From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jesper Dangaard Brouer Subject: Re: [PATCH net] bpf: don't select potentially stale ri->map from buggy xdp progs Date: Fri, 8 Sep 2017 13:52:42 +0200 Message-ID: <20170908135242.55f99177@redhat.com> References: <01d69254802986ad3a8b18a8650c45df3df95def.1504821825.git.daniel@iogearbox.net> <20170908070610.4b8e1df1@redhat.com> <59B27234.4050703@iogearbox.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: davem@davemloft.net, ast@fb.com, john.fastabend@gmail.com, netdev@vger.kernel.org, brouer@redhat.com To: Daniel Borkmann , andy@greyhouse.net Return-path: Received: from mx1.redhat.com ([209.132.183.28]:33476 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751577AbdIHLwt (ORCPT ); Fri, 8 Sep 2017 07:52:49 -0400 In-Reply-To: <59B27234.4050703@iogearbox.net> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, 08 Sep 2017 12:34:28 +0200 Daniel Borkmann wrote: > On 09/08/2017 07:06 AM, Jesper Dangaard Brouer wrote: > > On Fri, 8 Sep 2017 00:14:51 +0200 > > Daniel Borkmann wrote: > > > >> + /* This is really only caused by a deliberately crappy > >> + * BPF program, normally we would never hit that case, > >> + * so no need to inform someone via tracepoints either, > >> + * just bail out. > >> + */ > >> + if (unlikely(map_owner != xdp_prog)) > >> + return -EINVAL; > > > > IMHO we do need to call the tracepoint here. It is not just crappy > > BPF-progs that cause this situation, it is also drivers not implementing > > XDP_REDIRECT yet (which is all but ixgbe). Due to the level XDP > > operates at, tracepoints are the only way users can runtime troubleshoot > > their XDP programs. > > Drivers not implementing XDP_REDIRECT don't even get there in > the first place. What they will do is to hit the 'default' case > when they check for the action code from the BPF program. Then > call into bpf_warn_invalid_xdp_action(act), and fall-through > to hit the tracepoint at trace_xdp_exception() which is also > triggered by XDP_ABORTED usually. So when that happens we do > complain loudly and call a tracepoint already. We should probably > tweak the bpf_warn_invalid_xdp_action() message a little to make > it clear that the action could also just be unsupported by the > driver instead of being illegal. Yes. drivers not implementing XDP_REDIRECT will cause a tracepoint trace_xdp_exception() to be called for its _own_ packets. But it will still setup and leave map and map_owner pointer dangling. Another NIC can load an xdp_prog that return XDP_REDIRECT, which will hit above if-statement, and its packets will disappear, without getting recorded by a tracepoint (thus hard to debug!). The fundamental point is that tracepoints is the way we choose to handle debugging XDP programs. Thus, we must trigger a tracepoint when a packet gets dropped. Even in this unlikely case. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer