linux-kernel - Re: [PATCH v2 0/2] x86, refcount: Implement fast refcount overflow

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Date:   Wed, 26 Apr 2017 04:01:28 +0200
From:   "PaX Team" <pageexec@...email.hu>
To:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
CC:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 0/2] x86, refcount: Implement fast refcount overflow

On 25 Apr 2017 at 15:56, Kees Cook wrote:

> This protection is a modified version of the x86 PAX_REFCOUNT
> implementation from PaX/grsecurity. This speeds up the refcount_t API by
> duplicating the existing atomic_t implementation with a single instruction
> added to detect if the refcount has wrapped past INT_MAX (or below 0)
> resulting in a signed value.

'signed value' sounds somewhat ambiguous given that in C a signed type (such
as the one beneath refcount_t) can have both negative and positive values yet
you didn't mean the latter here i guess.

> Various differences from PaX:
> - uses "js" instead of "jo" to trap all signed results instead of just
>   under/overflow transitions

there're differences in my 4.11 port but this isn't one of them.