Subject: Re: [PATCH bpf v2 2/3] bpf_fib_lookup: optionally skip neighbour lookup

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 20 Oct 2020 20:10:27 +0200

Daniel Borkmann <[email-masked]> writes:

> On 10/20/20 3:49 PM, David Ahern wrote:
>> On 10/20/20 4:51 AM, Toke Høiland-Jørgensen wrote:
>>> From: Toke Høiland-Jørgensen <[email-masked]>
>>>
>>> The bpf_fib_lookup() helper performs a neighbour lookup for the destination
>>> IP and returns BPF_FIB_LKUP_NO_NEIGH if this fails, with the expectation
>>> that the BPF program will deal with this condition, either by passing the
>>> packet up the stack, or by using bpf_redirect_neigh().
>>>
>>> The neighbour lookup is done via a hash table (through ___neigh_lookup_noref()),
>>> which incurs some overhead. If the caller knows this is likely to fail
>>> anyway, it may want to skip that and go unconditionally to
>>> bpf_redirect_neigh(). For this use case, add a flag to bpf_fib_lookup()
>>> that will make it skip the neighbour lookup and instead always return
>>> BPF_FIB_LKUP_RET_NO_NEIGH (but still populate the gateway and target
>>> ifindex).
>>>
>>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>>> ---
>>>   include/uapi/linux/bpf.h       |   10 ++++++----
>>>   net/core/filter.c              |   16 ++++++++++++++--
>>>   tools/include/uapi/linux/bpf.h |   10 ++++++----
>>>   3 files changed, 26 insertions(+), 10 deletions(-)
>> 
>> Nack. Please don't.
>> 
>> As I mentioned in my reply to Daniel, I would prefer such logic be
>> pushed to the bpf programs. There is no reason for rare run time events
>> to warrant a new flag and new check in the existing FIB helpers. The bpf
>> programs can take the hit of the extra lookup.
>
> Fair enough, lets push it to progs then.

OK, with this and the other comments, this goes back to v1 + the
compilation fix. Will send that as v3...

-Toke