Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 17 Apr 2020 10:42:24 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Jakub Jelinek <jakub@redhat.com>
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: <20200417084224.GB7322@zn.tnic>
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>
 <20200417080726.GS2424@tucnak>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20200417080726.GS2424@tucnak>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Apr 17, 2020 at 10:07:26AM +0200, Jakub Jelinek wrote:
> 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 ("");

That sounds ok-ish to me too.

I know you probably can't tell the future :) but what stops gcc from
doing the tail-call optimization in the future?

Or are optimization decisions behind an inline asm a no-no and will
pretty much always stay that way?

And I hope the clang folks don't come around and say, err, nope, we're
much more aggressive here.

However, if we do it with the explicit disabling with
-fno-stack-protector for only this compilation unit, then it is

1. clear why we're doing this
2. no compiler would break it

So I'm still gravitating a bit towards the explicit thing...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette