linux-kernel - Re: [PATCH v6 0/2] x86: Implement fast refcount overflow protection

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Date:   Mon, 24 Jul 2017 22:09:32 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        "axboe\@kernel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86\@kernel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening\@lists.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v6 0/2] x86: Implement fast refcount overflow protection

Peter Zijlstra <peterz@...radead.org> writes:

> On Mon, Jul 24, 2017 at 04:38:06PM +1000, Michael Ellerman wrote:
>
>> What I'm not entirely clear on is what the best trade off is in terms of
>> overhead vs checks. The summary of behaviour between the fast and full
>> versions you promised Ingo will help there I think.
>
> That's something that's probably completely different for PPC than it is
> for x86.

Yeah definitely. I guess I see the x86 version as a lower bound on the
semantics we'd need to implement and still claim to implement the
refcount stuff.

> Both because your primitive is LL/SC and thus the saturation
> semantics we need a cmpxchg loop for are more natural in your case

Yay!

> anyway, and the fact that your LL/SC is horrendously slow in any case.

Boo :/

Just kidding. I suspect you're right that we can probably pack a
reasonable amount of tests in the body of the LL/SC and not notice.

> Also, I still haven't seen an actual benchmark where our cmpxchg loop
> actually regresses anything, just a lot of yelling about potential
> regressions :/

Heh yeah. Though I have looked at the code it generates on PPC and it's
not sleek, though I guess that's not a benchmark is it :)

cheers