linux-kernel - Re: [PATCH v4 3/3] x86/build: Specify stack alignment for clang

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Message-ID: <EAA687B2-ECD0-42C7-907E-BE8BDF311EC1@zytor.com>
Date:   Mon, 19 Jun 2017 13:17:03 -0700
From:   hpa@...or.com
To:     Matthias Kaehlcke <mka@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <mmarek@...e.com>
CC:     x86@...nel.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bernhard.Rosenkranzer@...aro.org,
        Peter Foley <pefoley2@...oley.com>,
        Behan Webster <behanw@...verseincode.com>,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH v4 3/3] x86/build: Specify stack alignment for clang

On June 19, 2017 11:37:57 AM PDT, Matthias Kaehlcke <mka@...omium.org> wrote:
>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 <mka@...omium.org>
>---
> arch/x86/Makefile | 26 +++++++++++++++++++++-----
> 1 file changed, 21 insertions(+), 5 deletions(-)
>
>diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>index b2dae639f778..9406d3670452 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, $(CC), $(REALMODE_CFLAGS),
>-ffreestanding)
>REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS),
>-fno-stack-protector)
>-REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS),
>-mpreferred-stack-boundary=2)
>+REALMODE_CFLAGS += $(call __cc-option, $(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)

Goddammit.

How many times do I have to say NAK to

>+ifeq ($(cc-name),clang)

... before it sinks in?
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.