netdev - Re: [bpf-next PATCH v2 3/4] bpf: sockmap, BPF_F_INGRESS flag for BPF_SK_SKB_STREAM

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Date:   Wed, 28 Mar 2018 17:46:53 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     John Fastabend <john.fastabend@...il.com>, ast@...nel.org
Cc:     netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [bpf-next PATCH v2 3/4] bpf: sockmap, BPF_F_INGRESS flag for
 BPF_SK_SKB_STREAM_VERDICT:

On 03/28/2018 05:45 PM, John Fastabend wrote:
> On 03/28/2018 07:21 AM, Daniel Borkmann wrote:
>> On 03/27/2018 07:23 PM, John Fastabend wrote:
>>> Add support for the BPF_F_INGRESS flag in skb redirect helper. To
>>> do this convert skb into a scatterlist and push into ingress queue.
>>> This is the same logic that is used in the sk_msg redirect helper
>>> so it should feel familiar.
>>>
>>> Signed-off-by: John Fastabend <john.fastabend@...il.com>
>>> ---
>>>  include/linux/filter.h |    1 +
>>>  kernel/bpf/sockmap.c   |   94 +++++++++++++++++++++++++++++++++++++++---------
>>>  net/core/filter.c      |    2 +
>>>  3 files changed, 78 insertions(+), 19 deletions(-)
>> [...]
>>>  		if (!sg->length && md->sg_start == md->sg_end) {
>>>  			list_del(&md->list);
>>> +			if (md->skb)
>>> +				consume_skb(md->skb);
>>>  			kfree(md);
>>>  		}
>>>  	}
>>> @@ -1045,27 +1048,72 @@ static int smap_verdict_func(struct smap_psock *psock, struct sk_buff *skb)
>>>  		__SK_DROP;
>>>  }
>>>  
>>> +static int smap_do_ingress(struct smap_psock *psock, struct sk_buff *skb)
>>> +{
>>> +	struct sock *sk = psock->sock;
>>> +	int copied = 0, num_sg;
>>> +	struct sk_msg_buff *r;
>>> +
>>> +	r = kzalloc(sizeof(struct sk_msg_buff), __GFP_NOWARN | GFP_ATOMIC);
>>> +	if (unlikely(!r))
>>> +		return -EAGAIN;
>>> +
>>> +	if (!sk_rmem_schedule(sk, skb, skb->len)) {
>>> +		kfree(r);
>>> +		return -EAGAIN;
>>> +	}
>>> +	sk_mem_charge(sk, skb->len);
>>
>> Usually mem accounting is based on truesize. This is not done here since
>> you need the exact length of the skb for the sg list later on, right?
> 
> Correct.
> 
>>
>>> +	sg_init_table(r->sg_data, MAX_SKB_FRAGS);
>>> +	num_sg = skb_to_sgvec(skb, r->sg_data, 0, skb->len);
>>> +	if (unlikely(num_sg < 0)) {
>>> +		kfree(r);
>>
>> Don't we need to undo the mem charge here in case of error?
>>
> 
> Actually, I'll just move the sk_mem_charge() down below this error
> then we don't need to unwind it.

Agree, good point.