netdev - Re: [PATCH net-next 0/9] net: Kernel side filtering for route dumps

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Date:   Thu, 11 Oct 2018 12:46:19 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     David Ahern <dsahern@...il.com>,
        Sowmini Varadhan <sowmini.varadhan@...cle.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Cc:     David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        davem@...emloft.net
Subject: Re: [PATCH net-next 0/9] net: Kernel side filtering for route dumps

On 2018-10-11 12:16 p.m., David Ahern wrote:

> IMO, bpf at the fill_info stage is not appropriate.
> 

Somewhere before the skb is formed (and nlmsg is built).
If you go as far as constructing it, then cBPF per what
Sowmini should work; but there will be constructs which
are trickier.

> 
>> skb->sk (hence attached filter) should be available at that point.
>> Classical bpf per Sowmini's example maybe trickier.
>> Better - why dont we have an ebpf hook at this stage and then
>> we dont have to make changes to the kernel when someone adds
>> one more field to the filter?
>>
>> BTW: useful for events as well - not just dumps (as the name
>> fib_dump_filter suggests)
> 
> you mean kernel notifications on internal events?
> 1. there is no user socket when notifications are created and the
> *_fill_info is invoked
> 
> 2. notifications are global going to potentially many sockets. For these
> cases the existing sk_filter is appropriate.

#2 - netlink being a broad/multicast bus with attached listeners.

Yes, you can do it with cBPF but some complexity may occur. Example:
if i was interested to netdevice events of "kind = vxlan &&
admin flag is down" then that is non trivial to do with classical but
would be reasonably comfortable to do with ebpf.
Note:
That filter will work fine for dumps as well since the semantics
are the same.
the win is:  in the future when you just wanna add that one new
filter attribute you dont need a kernel patch in and roll in a
new production kernel.

cheers,
jamal