netdev - Re: [PATCH bpf-next 02/17] bpf: Introduce SK_LOOKUP program type with a dedicated attach point

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Date:   Wed, 06 May 2020 15:53:35 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Lorenz Bauer <lmb@...udflare.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        dccp@...r.kernel.org, kernel-team <kernel-team@...udflare.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Gerrit Renker <gerrit@....abdn.ac.uk>,
        Jakub Kicinski <kuba@...nel.org>,
        Marek Majkowski <marek@...udflare.com>
Subject: Re: [PATCH bpf-next 02/17] bpf: Introduce SK_LOOKUP program type with a dedicated attach point

On Wed, May 06, 2020 at 03:16 PM CEST, Lorenz Bauer wrote:
> On Wed, 6 May 2020 at 13:55, Jakub Sitnicki <jakub@...udflare.com> wrote:

[...]

>> @@ -4012,4 +4051,18 @@ struct bpf_pidns_info {
>>         __u32 pid;
>>         __u32 tgid;
>>  };
>> +
>> +/* User accessible data for SK_LOOKUP programs. Add new fields at the end. */
>> +struct bpf_sk_lookup {
>> +       __u32 family;           /* AF_INET, AF_INET6 */
>> +       __u32 protocol;         /* IPPROTO_TCP, IPPROTO_UDP */
>> +       /* IP addresses allows 1, 2, and 4 bytes access */
>> +       __u32 src_ip4;
>> +       __u32 src_ip6[4];
>> +       __u32 src_port;         /* network byte order */
>> +       __u32 dst_ip4;
>> +       __u32 dst_ip6[4];
>> +       __u32 dst_port;         /* host byte order */
>
> Jakub and I have discussed this off-list, but we couldn't come to an
> agreement and decided to invite
> your opinion.
>
> I think that dst_port should be in network byte order, since it's one
> less exception to the
> rule to think about when writing BPF programs.
>
> Jakub's argument is that this follows __sk_buff->local_port precedent,
> which is also in host
> byte order.

Yes, would be great to hear if there is a preference here.

Small correction, proposed sk_lookup program doesn't have access to
__sk_buff, so perhaps that case matters less.

bpf_sk_lookup->dst_port, the packet destination port, is in host byte
order so that it can be compared against bpf_sock->src_port, socket
local port, without conversion.

But I also see how it can be a surprise for a BPF user that one field has
a different byte order.