[tip:x86/build] x86/build: Fix stack alignment for CLang

[tip-bot for Matthias Kaehlcke]

Commit-ID:  8f91869766c00622b2eaa8ee567db4f333b78c1a
Gitweb:     http://git.kernel.org/tip/8f91869766c00622b2eaa8ee567db4f333b78c1a
Author:     Matthias Kaehlcke <mka@xxxxxxxxxxxx>
AuthorDate: Wed, 16 Aug 2017 17:47:40 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 17 Aug 2017 10:27:19 +0200

x86/build: Fix stack alignment for CLang

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.

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: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Bernhard.Rosenkranzer@xxxxxxxxxx
Cc: Greg Hackmann <ghackmann@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Michael Davidson <md@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephen Hines <srhines@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dianders@xxxxxxxxxxxx
Link: http://lkml.kernel.org/r/20170817004740.170588-1-mka@xxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/Makefile | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 70b3aa9..48f3ae7 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -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)