Re: [PATCH 2/2] net: tun: track dropped skb via kfree_skb_reason()

[David Ahern <dsahern@gmail.com>]

On 2/7/22 7:55 PM, Dongli Zhang wrote:
> The TUN can be used as vhost-net backend. E.g, the tun_net_xmit() is the
> interface to forward the skb from TUN to vhost-net/virtio-net.
> 
> However, there are many "goto drop" in the TUN driver. Therefore, the
> kfree_skb_reason() is involved at each "goto drop" to help userspace
> ftrace/ebpf to track the reason for the loss of packets.
> 
> Cc: Joao Martins <joao.m.martins@oracle.com>
> Cc: Joe Jin <joe.jin@oracle.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---
>  drivers/net/tun.c          | 33 +++++++++++++++++++++++++--------
>  include/linux/skbuff.h     |  6 ++++++
>  include/trace/events/skb.h |  6 ++++++
>  3 files changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index fed85447701a..d67f2419dbb4 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1062,13 +1062,16 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	struct netdev_queue *queue;
>  	struct tun_file *tfile;
>  	int len = skb->len;
> +	int drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;


>  
>  	rcu_read_lock();
>  	tfile = rcu_dereference(tun->tfiles[txq]);
>  
>  	/* Drop packet if interface is not attached */
> -	if (!tfile)
> +	if (!tfile) {
> +		drop_reason = SKB_DROP_REASON_DEV_NOT_ATTACHED;

That is going to be a confusing reason code (tap device existed to get
here) and does not really explain this error.


>  		goto drop;
> +	}
>  
>  	if (!rcu_dereference(tun->steering_prog))
>  		tun_automq_xmit(tun, skb);
> @@ -1078,19 +1081,27 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	/* Drop if the filter does not like it.
>  	 * This is a noop if the filter is disabled.
>  	 * Filter can be enabled only for the TAP devices. */
> -	if (!check_filter(&tun->txflt, skb))
> +	if (!check_filter(&tun->txflt, skb)) {
> +		drop_reason = SKB_DROP_REASON_TAP_RUN_FILTER;

just SKB_DROP_REASON_TAP_FILTER

>  		goto drop;
> +	}
>  
>  	if (tfile->socket.sk->sk_filter &&
> -	    sk_filter(tfile->socket.sk, skb))
> +	    sk_filter(tfile->socket.sk, skb)) {
> +		drop_reason = SKB_DROP_REASON_SKB_TRIM;

SKB_DROP_REASON_SOCKET_FILTER

The remainder of your changes feels like another variant of your
previous "function / line" reason code. You are creating new reason
codes for every goto failure with a code based name. The reason needs to
be the essence of the failure in a user friendly label.