Date: Mon, 8 Jan 2018 07:42:20 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: David Woodhouse <dwmw@amazon.co.uk>
Cc: Andi Kleen <ak@linux.intel.com>, Paul Turner <pjt@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>, tglx@linutronix.de,
        Kees Cook <keescook@google.com>,
        Rik van Riel <riel@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>, gnomes@lxorguk.ukuu.org.uk
Subject: Re: [PATCH v6 01/10] x86/retpoline: Add initial retpoline support
Message-ID: <20180108134220.2for3pwgqbmkkq2s@treble>
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 <1515363085-4219-2-git-send-email-dwmw@amazon.co.uk>
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On Sun, Jan 07, 2018 at 10:11:16PM +0000, David Woodhouse wrote:
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index a20eacd..918e550 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -235,6 +235,16 @@ KBUILD_CFLAGS += -Wno-sign-compare
>  #
>  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
>  
> +# Avoid indirect branches in kernel to deal with Spectre
> +ifdef CONFIG_RETPOLINE
> +    RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
> +    ifneq ($(RETPOLINE_CFLAGS),)
> +        KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
> +    else
> +        $(warning Retpoline not supported in compiler. System may be insecure.)
> +    endif
> +endif

I wonder if an error might be more appropriate than a warning.  I
learned from experience that a lot of people don't see these Makefile
warnings, and this would be a dangerous one to miss.

Also if this were an error, you could get rid of the RETPOLINE define,
and that would be one less define cluttering up the already way-too-long
GCC arg list.

-- 
Josh