From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions Date: Mon, 29 May 2017 06:30:58 -0500 Message-ID: <87o9ubvr0t.fsf@xmission.com> References: <1487590189-18151-1-git-send-email-elena.reshetova@intel.com> <20170303162352.b6af1c0c3115b3f5f1e7aed3@linux-foundation.org> <20170529083903.GA17735@infradead.org> <87h904xc26.fsf@xmission.com> <20170529102442.gerbzxzixllen46q@hirez.programming.kicks-ass.net> <87a85wvsxa.fsf@xmission.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: Received: from out01.mta.xmission.com ([166.70.13.231]:39328 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750913AbdE2Lht (ORCPT ); Mon, 29 May 2017 07:37:49 -0400 In-Reply-To: <87a85wvsxa.fsf@xmission.com> (Eric W. Biederman's message of "Mon, 29 May 2017 05:49:53 -0500") Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Christoph Hellwig , Kees Cook , Andrew Morton , Elena Reshetova , Greg KH , Ingo Molnar , Alexey Dobriyan , "Serge E. Hallyn" , arozansk@redhat.com, Davidlohr Bueso , Manfred Spraul , "axboe@kernel.dk" , James Bottomley , "x86@kernel.org" , Ingo Molnar , Arnd Bergmann , "David S. Miller" , Rik van Riel , linux-arch , kernel-hardening@lists.openwall.com ebiederm@xmission.com (Eric W. Biederman) writes: > Peter Zijlstra writes: > >> On Mon, May 29, 2017 at 04:11:13AM -0500, Eric W. Biederman wrote: >> >>> Kees I I have a concern: >>> >>> __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r) >>> { >>> unsigned int new, val = atomic_read(&r->refs); >>> >>> do { >>> if (!val) >>> return false; >>> >>> if (unlikely(val == UINT_MAX)) >>> return true; >>> >>> new = val + i; >>> if (new < val) >>> new = UINT_MAX; >>> >>> } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); >>> >>> WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); >>> >>> return true; >>> } >>> >>> Why in the world do you succeed when you the value saturates???? >> >> Why not? On saturation the object will leak and returning a reference to >> it is always good. >> >>> From a code perspective that is bizarre. The code already has to handle >>> the case when the counter does not increment. >> >> I don't see it as bizarre, we turned an overflow/use-after-free into a >> leak. That's the primary mechanism here. >> >> As long as we have a reference to a leaked object, we might as well use >> it, its not going anywhere. >> >>> Fixing the return value would move refcount_t into the realm of >>> something that is desirable because it has bettern semantics and >>> is more useful just on a day to day correctness point of view. Even >>> ignoring the security implications. >> >> It changes the semantics between inc_not_zero() and inc(). It also >> complicates the semantics of inc_not_zero(), where currently the failure >> implies the count is 0 and means no-such-object, you complicate matters >> by basically returning 'busy'. > > Busy is not a state of a reference count. > > It is true I am suggesting treating something with a saturated reference > as not available. If that is what you mean by busy. But if it's > reference is zero it is also not available. So there is no practical > difference. > >> That is a completely new class of failure that is actually hard to deal >> with, not to mention that it completely destroys refcount_inc_not_zero() >> being a 'simple' replacement for atomic_inc_not_zero(). >> >> In case of the current failure, the no-such-object, we can fix that by >> creating said object. But what to do on 'busy' ? Surely you don't want >> to create another. You'd have to somehow retrofit something to wait on >> in every user. > > Using little words. > > A return of true from inc_not_zero means we took a reference. > A return of false means we did not take a reference. > > The code already handles I took a reference or I did not take a > reference. > > Therefore lying with refcount_t is not helpful. It takes failures > the code could easily handle and turns them into leaks. > > At least that is how I have seen reference counts used. And those > are definitely the plane obivous semantics. > > Your changes are definitely not drop in replacements for atomic_t in my > code. To clarify. If my code uses atomic_inc it does not expect a failure of any sort and saturate semantics are a fine replacement. If my code uses atomic_inc_not_zero it knows how to handle a failure to take a reference count. Making hiding the failure really bizarre. A must check function that hides a case I can handle and requires checking in a case where my code is built not to check is a drop in replacement for neither. So anyone who is proposing a refcount_t change as a drop in replacement for any code I maintain I will nack on sight because refcount_t is not currently a no-brain drop in replacement. Eric From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from out01.mta.xmission.com ([166.70.13.231]:39328 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750913AbdE2Lht (ORCPT ); Mon, 29 May 2017 07:37:49 -0400 From: ebiederm@xmission.com (Eric W. Biederman) References: <1487590189-18151-1-git-send-email-elena.reshetova@intel.com> <20170303162352.b6af1c0c3115b3f5f1e7aed3@linux-foundation.org> <20170529083903.GA17735@infradead.org> <87h904xc26.fsf@xmission.com> <20170529102442.gerbzxzixllen46q@hirez.programming.kicks-ass.net> <87a85wvsxa.fsf@xmission.com> Date: Mon, 29 May 2017 06:30:58 -0500 In-Reply-To: <87a85wvsxa.fsf@xmission.com> (Eric W. Biederman's message of "Mon, 29 May 2017 05:49:53 -0500") Message-ID: <87o9ubvr0t.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [PATCH 0/3] ipc subsystem refcounter conversions Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Christoph Hellwig , Kees Cook , Andrew Morton , Elena Reshetova , Greg KH , Ingo Molnar , Alexey Dobriyan , "Serge E. Hallyn" , arozansk@redhat.com, Davidlohr Bueso , Manfred Spraul , "axboe@kernel.dk" , James Bottomley , "x86@kernel.org" , Ingo Molnar , Arnd Bergmann , "David S. Miller" , Rik van Riel , linux-arch , "kernel-hardening@lists.openwall.com" , LKML Message-ID: <20170529113058.Z60M9DjV8LLj4CMWkbCtvd5V9bDjjeOPwK99z2Mf1Zg@z> ebiederm@xmission.com (Eric W. Biederman) writes: > Peter Zijlstra writes: > >> On Mon, May 29, 2017 at 04:11:13AM -0500, Eric W. Biederman wrote: >> >>> Kees I I have a concern: >>> >>> __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r) >>> { >>> unsigned int new, val = atomic_read(&r->refs); >>> >>> do { >>> if (!val) >>> return false; >>> >>> if (unlikely(val == UINT_MAX)) >>> return true; >>> >>> new = val + i; >>> if (new < val) >>> new = UINT_MAX; >>> >>> } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new)); >>> >>> WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n"); >>> >>> return true; >>> } >>> >>> Why in the world do you succeed when you the value saturates???? >> >> Why not? On saturation the object will leak and returning a reference to >> it is always good. >> >>> From a code perspective that is bizarre. The code already has to handle >>> the case when the counter does not increment. >> >> I don't see it as bizarre, we turned an overflow/use-after-free into a >> leak. That's the primary mechanism here. >> >> As long as we have a reference to a leaked object, we might as well use >> it, its not going anywhere. >> >>> Fixing the return value would move refcount_t into the realm of >>> something that is desirable because it has bettern semantics and >>> is more useful just on a day to day correctness point of view. Even >>> ignoring the security implications. >> >> It changes the semantics between inc_not_zero() and inc(). It also >> complicates the semantics of inc_not_zero(), where currently the failure >> implies the count is 0 and means no-such-object, you complicate matters >> by basically returning 'busy'. > > Busy is not a state of a reference count. > > It is true I am suggesting treating something with a saturated reference > as not available. If that is what you mean by busy. But if it's > reference is zero it is also not available. So there is no practical > difference. > >> That is a completely new class of failure that is actually hard to deal >> with, not to mention that it completely destroys refcount_inc_not_zero() >> being a 'simple' replacement for atomic_inc_not_zero(). >> >> In case of the current failure, the no-such-object, we can fix that by >> creating said object. But what to do on 'busy' ? Surely you don't want >> to create another. You'd have to somehow retrofit something to wait on >> in every user. > > Using little words. > > A return of true from inc_not_zero means we took a reference. > A return of false means we did not take a reference. > > The code already handles I took a reference or I did not take a > reference. > > Therefore lying with refcount_t is not helpful. It takes failures > the code could easily handle and turns them into leaks. > > At least that is how I have seen reference counts used. And those > are definitely the plane obivous semantics. > > Your changes are definitely not drop in replacements for atomic_t in my > code. To clarify. If my code uses atomic_inc it does not expect a failure of any sort and saturate semantics are a fine replacement. If my code uses atomic_inc_not_zero it knows how to handle a failure to take a reference count. Making hiding the failure really bizarre. A must check function that hides a case I can handle and requires checking in a case where my code is built not to check is a drop in replacement for neither. So anyone who is proposing a refcount_t change as a drop in replacement for any code I maintain I will nack on sight because refcount_t is not currently a no-brain drop in replacement. Eric