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From: Michael Davidson <md@google.com>
Date: Thu, 13 Jul 2017 12:38:32 -0700
Message-ID: <CA+=D-XWi45Oor6PKtR=OpEoVPD1isXDeECJ0mRG_aU0Z4aZRWA@mail.gmail.com>
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in
 get_user() inline asm"
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Matthias Kaehlcke <mka@chromium.org>,
        Chris J Arges <chris.j.arges@canonical.com>,
        Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Douglas Anderson <dianders@chromium.org>,
        Greg Hackmann <ghackmann@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Stephen Hines <srhines@google.com>, Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>, Bernhard.Rosenkranzer@linaro.org
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On Thu, Jul 13, 2017 at 12:25 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:

>
> Anyway this seems like a clang bug to me.  If I specify RSP as an input
> register then the compiler shouldn't overwrite it first.  For that
> matter it has no reason to overwrite it if it's an output register
> either.
>

It's certainly  a difference in behavior between clang and gcc.

My question is whether this particular construct is really a
"supported" (or, at least, reasonably guaranteed) way of forcing gcc
to create a stack frame if none exists. or whether it is something
that "just happens to work".

If someone could explain the rationale behind *why* this works the way
that it does on gcc that might help convince the clang people that
this is actually a bug rather than just a piece of undefined behavior
which gcc and clang happen to handle differently.