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 [off-list ref] wrote:

[...]

quoted

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.

quoted

Signed-off-by: Nitin Hande <redacted>

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