linux-kernel - Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get

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Date:   Thu, 13 Jul 2017 12:38:32 -0700
From:   Michael Davidson <md@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Chris J Arges <chris.j.arges@...onical.com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Bernhard.Rosenkranzer@...aro.org
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in
 get_user() inline asm"

On Thu, Jul 13, 2017 at 12:25 PM, Josh Poimboeuf <jpoimboe@...hat.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.