Subject: Re: [PATCH] af_packet: add interframe drop cmsg

From: Eric Dumazet <[email-masked]>
Date: Wed, 23 Sep 2009 22:55:12 +0200

Neil Horman a écrit :
>     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) attaching ancilliary data to any skb enqueued to a socket recieve queue for
>     which frames were lost between it and the previously enqueued frame.  Note I use
>     a ring buffer with a correlator value (the skb pointer) to do this.  This was
>     done because the skb->cb array is exhausted already for AF_PACKET

Hmm, how mmap() users can find this information ? I thought recent libpcap were
using mmap(), in order to reduce losses :)

>     
>     B) For any frame dequeued that has ancilliary data in the ring buffer (as
>     determined by the correlator value), we add a cmsg structure to the msghdr that
>     gets copied to user space, this cmsg structure is of cmsg_level AF_PACKET, and
>     cmsg_type PACKET_GAPDATA.  It contains a u32 value which counts the number of
>     frames lost between the reception of the frame being currently recevied and the
>     frame most recently preceding it.  Note this creates a situation in which if we
>     have packet loss followed immediately by a socket close operation we might miss
>     some gap information.  This situation is covered by the use of the
>     PACKET_AUXINFO socket option, which provides total loss stats (from which the
>     final gap can be computed).
>     
>     I've tested this patch myself, and it works well.

Okay :)

>     
>     Signed-off-by: Neil Horman <[email-masked]>
> 
> 
>  include/linux/if_packet.h |    2 +
>  net/packet/af_packet.c    |   90 +++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 91 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/if_packet.h b/include/linux/if_packet.h
> index dea7d6b..e5d200f 100644
> --- a/include/linux/if_packet.h
> +++ b/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
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index d3d52c6..b74a91c 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -179,6 +179,11 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg);
>  
>  static void packet_flush_mclist(struct sock *sk);
>  
> +struct packet_gap_record {
> +		struct sk_buff *skb;
> +		__u32 gap;
> +};
> +
>  struct packet_sock {
>  	/* struct sock has to be the first member of packet_sock */
>  	struct sock		sk;
> @@ -197,6 +202,11 @@ struct packet_sock {
>  	int			ifindex;	/* bound device		*/
>  	__be16			num;
>  	struct packet_mclist	*mclist;
> +	struct packet_gap_record *gaps;
> +	unsigned int gap_head;
> +	unsigned int gap_tail;
> +	unsigned int gap_list_size;
> +
>  #ifdef CONFIG_PACKET_MMAP
>  	atomic_t		mapped;
>  	enum tpacket_versions	tp_version;
> @@ -524,6 +534,55 @@ static inline unsigned int run_filter(struct sk_buff *skb, struct sock *sk,
>  }
>  
>  /*
> + * 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 void record_packet_gap(struct sk_buff *skb, struct packet_sock *po)
> +{
> +	/*
> +	 * do nothing if there is no gap
> +	 */
> +	if (!po->stats.tp_gap)
> +		return;
> +
> +	/*
> +	 * If there is, check the gap list tail to make sure we
> +	 * have an open entry
> +	 */
> +	if (po->gaps[po->gap_tail].skb != NULL) {
> +		if (net_ratelimit())
> +			printk(KERN_WARNING "packet socket gap list is full!\n");

New code can use pr_warning() macro

> +		return;
> +	}
> +
> +	/*
> +	 * We have a free entry, record it
> +	 */
> +	po->gaps[po->gap_tail].skb = skb;
> +	po->gaps[po->gap_tail].gap = po->stats.tp_gap;
> +	po->gap_tail = (po->gap_tail+1) % po->gap_list_size;

you could avoid this divide

	if (++po->gap_tail == po->gap_list_size)
		po->gap_tail = 0;

> +	po->stats.tp_gap = 0;
> +	return;
> +
> +}
> +
> +static __u32 check_packet_gap(struct sk_buff *skb, struct packet_sock *po)
> +{
> +	__u32 gap = 0;
> +
> +	if (po->gaps[po->gap_head].skb != skb)
> +		return 0;
> +
> +	gap = po->gaps[po->gap_head].gap;
> +	po->gaps[po->gap_head].skb = NULL;
> +	po->gap_head = (po->gap_head + 1) % po->gap_list_size;

ditto

> +	return gap;
> +}
> +
> +
> +/*
>     This function makes lazy skb cloning in hope that most of packets
>     are discarded by BPF.
>  
> @@ -626,6 +685,7 @@ 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 +694,7 @@ 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 +872,7 @@ drop:
>  
>  ring_is_full:
>  	po->stats.tp_drops++;
> +	po->stats.tp_gap++;
>  	spin_unlock(&sk->sk_receive_queue.lock);
>  
>  	sk->sk_data_ready(sk, 0);
> @@ -1223,6 +1285,8 @@ static int packet_release(struct socket *sock)
>  	skb_queue_purge(&sk->sk_receive_queue);
>  	sk_refcnt_debug_release(sk);
>  
> +	kfree(po->gaps);
> +
>  	sock_put(sk);
>  	return 0;
>  }
> @@ -1350,6 +1414,7 @@ static int packet_create(struct net *net, struct socket *sock, int protocol)
>  	struct packet_sock *po;
>  	__be16 proto = (__force __be16)protocol; /* weird, but documented */
>  	int err;
> +	unsigned int num_records = PAGE_SIZE/sizeof(struct packet_gap_record);
>  
>  	if (!capable(CAP_NET_RAW))
>  		return -EPERM;
> @@ -1360,6 +1425,7 @@ static int packet_create(struct net *net, struct socket *sock, int protocol)
>  	sock->state = SS_UNCONNECTED;
>  
>  	err = -ENOBUFS;
> +
>  	sk = sk_alloc(net, PF_PACKET, GFP_KERNEL, &packet_proto);
>  	if (sk == NULL)
>  		goto out;
> @@ -1374,6 +1440,19 @@ static int packet_create(struct net *net, struct socket *sock, int protocol)
>  	sk->sk_family = PF_PACKET;
>  	po->num = proto;
>  
> +	err = -ENOMEM;
> +	po->gaps = kmalloc(sizeof(struct packet_gap_record)*num_records,
> +				GFP_KERNEL);

kzalloc(), and no need for some following lines

> +	if (!po->gaps)
> +		goto out_free;
> +	po->gap_tail = po->gap_head = 0;
> +	po->gap_list_size = num_records;
> +
> +	for (num_records = 0; num_records < po->gap_list_size; num_records++) {
> +		po->gaps[num_records].skb = NULL;
> +		po->gaps[num_records].gap = 0;
> +	}
> +
>  	sk->sk_destruct = packet_sock_destruct;
>  	sk_refcnt_debug_inc(sk);
>  
> @@ -1402,6 +1481,9 @@ static int packet_create(struct net *net, struct socket *sock, int protocol)
>  	sock_prot_inuse_add(net, &packet_proto, 1);
>  	write_unlock_bh(&net->packet.sklist_lock);
>  	return 0;
> +
> +out_free:
> +	sk_free(sk);
>  out:
>  	return err;
>  }
> @@ -1418,6 +1500,7 @@ 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))
> @@ -1492,10 +1575,15 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
>  		aux.tp_mac = 0;
>  		aux.tp_net = skb_network_offset(skb);
>  		aux.tp_vlan_tci = skb->vlan_tci;
> -

Please dont mix cleanups 

>  		put_cmsg(msg, SOL_PACKET, PACKET_AUXDATA, sizeof(aux), &aux);
>  	}
>  
> +	lock_sock(sk);

strange locking here. this doesnt match locking used at record time.

( spin_lock(&sk->sk_receive_queue.lock);)

> +	gap = check_packet_gap(skb, pkt_sk(sk));
> +	release_sock(sk);
> +	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.

Thanks