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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Wed, 13 May 2020 20:50:03 -0700
Message-ID: <CALCETrUYA60fWu+=MviKx0NmW+_ppsOcv-ShUXdbyM4EjyfzHQ@mail.gmail.com>
Subject: Re: gcc-10: kernel stack is corrupted and fails to boot
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Borislav Petkov <bp@suse.de>, Arnd Bergmann <arnd@arndb.de>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        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>
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> On May 13, 2020, at 7:20 PM, Linus Torvalds <torvalds@linux-foundation.or=
g> wrote:
>
> =EF=BB=BFOn Wed, May 13, 2020 at 5:51 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
>>
>> Are you sure LTO treats empty asm statements differently than full
>> memory barriers in regards to preventing tail calls?
>
> It had better.
>
> At link-time, there is nothing left of an empty asm statement. So by
> the time the linker runs, it only sees
>
>    call xyz
>    ret
>
> in the object code. At that point, it's somewhat reasonable for any
> link-time optimizer (or an optimizing assembler, for that matter) to
> say "I'll just turn that sequence into a simple 'jmp xyz' instead".
>

What, what?

LTO isn=E2=80=99t a linker taking regular .o files full of regular machine
code and optimizing it. That=E2=80=99s nuts.  LTO takes an intermediate
representation and optimizes *that*. This will contain actual
indications that something is inline asm.  If LTO starts rewriting
inline asm, then I bet all kinds of things will go wrong and this is
the least of our worries.  Also, trying to do the kinds of stuff LTO
does by looking at just machine code isn't going to work.

So the difference between:

asm volatile ("nop");

and

asm volatile ("");

will be, literally, the absence of the nop.  (And alignment changes, etc.)