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

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Date:   Thu, 14 May 2020 10:40:44 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     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 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