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Message-ID: <20170529122316.giceifwjnuz6djr4@hirez.programming.kicks-ass.net>
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.
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