netdev - Re: [RFC bpf-next 0/7] net: flow_dissector: trigger BPF hook when called from eth_get

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Date:   Tue, 5 Feb 2019 15:18:07 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        simon.horman@...ronome.com, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [RFC bpf-next 0/7] net: flow_dissector: trigger BPF hook when
 called from eth_get_headlen

On Tue, Feb 5, 2019 at 12:57 PM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> Currently, when eth_get_headlen calls flow dissector, it doesn't pass any
> skb. Because we use passed skb to lookup associated networking namespace
> to find whether we have a BPF program attached or not, we always use
> C-based flow dissector in this case.
>
> The goal of this patch series is to add new networking namespace argument
> to the eth_get_headlen and make BPF flow dissector programs be able to
> work in the skb-less case.
>
> The series goes like this:
> 1. introduce __init_skb and __init_skb_shinfo; those will be used to
>    initialize temporary skb
> 2. introduce skb_net which can be used to get networking namespace
>    associated with an skb
> 3. add new optional network namespace argument to __skb_flow_dissect and
>    plumb through the callers
> 4. add new __flow_bpf_dissect which constructs temporary on-stack skb
>    (using __init_skb) and calls BPF flow dissector program

The main concern I see with this series is this cost of skb zeroing
for every packet in the device driver receive routine, *independent*
from the real skb allocation and zeroing which will likely happen
later.