linux-kernel - Re: [PATCH v9 04/10] x86: refcount: prevent gcc distortions

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Message-ID: <CALCETrUm255shi0ba8j+ZJhkxv9HdDJ9A70U1T28bEHEMQi+8Q@mail.gmail.com>
Date:   Thu, 4 Oct 2018 13:29:20 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jan Beulich <JBeulich@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v9 04/10] x86: refcount: prevent gcc distortions

On Thu, Oct 4, 2018 at 12:33 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> On 10/04/18 02:16, Ingo Molnar wrote:
> >
> > * hpa@...or.com <hpa@...or.com> wrote:
> >
> >> Ingo: I wasn't talking necessarily about the specifics of each bit, but rather the general
> >> concept about being able to use macros in inlines...
> >
> > Ok, agreed about that part - and some of the patches did improve readability.
> >
> > Also, the 275 lines macros.s is a lot nicer than the 4,200 lines macros.S.
> >
> > Also, I'm not against using workarounds when the benefits are larger than the costs, but I am
> > against *hiding* the fact that these are workarounds and that for some of them there are costs.
> >
>
> Agreed, of course.
>
> >> I can send you something I have been working on in the background, but have been holding off
> >> on because of this, in the morning my time.
> >
> > BTW., I have applied most of the series to tip:x86/kbuild already, and will push them out later
> > today after some testing. I didn't apply the final 3 patches as they have dependencies, but
> > applied the basics and fixed up the changelogs.
> >
> > So you can rely on this.
> >
>
> Wonderful.
>
> Here is the horrible code I mentioned yesterday.  This is about
> implementing the immediate-patching framework that Linus and others have
> discussed (it helps both performance and kernel hardening):
>

I'm wondering if a production version should look more like:

patch_point:
mov $whatever, [target]
.pushsection ".immediate_patches"
.quad .Lpatch_point
.popsection

and let objtool parse the resulting assembled output and emit an entry
in some table mapping patch_point to the actual address and size of
the immediate to be patched.