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:   Sat, 25 Apr 2020 19:31:40 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
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>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kees Cook <keescook@chromium.org>,
        Martin =?utf-8?B?TGnFoWth?= <mliska@suse.cz>,
        =?utf-8?Q?Fr=C3=A9d=C3=A9ric_Pierret_=28fepitre=29?= 
        <frederic.pierret@qubes-os.org>, boris.ostrovsky@oracle.com,
        jgross@suse.com, linuxppc-dev@lists.ozlabs.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH] x86: Fix early boot crash on gcc-10, next try
Message-ID: <20200425173140.GB24294@zn.tnic>
References: <CAKwvOd=Dza3UBfeUzs2RW6ko5fDr3jYeGQAYpJXqyEVns6DJHg@mail.gmail.com>
 <20200422192113.GG26846@zn.tnic>
 <CAKwvOdkbcO8RzoafON2mGiSy5P96P5+aY8GySysF2my7q+nTqw@mail.gmail.com>
 <20200422212605.GI26846@zn.tnic>
 <CAKwvOd=exxhfb8N6=1Q=wBUaYcRDEq3L1+TiHDLz+pxWg8OuwQ@mail.gmail.com>
 <20200423125300.GC26021@zn.tnic>
 <20200423161126.GD26021@zn.tnic>
 <20200425014657.GA2191784@rani.riverdale.lan>
 <20200425085759.GA24294@zn.tnic>
 <20200425150440.GA470719@rani.riverdale.lan>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20200425150440.GA470719@rani.riverdale.lan>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Apr 25, 2020 at 11:04:40AM -0400, Arvind Sankar wrote:
> I'd put the clause about stack protector being disabled and the
> tail-call one together, to make clear that you still need the never
> return and always inline bits.

Done.

> Also, this function is implemented by multiple arch's and they all
> have similar comments -- might be better to consolidate the comment in
> the generic (dummy) one in include/linux/stackprotector.h laying out
> the restrictions that arch implementations should follow?

I'm not sure gcc-10 does the same thing on other arches - I'd let gcc
guys chime in here and other arch maintainers to decide what to do.

> There's also the one in init/main.c which is used by multiple
> architectures. On x86 at least, the call to arch_call_rest_init at the
> end of start_kernel does not get tail-call optimized by gcc-10, but I
> don't see anything that actually prevents that from happening. We should
> add the asm("") there as well I think, unless the compiler guys see
> something about this function that will always prevent the optimization?

Hmm, that's what I was afraid of - having to sprinkle this around. Yah, let's
wait for compiler guys to have a look here and then maybe I'll convert that
thing to a macro called

	compiler_prevent_tail_call_opt()

or so, so that it can be sprinkled around. ;-\

-- 
Regards/Gruss,
    Boris.

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