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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