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

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Date:   Fri, 8 Jun 2018 15:24:12 -0700
From:   Tushar Dave <tushar.n.dave@...cle.com>
To:     Daniel Borkmann <daniel@...earbox.net>, 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 02:46 PM, Tushar Dave wrote:
> 
> 
> On 06/08/2018 02:27 PM, Daniel Borkmann wrote:
>> 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.
> Thanks Daniel for your reply.
>>
>> 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.
> I'm looking into bpf_skb_load_bytes and friends.

Daniel,

While I am trying to see if I can use exiting bpf_skb_load helpers, I am
wondering socket filter based ebpf program are allowed to change packet
data? In other words, can we use them to build firewall?

Thanks.

-Tushar
> 
> Thanks.
> -Tushar
>>
>> Thanks,
>> Daniel
>>
>