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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 23 Apr 2020 11:02:09 -0700
Message-ID: <CAKwvOdmmuE=o-bGpraR1rJdzDgf2UYvgcx2-S7cdGSFWWkWurA@mail.gmail.com>
Subject: Re: [PATCH] x86: Fix early boot crash on gcc-10, next try
To:     Borislav Petkov <bp@alien8.de>
Cc:     Michael Matz <matz@suse.de>, Jakub Jelinek <jakub@redhat.com>,
        Sergei Trofimovich <slyfox@gentoo.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kees Cook <keescook@chromium.org>,
        =?UTF-8?Q?Martin_Li=C5=A1ka?= <mliska@suse.cz>,
        Masahiro Yamada <masahiroy@kernel.org>
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On Thu, Apr 23, 2020 at 10:31 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Thu, Apr 23, 2020 at 06:12:24PM +0200, Borislav Petkov wrote:
> > Ok,
> >
> > I have tried to summarize our odyssey so far and here's what I came up
> > with. Just built latest gcc from the git repo and it seems to work.
> >
> > Next I need to come up with a slick way of testing the compiler...
>
> Maybe something like this. Seems to work with both.
>
> ---
> From: Borislav Petkov <bp@suse.de>
> Date: Thu, 23 Apr 2020 19:28:28 +0200
> Subject: [PATCH] Check compiler
>
> Signed-off-by: Borislav Petkov <bp@suse.de>

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

It's too bad that $(CC) isn't exported yet; IIUC we include the arch
specific Makefiles then later export $(CC).  If that was the case, we
could just use $(CC) in the shell script, rather than passing it along
as an argument.  Oh well.

> ---
>  arch/x86/Makefile             | 4 ++++
>  scripts/x86-check-compiler.sh | 9 +++++++++
>  2 files changed, 13 insertions(+)
>  create mode 100755 scripts/x86-check-compiler.sh
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 00e378de8bc0..38d3eec5062e 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -1,6 +1,10 @@
>  # SPDX-License-Identifier: GPL-2.0
>  # Unified Makefile for i386 and x86_64
>
> +#  Check the compiler
> +sane_compiler := $(shell $(srctree)/scripts/x86-check-compiler.sh $(CC))
> +$(if $(sane_compiler),$(error $(CC) check failed. Aborting),)

If I add `echo "hello world"` to the end of
scripts/x86-check-compiler.sh to verify this stops a build, this is
the error message I would observe:
arch/x86/Makefile:6: *** clang check failed. Aborting.  Stop.

> +
>  # select defconfig based on actual architecture
>  ifeq ($(ARCH),x86)
>    ifeq ($(shell uname -m),x86_64)
> diff --git a/scripts/x86-check-compiler.sh b/scripts/x86-check-compiler.sh
> new file mode 100755
> index 000000000000..b2b5b54b6939
> --- /dev/null
> +++ b/scripts/x86-check-compiler.sh
> @@ -0,0 +1,9 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +
> +# Check whether the compiler tail-call optimizes across an asm() statement.
> +# Fail the build if it does.
> +
> +echo "int foo(int a); int bar(int a) { int r = foo(a); asm(\"\"); return r; }" |\
> +            $* -O2 -x c -c -S - -o - 2>/dev/null |\
> +            grep -E "^[[:blank:]]+jmp[[:blank:]]+.*"
> --
> 2.21.0
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette


-- 
Thanks,
~Nick Desaulniers