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

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Date:   Mon, 12 Jun 2017 09:17:14 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "H . J . Lu" <hjl.tools@...il.com>, x86@...nel.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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86/build: Specify stack alignment for clang

El Sat, Jun 10, 2017 at 09:03:59AM +0200 Ingo Molnar ha dit:

> 
> * 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 for 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 | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index 5851411e60fb..a32badbe87ad 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -27,7 +27,8 @@ REALMODE_CFLAGS	:= $(M16_CFLAGS) -g -Os -D__KERNEL__ \
> >  		   -mno-mmx -mno-sse \
> >  		   $(call cc-option, -ffreestanding) \
> >  		   $(call cc-option, -fno-stack-protector) \
> > -		   $(call cc-option, -mpreferred-stack-boundary=2)
> > +		   $(call cc-option, -mpreferred-stack-boundary=2) \
> > +		   $(call cc-option, -mstack-alignment=2)
> >  export REALMODE_CFLAGS
> >  
> >  # BITS is used as extension for files which are available in a 32 bit
> > @@ -64,8 +65,9 @@ ifeq ($(CONFIG_X86_32),y)
> >          # with nonstandard options
> >          KBUILD_CFLAGS += -fno-pic
> >  
> > -        # prevent gcc from keeping the stack 16 byte aligned
> > +        # prevent the compiler from keeping the stack 16 byte aligned
> >          KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=2)
> > +        KBUILD_CFLAGS += $(call cc-option,-mstack-alignment=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:
> > @@ -97,8 +99,9 @@ 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.
> > +	# Align the stack to 8 bytes if supported.
> >  	KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)
> > +	KBUILD_CFLAGS += $(call cc-option,-mstack-alignment=3)
> >  
> >  	# Use -mskip-rax-setup if supported.
> >  	KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
> 
> That's really ugly and the duplicated options are repeated. A cleaner solution 
> would be introduce a variable that stores the option, and use that later on. Also 
> describe the variable, pointing out that -mpreferred-stack-boundary is a GCC 
> option, while -mstack-alignment is a Clang one.

Thanks for your comments, I will send out an updated patch shortly.