linux-kernel - Re: gcc-10: kernel stack is corrupted and fails to boot

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Message-ID: <20200514155028.GA2336285@rani.riverdale.lan>
Date:   Thu, 14 May 2020 11:50:28 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Borislav Petkov <bp@...e.de>,
        Kalle Valo <kvalo@...eaurora.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: gcc-10: kernel stack is corrupted and fails to boot

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@...m.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@...capital.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.