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

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Message-ID: <20170719233024.nt5yr5xoc5woelbk@treble>
Date:   Wed, 19 Jul 2017 18:30:24 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86@...nel.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@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v6 2/2] x86/refcount: Implement fast refcount overflow
 protection

On Wed, Jul 19, 2017 at 04:12:37PM -0700, Kees Cook wrote:
> On Wed, Jul 19, 2017 at 12:37 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > +#define ASM_UNREACHABLE                                                        \
> > +       "999: .pushsection .discard.unreachable\n\t"                    \
> > +       ".long 999b - .\n\t"                                            \
> > +       ".popsection\n\t"
> 
> Will this end up running into the same bug fixed with
> 3d1e236022cc1426b0834565995ddee2ca231cee and the __LINE__ macro? I
> could tell under which conditions gcc would do this kind of merging...

I hope not, but objtool should complain about it if it does.

We could involve the __LINE__ macro, or some other way to make the
labels unique, but that might complicate the code a bit, so we can delay
doing that unless necessary.

-- 
Josh