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

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Date:   Mon, 21 Feb 2022 20:45:43 -0800
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     David Ahern <dsahern@...il.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()

Hi David,

On 2/21/22 7:28 PM, David Ahern wrote:
> 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.

I will move this up to before "int txq = skb->queue_mapping;".

> 
>>  
>>  	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.
> 

While there is a diff between BPF_FILTER (here) and SOCKET_FILTER ...

... indeed the issue is: there is NO diff between BPF_FILTER (here) and
DEV_FILTER (introduced by the patch).

The run_ebpf_filter() is to run the bpf filter attached to the TUN device (not
socket). This is similar to DEV_FILTER, which is to run a device specific filter.

Initially, I would use DEV_FILTER at both locations. This makes trouble to me as
there would be two places with same reason=DEV_FILTER. I will not be able to
tell where the skb is dropped.

I was thinking about to introduce a SKB_DROP_REASON_DEV_BPF. While I have
limited experience in device specific bpf, the TUN is the only device I know
that has a device specific ebpf filter (by commit aff3d70a07ff ("tun: allow to
attach ebpf socket filter")). The SKB_DROP_REASON_DEV_BPF is not generic enough
to be re-used by other drivers.

Would you mind sharing your suggestion if I would re-use (1)
SKB_DROP_REASON_DEV_FILTER or (2) introduce a new SKB_DROP_REASON_DEV_BPF, which
is for sk_buff dropped by ebpf attached to device (not socket).

To answer your question, the SOCKET_FILTER is for filter attached to socket, the
BPF_FILTER was supposed for ebpf filter attached to device (tun->filter_prog).

Thank you very much!

Dongli Zhang