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

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Message-ID: <e21dbc13-40e5-61e0-4aa1-5bfe47fa6415@zytor.com>
Date:   Thu, 4 Oct 2018 16:11:45 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andy Lutomirski <luto@...nel.org>
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>
Subject: Re: [PATCH v9 04/10] x86: refcount: prevent gcc distortions

On 10/04/18 13:29, Andy Lutomirski wrote:
>>
>> 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.
> 

Putting the generation of the ersatz code in objtool is an interesting
idea, although I'm wondering if these macros, as awful as they are,
wouldn't have generic applicability (what they do is they allow the cold
branch of an asm to push temporaries to the stack rather than having to
have gcc clobber them.)

I'll think about it.

	-hpa