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Date:   Wed, 22 Apr 2020 13:40:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        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>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10
Message-ID: <20200422114007.GC20730@hirez.programming.kicks-ass.net>
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On Wed, Apr 22, 2020 at 12:23:09PM +0200, Borislav Petkov wrote:
> Ok,
> 
> let's try the simple and clean fix first. Nick, would that work on LLVM
> too?
> 
> And I hope this will remain working and the compiler won't jump over an
> inline asm and go nuts.
> 
> Thx.
> 
> ---
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 3b9bf8c7e29d..06d2e16bedbb 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -266,6 +266,13 @@ static void notrace start_secondary(void *unused)
>  
>  	wmb();
>  	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> +
> +	/*
> +	 * Prevent tail call to cpu_startup_entry() because the stack protector
> +	 * guard has been changed in the middle of this function and must not be
> +	 * checked before tail calling another function.
> +	 */
> +        asm ("");
>  }

You haz a whitespace issue there.

Also, can we get this in writing, signed in blood, from the various
compiler teams ;-)