af_packet: add interframe drop cmsg (v6)

Add Ancilliary data to better represent loss information

I've had a few requests recently to provide more detail regarding frame loss
during an AF_PACKET packet capture session.  Specifically the requestors want to
see where in a packet sequence frames were lost, i.e. they want to see that 40
frames were lost between frames 302 and 303 in a packet capture file.  In order
to do this we need:

1) The kernel to export this data to user space
2) The applications to make use of it

This patch addresses item (1).  It does this by doing the following:

A) Anytime we drop a frame for which we would increment po->stats.tp_drops, we
also no increment a stats called po->stats.tp_gap.

B) Every time we successfully enqueue a frame to sk_receive_queue, we record the
value of po->stats.tp_gap in skb->mark.  skb->cb would nominally be the place to
record this, but since all the space there is used up, we're overloading
skb->mark.  Its safe to do since any enqueued packet is guaranteed to be
unshared at this point, and skb->mark isn't used for anything else in the rx
path to the application.  After we record tp_gap in the skb, we zero
po->stats.tp_gap.  This allows us to keep a counter of the number of frames lost
between any two enqueued packets

C) When the application goes to dequeue a frame from the packet socket, we look
at skb->mark for that frame.  If it is non-zero, we add a cmsg chunk to the
msghdr of level SOL_PACKET and type PACKET_GAPDATA.  Its a 32 bit integer that
represents the number of frames lost between this packet and the last previous
frame received.

Note there is a chance that if there is frame loss after a receive, and then the
socket is closed, some gap data might be lost.  This is covered by the use of
the PACKET_AUXDATA socket option, which gives total loss data.  With a bit of
math, the final gap can be determined that way.

I've tested this patch myself, and it works well.
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>

 include/linux/if_packet.h |    2 ++
 net/packet/af_packet.c    |   33 +++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_packet.h

@@ -48,11 +48,13 @@ struct sockaddr_ll
 #define PACKET_RESERVE			12
 #define PACKET_TX_RING			13
 #define PACKET_LOSS			14
+#define PACKET_GAPDATA			15
 
 struct tpacket_stats
 {
 	unsigned int	tp_packets;
 	unsigned int	tp_drops;
+	unsigned int    tp_gap;
 };
 
 struct tpacket_auxdata

net/packet/af_packet.c

@@ -523,6 +523,31 @@ static inline unsigned int run_filter(struct sk_buff *skb, struct sock *sk,
 	return res;
 }
 
+/*
+ * If we've lost frames since the last time we queued one to the
+ * sk_receive_queue, we need to record it here.
+ * This must be called under the protection of the socket lock
+ * to prevent racing with other softirqs and user space
+ */
+static inline void record_packet_gap(struct sk_buff *skb,
+					struct packet_sock *po)
+{
+	/*
+	 * We overload the mark field here, since we're about
+	 * to enqueue to a receive queue and no body else will
+	 * use this field at this point
+	 */
+	skb->mark = po->stats.tp_gap;
+	po->stats.tp_gap = 0;
+	return;
+
+}
+
+static inline __u32 check_packet_gap(struct sk_buff *skb)
+{
+	return skb->mark;
+}
+
 /*
    This function makes lazy skb cloning in hope that most of packets
    are discarded by BPF.
@@ -626,6 +651,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 
 	spin_lock(&sk->sk_receive_queue.lock);
 	po->stats.tp_packets++;
+	record_packet_gap(skb, po);
 	__skb_queue_tail(&sk->sk_receive_queue, skb);
 	spin_unlock(&sk->sk_receive_queue.lock);
 	sk->sk_data_ready(sk, skb->len);
@@ -634,6 +660,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 drop_n_acct:
 	spin_lock(&sk->sk_receive_queue.lock);
 	po->stats.tp_drops++;
+	po->stats.tp_gap++;
 	spin_unlock(&sk->sk_receive_queue.lock);
 
 drop_n_restore:
@@ -811,6 +838,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 
 ring_is_full:
 	po->stats.tp_drops++;
+	po->stats.tp_gap++;
 	spin_unlock(&sk->sk_receive_queue.lock);
 
 	sk->sk_data_ready(sk, 0);
@@ -1418,6 +1446,7 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
 	struct sk_buff *skb;
 	int copied, err;
 	struct sockaddr_ll *sll;
+	__u32 gap;
 
 	err = -EINVAL;
 	if (flags & ~(MSG_PEEK|MSG_DONTWAIT|MSG_TRUNC|MSG_CMSG_COMPAT))
@@ -1496,6 +1525,10 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
 		put_cmsg(msg, SOL_PACKET, PACKET_AUXDATA, sizeof(aux), &aux);
 	}
 
+	gap = check_packet_gap(skb);
+	if (gap)
+		put_cmsg(msg, SOL_PACKET, PACKET_GAPDATA, sizeof(__u32), &gap);
+
 	/*
 	 *	Free or return the buffer as appropriate. Again this
 	 *	hides all the races and re-entrancy issues from us.