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

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Date:   Wed, 13 May 2020 17:11:21 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Borislav Petkov <bp@...e.de>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: gcc-10: kernel stack is corrupted and fails to boot

On Wed, May 13, 2020 at 4:36 PM Borislav Petkov <bp@...e.de> wrote:
>
>
> Looking at them, they do have an mb() too so how about this then
> instead?
>
> #define prevent_tail_call_optimization()        mb()

Yeah, I think a full mb() is likely safe, because that's pretty much
always going to be a real instruction with real semantics, and no
amount of link-time optimizations can move it around a call
instruction.

I could imagine some completely UP in-order CPU that doesn't need to
serialize with anything at all, and even "mb()" might be empty.  I
think you can compile old ARM kernels for that. But realistically I
think we can ignore them at least for now - I'm not sure the link-time
optimization will even do things like that tailcall conversion, and
I'm not convinced that old pre-ARMv7 systems will be relevant by the
time (if) it ever does.

                   Linus