linux-kernel - Re: [PATCH 5/8] x86/head: remove useless zeroed word

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Message-ID: <CA+55aFxQLfMcU4ty1_uJ=8fRO5n_DN_83Sek2XDC9vHeR2jJfg@mail.gmail.com>
Date:   Mon, 22 Aug 2016 15:47:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH 5/8] x86/head: remove useless zeroed word

On Mon, Aug 22, 2016 at 2:48 PM, Andi Kleen <andi@...stfloor.org> wrote:
>
> Seems dangerous. It wouldn't surprise me if some CPUs or x86 emulations
> load it anyways and trigger page faults if there is really nothing
> there.

Don't be silly, Andi and Peter.

We don't actually *use* lss any more. Not even on 32-bit. The zero is
never accessed. And on x86-64 we never did, obviously.

On 32-bit that zero doesn't even exist any more. On x86-64, it was
never even used at all. On x86-32, it was removed in commit
11d4c3f9b671 ("x86-32: Make sure the stack is set up before we use
it"), when the code stopped doing lss.

On x86-64, it has never made sense. It was added in commit
9cf4f298e29a ("x86: use stack_start in x86_64") to match x86-32 at the
time, but it didn't actually make sense even then, because x86-64
didn't use lss. 32-bit did, but 64-but just did a simple

    movq stack_start(%rip),%rsp

like a good user should.

             Linus