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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 <>
Signed-off-by: Eric Dumazet <>

 include/linux/if_packet.h |    2 ++
 net/packet/af_packet.c    |   33 +++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)
Signed-off-by: David S. Miller <>
Neil Horman 11 years ago
committed by David S. Miller
2 changed files with 35 additions and 0 deletions
  1. +2
  2. +33

+ 2
- 0
include/linux/if_packet.h View File

@ -48,11 +48,13 @@ struct sockaddr_ll
#define PACKET_TX_RING 13
#define PACKET_LOSS 14
struct tpacket_stats
unsigned int tp_packets;
unsigned int tp_drops;
unsigned int tp_gap;
struct tpacket_auxdata

+ 33
- 0
net/packet/af_packet.c View File

@ -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;
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,
record_packet_gap(skb, po);
__skb_queue_tail(&sk->sk_receive_queue, skb);
sk->sk_data_ready(sk, skb->len);
@ -634,6 +660,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
@ -811,6 +838,7 @@ drop:
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;
@ -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.