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From: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Date: Thu, 20 Jul 2017 13:01:39 +0300
Message-ID: <CAPAsAGyyzLC24cj=-EUE4WGPQ_UQTCJNvWKVUKxPQmJmAtU3HA@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@kernel.org" <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Douglas Anderson <dianders@chromium.org>,
        Michael Davidson <md@google.com>, 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>,
        =?UTF-8?Q?Bernhard_Rosenkr=C3=A4nzer?= 
        <Bernhard.Rosenkranzer@linaro.org>
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2017-07-19 20:46 GMT+03:00 Josh Poimboeuf <jpoimboe@redhat.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.

> It would be nice if both compilers could agree on a way to support this.
>
> --
> Josh