ 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 += $(cc_stack_align4)

export REALMODE_CFLAGS



# BITS is used as extension for files which are available in a 32 bit
@@ -76,7 +78,7 @@ 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 += $(cc_stack_align4)



        # 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 +117,7 @@ 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 += $(cc_stack_align8)



	# Use -mskip-rax-setup if supported.

	KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)

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