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Date:   Fri, 17 Apr 2020 10:07:26 +0200
From:   Jakub Jelinek <jakub@redhat.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Sergei Trofimovich <slyfox@gentoo.org>,
        Michael Matz <matz@suse.de>, 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>, x86@kernel.org
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10
Message-ID: <20200417080726.GS2424@tucnak>
Reply-To: Jakub Jelinek <jakub@redhat.com>
References: <20200326223501.GK11398@zn.tnic>
 <20200328084858.421444-1-slyfox@gentoo.org>
 <20200413163540.GD3772@zn.tnic>
 <alpine.LSU.2.21.2004141343370.11688@wotan.suse.de>
 <20200415074842.GA31016@zn.tnic>
 <alpine.LSU.2.21.2004151445520.11688@wotan.suse.de>
 <20200415231930.19755bc7@sf>
 <20200417075739.GA7322@zn.tnic>
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On Fri, Apr 17, 2020 at 09:57:39AM +0200, Borislav Petkov wrote:
> On Wed, Apr 15, 2020 at 11:19:30PM +0100, Sergei Trofimovich wrote:
> > Ah, that makes sense. Borislav, should I send a fix forward against
> > x86 tree to move -fno-stack-protector as it was in v1 patch?
> > Or you'll revert v2 and apply v1 ~as is? Or should I send those myself?
> 
> Yeah, Peter and I have been discussing something like the below
> yesterday. I don't like the additional exports too much but would
> disable stack protector only for the one function...

If you want minimal changes, you can as I said earlier either
mark cpu_startup_entry noreturn (in the declaration in some header so that
smpboot.c sees it), or you could add something after the cpu_startup_entry
call to ensure it is not tail call optimized (e.g. just
	/* 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 ("");
would do, or for (;;); , or combine both, mark cpu_startup_entry noreturn and
add asm (""); (which most GCC versions will optimize away as unreachable).

	Jakub