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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Stanislav Fomichev <sdf@fomichev.me>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
	Stanislav Fomichev <sdf@google.com>,
	Network Development <netdev@vger.kernel.org>,
	David Miller <davem@davemloft.net>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	simon.horman@netronome.com, Willem de Bruijn <willemb@google.com>
Subject: Re: [RFC bpf-next 0/7] net: flow_dissector: trigger BPF hook when called from eth_get_headlen
Date: Wed, 13 Feb 2019 22:38:50 -0800	[thread overview]
Message-ID: <20190214063848.7zr5ph3ijp3wmjgx@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20190214055725.GC10595@mini-arch>

On Wed, Feb 13, 2019 at 09:57:25PM -0800, Stanislav Fomichev wrote:
> 
> > That 'stuck with __sk_buff' is what bothers me.
> I might have use the wrong word here. I don't think there is another
> option to be honest. Using __sk_buff makes flow dissector programs work
> with fragmented packets;

good point. indeed real skb is essential.

> > It's an indication that api wasn't thought through if first thing
> > it needs is this fake skb hack.
> > If bpf_flow.c is a realistic example of such flow dissector prog
> > it means that real skb fields are accessed.
> > In particular skb->vlan_proto, skb->protocol.
> I do manually set skb->protocol to eth->h_proto in my proposal. This is later
> correctly handled by bpf_flow.c: parse_eth_proto() is called on skb->protocol
> and we correctly handle bpf_htons(ETH_P_8021Q) there. So existing
> bpf_flow.c works as expected.
...
> The goal of this patch series was to essentially make this skb/no-skb
> context transparent to the bpf_flow.c (i.e. no changes from the user
> flow programs). Adding another flow dissector for eth_get_headlen case
> also seems as a no go.

The problem with this thinking is assumption that bpf_flow.c is the only program.
Since ctx of flow_dissector prog type is 'struct __sk_buff'
all fields should be valid or the verifier has to reject access
to fields that were not set.
You cannot "manually set skb->protocol to eth->h_proto" in fake skb
and ignore the rest.


  reply	other threads:[~2019-02-14  6:38 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-05 17:36 [RFC bpf-next 0/7] net: flow_dissector: trigger BPF hook when called from eth_get_headlen Stanislav Fomichev
2019-02-05 17:36 ` [RFC bpf-next 1/7] net: introduce __init_skb and __init_skb_shinfo helpers Stanislav Fomichev
2019-02-05 20:18   ` Willem de Bruijn
2019-02-05 17:36 ` [RFC bpf-next 2/7] net: introduce skb_net helper Stanislav Fomichev
2019-02-05 20:19   ` Willem de Bruijn
2019-02-05 20:42     ` Stanislav Fomichev
2019-02-05 17:36 ` [RFC bpf-next 3/7] net: plumb network namespace into __skb_flow_dissect Stanislav Fomichev
2019-02-05 20:19   ` Willem de Bruijn
2019-02-05 20:40     ` Stanislav Fomichev
2019-02-05 17:36 ` [RFC bpf-next 4/7] net: flow_dissector: handle no-skb use case Stanislav Fomichev
2019-02-05 20:19   ` Willem de Bruijn
2019-02-05 20:45     ` Stanislav Fomichev
2019-02-05 17:36 ` [RFC bpf-next 5/7] bpf: when doing BPF_PROG_TEST_RUN for flow dissector use no-skb mode Stanislav Fomichev
2019-02-05 20:19   ` Willem de Bruijn
2019-02-05 17:36 ` [RFC bpf-next 6/7] selftests/bpf: add flow dissector bpf_skb_load_bytes helper test Stanislav Fomichev
2019-02-05 17:36 ` [RFC bpf-next 7/7] net: flow_dissector: pass net argument to the eth_get_headlen Stanislav Fomichev
2019-02-05 20:18 ` [RFC bpf-next 0/7] net: flow_dissector: trigger BPF hook when called from eth_get_headlen Willem de Bruijn
2019-02-05 20:40   ` Stanislav Fomichev
2019-02-06  0:47     ` Alexei Starovoitov
2019-02-06  0:59       ` Stanislav Fomichev
2019-02-06  3:12         ` Alexei Starovoitov
2019-02-06  3:56           ` Stanislav Fomichev
2019-02-06  4:11             ` Alexei Starovoitov
2019-02-06  5:49               ` Stanislav Fomichev
2019-02-12 17:02                 ` Stanislav Fomichev
2019-02-14  4:39                   ` Alexei Starovoitov
2019-02-14  5:57                     ` Stanislav Fomichev
2019-02-14  6:38                       ` Alexei Starovoitov [this message]
2019-02-14 17:35                         ` Stanislav Fomichev
2019-02-25 20:33                           ` Stanislav Fomichev

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