From mboxrd@z Thu Jan 1 00:00:00 1970 From: Guillaume Nault Subject: Re: [PATCH] l2tp: Restore socket refcount when sendmsg succeeds Date: Tue, 12 Mar 2013 11:36:50 +0100 Message-ID: <20130312103650.GA4512@alphalink.fr> References: <20130301150202.GA3649@alphalink.fr> <20130301.141252.1152926224695074046.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: jchapman@katalix.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org To: David Miller Return-path: Content-Disposition: inline In-Reply-To: <20130301.141252.1152926224695074046.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Fri, Mar 01, 2013 at 02:12:52PM -0500, David Miller wrote: > From: Guillaume Nault > Date: Fri, 1 Mar 2013 16:02:02 +0100 > > > The sendmsg() syscall handler for PPPoL2TP doesn't decrease the socket > > reference counter after successful transmissions. Any successful > > sendmsg() call from userspace will then increase the reference counter > > forever, thus preventing the kernel's session and tunnel data from > > being freed later on. > > > > The problem only happens when writing directly on L2TP sockets. > > PPP sockets attached to L2TP are unaffected as the PPP subsystem > > uses pppol2tp_xmit() which symmetrically increase/decrease reference > > counters. > > > > This patch adds the missing call to sock_put() before returning from > > pppol2tp_sendmsg(). > > > > Cc: > > Signed-off-by: Guillaume Nault > > Looking at how this code works, it is such a terrible design. This > whole reference counting issue exists purely because > pppol2tp_sock_to_session() grabs the 'sk' reference. > > In all but one case, it need not do this. > > The socket system calls have an implicit reference to 'sk' via > socket->sk. If you can get into the system call and socket->sk > is non-NULL then 'sk' is NOT going anywhere. > > And all of these system call handlers have this pattern: > > session = pppol2tp_sock_to_session(sk); > ... > sock_put(sk); > > The only case where the reference count is really needed is that > sequence in pppol2tp_release(). > > Long term the right thing to do here is stop having this session > grabber function take the 'sk' reference. Then in pppol2tp_release > we'll grab a reference explicitly. At all the other call sites we > then blast aweay all of the sock_put(sk) paths. > Could this also apply to l2tp_sock_to_tunnel() (in l2tp_core.h)? As per my understanding, none of its callers needs to take a socket reference. So sock_hold() could be removed in both pppol2tp_sock_to_session() and l2tp_sock_to_tunnel() functions. The corresponding sock_put() calls would then be removed from all calling functions but pppol2tp_release(). If this is correct, I'll send a patch for net-next.