Received-SPF: pass (google.com: domain of linux-wireless-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Thu, 14 May 2020 11:50:28 -0400
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Borislav Petkov <bp@suse.de>,
        Kalle Valo <kvalo@codeaurora.org>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: gcc-10: kernel stack is corrupted and fails to boot
Message-ID: <20200514155028.GA2336285@rani.riverdale.lan>
References: <20200513222038.GC6733@zn.tnic>
 <CAHk-=wgybuOF+Jp2XYWqM7Xn1CW6szWQw_FgVoFh5jx_4YoCVw@mail.gmail.com>
 <20200513233616.GD6733@zn.tnic>
 <CAHk-=wjZXFe08MiNRevJFGDvX0O6kcQTiK8GFBS7hwUAzB+LQw@mail.gmail.com>
 <CAKwvOd=o_wuiVpw5KVzLEt25W-A9Ah9fzftPZLG+yutqJmWbOg@mail.gmail.com>
 <CAHk-=wg6G+p1RRjR6UZBEuSCDs9=iWBsxrDPTEwqh+y5RayqKA@mail.gmail.com>
 <CALCETrUYA60fWu+=MviKx0NmW+_ppsOcv-ShUXdbyM4EjyfzHQ@mail.gmail.com>
 <CAHk-=wgiGxRgJGS-zyer1C_x2MQUVo6iZn0=aJyuFTqJWk-mpA@mail.gmail.com>
 <20200514052234.GA1894416@rani.riverdale.lan>
 <CAK8P3a1Cfzu7L30bFP-Sf2_GbkN_10CCJsbefTXyfnGF16uHMA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAK8P3a1Cfzu7L30bFP-Sf2_GbkN_10CCJsbefTXyfnGF16uHMA@mail.gmail.com>
Sender: linux-wireless-owner@vger.kernel.org
Precedence: bulk

On Thu, May 14, 2020 at 10:40:44AM +0200, Arnd Bergmann wrote:
> On Thu, May 14, 2020 at 7:22 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > On Wed, May 13, 2020 at 09:52:07PM -0700, Linus Torvalds wrote:
> > > On Wed, May 13, 2020, 20:50 Andy Lutomirski <luto@amacapital.net> wrote:
> > The gcc docs [1,2] at least don't inspire much confidence that this will
> > continue working with plain asm("") though:
> >
> > "Note that GCC’s optimizers can move asm statements relative to other
> > code, including across jumps."
> > ...
> > "Note that the compiler can move even volatile asm instructions relative
> > to other code, including across jump instructions."
> >
> > Even if we don't include an instruction in it I think it should at least
> > have a memory clobber, to stop the compiler from deciding that it can be
> > moved before the call so it can do the tail-call optimization.
> 
> I think LTO would still be able to notice that cpu_startup_entry() can
> be annotated __attribute__((noreturn)) and optimize the callers
> accordingly, which in turn would allow a tail call again after dead code
> elimination.
> 
>      Arnd

Yes, with LTO the only solution is to actually compile the caller
without stack checking I think.  Although at present gcc actually
doesn't tail-call optimize calls to noreturn functions that could easily
change.