netdev - Re: [RFC PATCH 1/3] ebpf: add next_skb

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Date:   Fri, 8 Jun 2018 23:27:59 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Tushar Dave <tushar.n.dave@...cle.com>, netdev@...r.kernel.org,
        ast@...nel.org, davem@...emloft.net, john.fastabend@...il.com,
        jakub.kicinski@...ronome.com, kafai@...com, rdna@...com,
        quentin.monnet@...ronome.com, brakmo@...com, acme@...hat.com
Subject: Re: [RFC PATCH 1/3] ebpf: add next_skb_frag bpf helper for sk filter

On 06/08/2018 11:00 PM, Tushar Dave wrote:
> Today socket filter only deals with linear skbs. This change allows
> ebpf programs to look into non-linear skb e.g. skb frags. This will be
> useful when users need to look into data which is not contained in the
> linear part of skb.

Hmm, I don't think this statement is correct in its form here ... they
can handle non-linear skbs just fine.

Straight forward way is to use bpf_skb_load_bytes(). It's simple and uses
internally skb_header_pointer(), and that one of course walks everything
if it really has to via skb_copy_bits() (page frags _and_ frag list). And
if you need to look into mac/net headers that may otherwise not be accessible
anymore from socket layer, there's bpf_skb_load_bytes_relative() helper
which is effectively doing the negative offset trick from ld_abs/ind more
efficient for multi-byte loads.

Thanks,
Daniel