netdev - Re: [PATCH net-next v3 4/4] net: tun: track dropped skb via kfree_skb

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Date:   Mon, 21 Feb 2022 20:28:33 -0700
From:   David Ahern <dsahern@...il.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        rostedt@...dmis.org, mingo@...hat.com, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, imagedong@...cent.com,
        joao.m.martins@...cle.com, joe.jin@...cle.com, edumazet@...gle.com
Subject: Re: [PATCH net-next v3 4/4] net: tun: track dropped skb via
 kfree_skb_reason()

On 2/20/22 10:34 PM, Dongli Zhang wrote:
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index aa27268..bf7d8cd 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;
> +	enum skb_drop_reason drop_reason;

this function is already honoring reverse xmas tree style, so this needs
to be moved up.

>  
>  	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_READY;
>  		goto drop;
> +	}
>  
>  	if (!rcu_dereference(tun->steering_prog))
>  		tun_automq_xmit(tun, skb);
> @@ -1078,22 +1081,32 @@ 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_DEV_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_SOCKET_FILTER;
>  		goto drop;
> +	}
>  
>  	len = run_ebpf_filter(tun, skb, len);
> -	if (len == 0)
> +	if (len == 0) {
> +		drop_reason = SKB_DROP_REASON_BPF_FILTER;

how does this bpf filter differ from SKB_DROP_REASON_SOCKET_FILTER? I
think the reason code needs to be a little clearer on the distinction.