Re: When does Linux drop UDP packets?

[Steven Rostedt]

Oh, btw, you don't need to strip LKML from replys (or any Cc), unless it 
is off topic.  LKML is a public forum, and as long as things are on topic, 
it is nice to show others the solutions that are found.

> 
> On Thu, 4 Jun 2009, Philipp Reh wrote:
> 
> > I'm not really sure it is dropped but tcpdump sees the packet and
> > recvfrom doesn't see it, so I think that the kernel might drop it
> > somewhere when processing the IP stack logic.
> 
> Looking at the udp stack, selinux plays a role, and looks like it will 
> drop a packet if it is not in the routing table.
> 
> Try:
> 
> # setenforce 0
> 
> and see if you start seeing packets.

BTW, I used ftrace to look into this:

Enable:

	CONFIG_FTRACE
	CONFIG_FUNCTION_TRACER
	CONFIG_FUNCITON_GRAPH_TRACER
	CONFIG_DYNAMIC_FTRACE

Compile and boot:

# mount -t debugfs nodev /sys/kernel/debug
# cd /sys/kernel/debug
# echo udp_rcv > set_graph_function
# echo function_graph > current_tracer
# cat trace

and this is what I see:

# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
 1)               |  udp_rcv() {
 1)               |    __udp4_lib_rcv() {
 1)               |      __udp4_lib_lookup() {
 1)   0.584 us    |        T.757();
 1)   2.905 us    |      }
 1)               |      udp_queue_rcv_skb() {
 1)   0.584 us    |        nf_reset();
 1)   0.996 us    |        _spin_lock();
 1)               |        __udp_queue_rcv_skb() {
 1)               |          sock_queue_rcv_skb() {
 1)               |            sk_filter() {
 1)               |              security_sock_rcv_skb() {
 1)               |                selinux_socket_sock_rcv_skb() {
 1)               |                  selinux_parse_skb() {
 1)   0.584 us    |                    skb_header_pointer();
 1)   0.569 us    |                    skb_header_pointer();
 1)   3.074 us    |                  }
 1)               |                  selinux_netlbl_sock_rcv_skb() {
 1)   0.561 us    |                    netlbl_enabled();
 1)   1.748 us    |                  }
 1)   6.792 us    |                }
 1)   8.095 us    |              }
 1)               |              local_bh_disable() {
 1)   0.599 us    |                __local_bh_disable();
 1)   1.790 us    |              }
 1)               |              local_bh_enable() {
 1)               |                _local_bh_enable_ip() {
 1)   0.566 us    |                  T.445();
 1)   1.906 us    |                }
 1)   3.149 us    |              }
 1) + 15.722 us   |            }
 1)               |            skb_queue_tail() {
 1)   1.059 us    |              _spin_lock_irqsave();
 1)               |              _spin_unlock_irqrestore() {
 1)   0.596 us    |                T.263();
 1)   1.913 us    |              }
 1)   4.718 us    |            }
 1)   0.565 us    |            sock_flag();
 1)               |            sock_def_readable() {
 1)   0.981 us    |              _read_lock();
 1)               |              __wake_up_sync_key() {
 1)   0.887 us    |                _spin_lock_irqsave();
 1)               |                __wake_up_common() {
 1)               |                  pollwake() {
 1)               |                    default_wake_function() {
 1)               |                      try_to_wake_up() {
 1)               |                        task_rq_lock() {
 1)   0.951 us    |                          _spin_lock();
 1)   2.030 us    |                        }
 1)   0.768 us    |                        update_rq_clock();
 1)               |                        select_task_rq_fair() {
 1)   0.501 us    |                          effective_load();
 1)   0.494 us    |                          effective_load();

-- Steve

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