linux-kernel - Re: [PATCH] x86: only use ERMS for user copies for larger sizes

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Date:   Thu, 22 Nov 2018 09:26:16 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>, pabeni@...hat.com,
        Jens Axboe <axboe@...nel.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Brian Gerst <brgerst@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes

On Thu, Nov 22, 2018 at 8:56 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Nov 22, 2018 at 2:32 AM Ingo Molnar <mingo@...nel.org> wrote:
> > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > >
> > > Random patch (with my "asm goto" hack included) attached, in case
> > > people want to play with it.
> >
> > Doesn't even look all that hacky to me. Any hack in it that I didn't
> > notice? :-)
>
> The code to use asm goto sadly doesn't have any fallback at all for
> the "no asm goto available".
>
> I guess we're getting close to "we require asm goto support", but I
> don't think we're there yet.

commit e501ce957a786ecd076ea0cfb10b114e6e4d0f40
Author: Peter Zijlstra <peterz@...radead.org>
Date:   Wed Jan 17 11:42:07 2018 +0100

    x86: Force asm-goto

    We want to start using asm-goto to guarantee the absence of dynamic
    branches (and thus speculation).

    A primary prerequisite for this is of course that the compiler
    supports asm-goto. This effecively lifts the minimum GCC version to
    build an x86 kernel to gcc-4.5.

This is basically the only good outcome from the speculation crap as
far as I'm concerned :)

So I think your patch is viable.  Also, with that patch applied,
put_user_ex() should become worse than worthless -- if gcc is any
good, plain old:

if (unsafe_put_user(...) != 0)
  goto err;
if (unsafe_put_user(...) != 0)
  goto err;
etc.

will generate *better* code than a series of put_user_ex() calls.