From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 0/4] RCU: introduce noref debug Date: Wed, 11 Oct 2017 08:45:32 -0700 Message-ID: <20171011154532.GD3521@linux.vnet.ibm.com> References: <20171006133436.GY3521@linux.vnet.ibm.com> <1507302609.2793.16.camel@redhat.com> <20171006163414.GC3521@linux.vnet.ibm.com> <1507567992.21825.9.camel@redhat.com> <20171011040225.GU3521@linux.vnet.ibm.com> <1507733436.2487.32.camel@redhat.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , "David S. Miller" , Eric Dumazet , Hannes Frederic Sowa , netdev@vger.kernel.org To: Paolo Abeni Return-path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:58726 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756560AbdJKPph (ORCPT ); Wed, 11 Oct 2017 11:45:37 -0400 Received: from pps.filterd (m0098394.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v9BFiMRO002479 for ; Wed, 11 Oct 2017 11:45:37 -0400 Received: from e18.ny.us.ibm.com (e18.ny.us.ibm.com [129.33.205.208]) by mx0a-001b2d01.pphosted.com with ESMTP id 2dhnrssk9f-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 11 Oct 2017 11:45:36 -0400 Received: from localhost by e18.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Oct 2017 11:45:35 -0400 Content-Disposition: inline In-Reply-To: <1507733436.2487.32.camel@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, Oct 11, 2017 at 04:50:36PM +0200, Paolo Abeni wrote: > On Tue, 2017-10-10 at 21:02 -0700, Paul E. McKenney wrote: > > Linus and Ingo will ask me how users decide how they should set that > > additional build flag. Especially given that if there is code that > > requires non-strict checking, isn't everyone required to set up non-strict > > checking to avoid false positives? Unless you can also configure out > > all the code that requires non-strict checking, I suppose, but how > > would you keep track of what to configure out? > > I'm working to a new version using a single compile flag - without > additional strict option. > > I don't know of any other subsytem that stores rcu pointer in > datastructures for a longer amount of time. That having said, I wonder > if the tests should completely move to the networking subsystem for the > time being. The Kconfig option would thus be called NET_DEBUG or > something along the lines. For abstraction it would be possible to add > an atomic_notifier_chain to the rcu_read/unlock methods, where multiple > users or checkers could register for. That way we keep the users > seperate from the implementation for the cost of a bit more layering > and more indirect calls. But given that this will anyway slow down > execution a lot, it will anyway only be suitable in > testing/verification/debugging environments. I like this approach. And if it does a good job of finding networking bugs over the next year or so, I would be quite happy to consider something for all RCU users. > > OK. There will probably be some discussion about the API in that case. > > I'll drop noref parameter, the key will became mandatory - the exact > position of where the reference of RCU managed object is stored. In the > case of noref dst it is &skb->_skb_refdst. With this kind of API it > should suite more subsystems. Interesting. Do you intend to allow rcu_track_noref() to refuse to record a pointer? Other than for the array-full case. > > True enough. Except that if people were good about always clearing the > > pointer, then the pointer couldn't leak, right? Or am I missing something > > in your use cases? > > This is correct. The dst_entry checking in skb, which this patch series > implements there are strict brackets in the sense of skb_dst_set, > skb_dst_set_noref, skb_dst_force, etc., which form brackets around the > safe uses of those dst_entries. This patch series validates that the > correct skb_dst_* functions are being called before the sk_buff leaves > the rcu protected section. Thus we don't need to modify and review a > lot of code but we can just patch into those helpers already. Makes sense. Those willing to strictly bracket uses gain some debug assist. > > Or to put it another way -- have you been able to catch any real > > pointer-leak bugs with thister-leak bugs with this? The other RCU > > debug options have had pretty long found-bug lists before we accepted > > them. > > There have been two problems found so far, one is a rather minor one > while the other one seems like a normal bug. The patches for those are > part of this series (3/4 and 4/4). I agree that you have started gathering evidence and that the early indications are positive, if quite mildly so. If this time next year there are a few tens of such bugs found, preferably including some serious ones, I would very likely look favorably on pulling this in to allow others to use it. Thanx, Paul