linux-kernel - Re: [PATCH] x86: Fix early boot crash on gcc-10, next try

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Message-ID: <20200423173058.GE26021@zn.tnic>
Date:   Thu, 23 Apr 2020 19:30:58 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Michael Matz <matz@...e.de>, Jakub Jelinek <jakub@...hat.com>,
        Sergei Trofimovich <slyfox@...too.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>,
        Martin Liška <mliska@...e.cz>
Subject: Re: [PATCH] x86: Fix early boot crash on gcc-10, next try

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@...e.de>
Date: Thu, 23 Apr 2020 19:28:28 +0200
Subject: [PATCH] Check compiler

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 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 $(CC) check failed. Aborting),)
+
 # select defconfig based on actual architecture
 ifeq ($(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.

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