netdev - Re: [PATCH bpf-next] bpf: Extend the sk_lookup() helper to XDP hookpoint.

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Date:   Thu, 18 Oct 2018 23:20:17 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Joe Stringer <joe@...d.net.nz>, Nitin Hande <nitin.hande@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, ast@...nel.org,
        Jesper Brouer <brouer@...hat.com>,
        john fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH bpf-next] bpf: Extend the sk_lookup() helper to XDP
 hookpoint.

On 10/18/2018 11:06 PM, Joe Stringer wrote:
> On Thu, 18 Oct 2018 at 11:54, Nitin Hande <nitin.hande@...il.com> wrote:
[...]
>> Open Issue
>> * The underlying code relies on presence of an skb to find out the
>> right sk for the case of REUSEPORT socket option. Since there is
>> no skb available at XDP hookpoint, the helper function will return
>> the first available sk based off the 5 tuple hash. If the desire
>> is to return a particular sk matching reuseport_cb function, please
>> suggest way to tackle it, which can be addressed in a future commit.

>> Signed-off-by: Nitin Hande <Nitin.Hande@...il.com>
> 
> Thanks Nitin, LGTM overall.
> 
> The REUSEPORT thing suggests that the usage of this helper from XDP
> layer may lead to a different socket being selected vs. the equivalent
> call at TC hook, or other places where the selection may occur. This
> could be a bit counter-intuitive.
> 
> One thought I had to work around this was to introduce a flag,
> something like BPF_F_FIND_REUSEPORT_SK_BY_HASH. This flag would
> effectively communicate in the API that the bpf_sk_lookup_xxx()
> functions will only select a REUSEPORT socket based on the hash and
> not by, for example BPF_PROG_TYPE_SK_REUSEPORT programs. The absence
> of the flag would support finding REUSEPORT sockets by other
> mechanisms (which would be allowed for now from TC hooks but would be
> disallowed from XDP, since there's no specific plan to support this).

Hmm, given skb is NULL here the only way to lookup the socket in such
scenario is based on hash, that is, inet_ehashfn() / inet6_ehashfn(),
perhaps alternative is to pass this hash in from XDP itself to the
helper so it could be custom selector. Do you have a specific use case
on this for XDP (just curious)?

Thanks,
Daniel