From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: David Ahern <dsahern@gmail.com>, daniel@iogearbox.net, ast@fb.com
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH bpf-next] bpf_fib_lookup: return target ifindex even if neighbour lookup fails
Date: Thu, 08 Oct 2020 22:57:47 +0200 [thread overview]
Message-ID: <87d01se8qc.fsf@toke.dk> (raw)
In-Reply-To: <da1b5e5f-edb3-4384-c748-8170f51f6f6d@gmail.com>
David Ahern <dsahern@gmail.com> writes:
> On 10/8/20 7:53 AM, Toke Høiland-Jørgensen wrote:
>> 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 pass the packet up the stack in this case.
>> However, with the addition of bpf_redirect_neigh() that can be used instead
>> to perform the neighbour lookup.
>>
>> However, for that we still need the target ifindex, and since
>> bpf_fib_lookup() already has that at the time it performs the neighbour
>> lookup, there is really no reason why it can't just return it in any case.
>> With this fix, a BPF program can do the following to perform a redirect
>> based on the routing table that will succeed even if there is no neighbour
>> entry:
>>
>> ret = bpf_fib_lookup(skb, &fib_params, sizeof(fib_params), 0);
>> if (ret == BPF_FIB_LKUP_RET_SUCCESS) {
>> __builtin_memcpy(eth->h_dest, fib_params.dmac, ETH_ALEN);
>> __builtin_memcpy(eth->h_source, fib_params.smac, ETH_ALEN);
>>
>> return bpf_redirect(fib_params.ifindex, 0);
>> } else if (ret == BPF_FIB_LKUP_RET_NO_NEIGH) {
>> return bpf_redirect_neigh(fib_params.ifindex, 0);
>> }
>>
>
> There are a lot of assumptions in this program flow and redundant work.
> fib_lookup is generic and allows the caller to control the input
> parameters. direct_neigh does a fib lookup based on network header data
> from the skb.
>
> I am fine with the patch, but users need to be aware of the subtle details.
Yeah, I'm aware they are not equivalent; the code above was just meant
as a minimal example motivating the patch. If you think it's likely to
confuse people to have this example in the commit message, I can remove
it?
-Toke
next prev parent reply other threads:[~2020-10-08 20:57 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-08 14:53 [PATCH bpf-next] bpf_fib_lookup: return target ifindex even if neighbour lookup fails Toke Høiland-Jørgensen
2020-10-08 15:08 ` Daniel Borkmann
2020-10-08 20:59 ` Toke Høiland-Jørgensen
2020-10-08 21:02 ` Daniel Borkmann
2020-10-08 21:04 ` Toke Høiland-Jørgensen
2020-10-08 17:22 ` David Ahern
2020-10-08 20:57 ` Toke Høiland-Jørgensen [this message]
2020-10-08 21:58 ` David Ahern
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