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

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Date:   Thu, 20 Jul 2017 10:18:13 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andrey Ryabinin <ryabinin.a.a@...il.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@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Michael Davidson <md@...gle.com>,
        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 Rosenkränzer 
        <Bernhard.Rosenkranzer@...aro.org>
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in
 get_user() inline asm"

On Thu, Jul 20, 2017 at 01:01:39PM +0300, Andrey Ryabinin wrote:
> 2017-07-19 20:46 GMT+03:00 Josh Poimboeuf <jpoimboe@...hat.com>:
> 
> >
> > After doing some testing, I don't think this approach is going to work
> > after all.  In addition to forcing the stack frame, it also causes GCC
> > to add an unnecessary extra instruction to the epilogue of each affected
> > function:
> >
> >   lea    -0x10(%rbp),%rsp
> >
> > We shouldn't be inserting extra instructions like that.  I also don't
> > think the other suggestion of turning the '__sp' register variable into
> > a global variable is a very good solution either, as that just wastes
> > memory for no reason.
> >
> 
> Wastes memory? How is that wastes memory? That doesn't make any sense.

Yes, you're right, that doesn't make any sense.  I think I was trying to
wrap my head around what it means to have a global register variable --
in a header file no less -- and why clang would treat that differently
than a local register variable.

-- 
Josh