linux-kernel - Re: [PATCH 0/3] ipc subsystem refcounter conversions

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Date:   Mon, 29 May 2017 14:23:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        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>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions

On Mon, May 29, 2017 at 06:39:44AM -0500, Eric W. Biederman wrote:
> I failed to see that there is a refcount_inc.  Too much noise in
> the header file I suppose.
> 
> But implementing refcount_inc in terms of refcount_inc_not_zero is
> totally broken.  The two operations are not the same and the go to
> different assumptions the code is making.
> 
> That explains why you think refcount_inc_not_zero should lie because
> you are implementing refcount_inc with it.  They are semantically very
> different operations.  Please separate them.

There has been much debate about this. And the best I'll do is add a
comment and/or retain these exact semantics.

What is done is:

	refcount_inc() := WARN_ON(!refcount_inc_not_zero())

Because incrementing a zero reference count is a use-after-free and
something we should not do ever.

This is where the whole usage count vs reference count pain comes from.

Once there are no more _references_ to an object, a reference count
frees the object. Therefore a zero reference count means a dead object
and incrementing from that is fail.

The usage count model otoh counts how many (active) users there are of
an object, and no active users is a good and expected situation. But it
is very explicitly not a reference count. Because even in the no users
case do we have a reference to the object (we've not leaked it after
all, we just don't track all references).

Similarly, refcount_dec() is implemented using dec_and_test() and will
WARN when it hits 0, because this is a leak and we don't want those
either.

A usage count variant otoh would be fine with hitting 0.