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;
MIME-Version: 1.0
References: <20200513214128.GB6733@zn.tnic> <CAK8P3a3XPCyNM7s3vbn8JYK6swA3ZpPtTWB+uhmAE3YEX-nmig@mail.gmail.com>
 <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>
In-Reply-To: <20200514052234.GA1894416@rani.riverdale.lan>
From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 14 May 2020 10:40:44 +0200
Message-ID: <CAK8P3a1Cfzu7L30bFP-Sf2_GbkN_10CCJsbefTXyfnGF16uHMA@mail.gmail.com>
Subject: Re: gcc-10: kernel stack is corrupted and fails to boot
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     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>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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