linux-kernel - Re: [RFC][PATCH] locking/refcount: Provide _

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Message-ID: <202007291352.8775B08DA@keescook>
Date:   Wed, 29 Jul 2020 13:52:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     David Howells <dhowells@...hat.com>
Cc:     peterz@...radead.org, Xiaoming Ni <nixiaoming@...wei.com>,
        David Windsor <dwindsor@...il.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Paul Moore <paul@...l-moore.com>, edumazet@...gle.com,
        paulmck@...nel.org, shakeelb@...gle.com,
        James Morris <jamorris@...ux.microsoft.com>,
        alex.huangjianhui@...wei.com, dylix.dailei@...wei.com,
        chenzefeng2@...wei.com, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC][PATCH] locking/refcount: Provide __refcount API to obtain
 the old value

On Wed, Jul 29, 2020 at 09:41:37PM +0100, David Howells wrote:
> peterz@...radead.org wrote:
> 
> > I'm not entirely sure what you mean with interpret, provided you don't
> > trigger a refcount fail, the number will be just what you expect and
> > would get from refcount_read(). If you do trigger a fail, you'll get a
> > negative value.
> 
> That's fine.  I seem to remember talk about the possibility that the number
> wouldn't necessarily bottom out at zero - for instance if it was arranged such
> that the overflow flag was set on an overflow or underflow so that it could be
> trapped on (using INTO or TRAPV, for example).

The trap is an internal detail. The saturation value will be negative,
though.

-- 
Kees Cook