On Fri, 08 Sep 2017 12:34:28 +0200 Daniel Borkmann <dan...@iogearbox.net> wrote:
> On 09/08/2017 07:06 AM, Jesper Dangaard Brouer wrote: > > On Fri, 8 Sep 2017 00:14:51 +0200 > > Daniel Borkmann <dan...@iogearbox.net> 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
