Re: [PATCH bpf-next] xsk: support AF_PACKET

[Daniel Borkmann <daniel@iogearbox.net>]

On 5/28/21 12:00 PM, Magnus Karlsson wrote:
> On Fri, May 28, 2021 at 11:52 AM Jesper Dangaard Brouer
> <brouer@redhat.com> wrote:
>> On Fri, 28 May 2021 17:02:01 +0800
>> Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>>> On Fri, 28 May 2021 10:55:58 +0200, Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>>> Xuan Zhuo <xuanzhuo@linux.alibaba.com> writes:
>>>>
>>>>> In xsk mode, users cannot use AF_PACKET(tcpdump) to observe the current
>>>>> rx/tx data packets. This feature is very important in many cases. So
>>>>> this patch allows AF_PACKET to obtain xsk packages.
>>>>
>>>> You can use xdpdump to dump the packets from the XDP program before it
>>>> gets redirected into the XSK:
>>>> https://github.com/xdp-project/xdp-tools/tree/master/xdp-dump
>>>
>>> Wow, this is a good idea.
>>
>> Yes, it is rather cool (credit to Eelco).  Notice the extra info you
>> can capture from 'exit', like XDP return codes, if_index, rx_queue.
>>
>> The tool uses the perf ring-buffer to send/copy data to userspace.
>> This is actually surprisingly fast, but I still think AF_XDP will be
>> faster (but it usually 'steals' the packet).
>>
>> Another (crazy?) idea is to extend this (and xdpdump), is to leverage
>> Hangbin's recent XDP_REDIRECT extension e624d4ed4aa8 ("xdp: Extend
>> xdp_redirect_map with broadcast support").  We now have a
>> xdp_redirect_map flag BPF_F_BROADCAST, what if we create a
>> BPF_F_CLONE_PASS flag?
>>
>> The semantic meaning of BPF_F_CLONE_PASS flag is to copy/clone the
>> packet for the specified map target index (e.g AF_XDP map), but
>> afterwards it does like veth/cpumap and creates an SKB from the
>> xdp_frame (see __xdp_build_skb_from_frame()) and send to netstack.
>> (Feel free to kick me if this doesn't make any sense)
> 
> This would be a smooth way to implement clone support for AF_XDP. If
> we had this and someone added AF_XDP support to libpcap, we could both
> capture AF_XDP traffic with tcpdump (using this clone functionality in
> the XDP program) and speed up tcpdump for dumping traffic destined for
> regular sockets. Would that solve your use case Xuan? Note that I have
> not looked into the BPF_F_CLONE_PASS code, so do not know at this
> point what it would take to support this for XSKMAPs.

Recently also ended up with something similar for our XDP LB to record pcaps [0] ;)
My question is.. tcpdump doesn't really care where the packet data comes from,
so why not extending libpcap's Linux-related internals to either capture from
perf RB or BPF ringbuf rather than AF_PACKET sockets? Cloning is slow, and if
you need to end up creating an skb which is then cloned once again inside AF_PACKET
it's even worse. Just relying and reading out, say, perf RB you don't need any
clones at all.

Thanks,
Daniel

   [0] https://cilium.io/blog/2021/05/20/cilium-110#pcap