netdev - Re: [PATCH net-next 2/2] devmap: Allow map lookups from eBPF

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Date:   Tue, 04 Jun 2019 12:41:20 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Jesper Dangaard Brouer" <brouer@...hat.com>
Cc:     "Toke Høiland-Jørgensen" <toke@...hat.com>,
        "David Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "Alexei Starovoitov" <ast@...nel.org>,
        "Arnaldo Carvalho de Melo" <acme@...hat.com>,
        "Jiri Olsa" <jolsa@...hat.com>
Subject: Re: [PATCH net-next 2/2] devmap: Allow map lookups from eBPF

On 4 Jun 2019, at 9:35, Jesper Dangaard Brouer wrote:

> On Tue, 04 Jun 2019 17:24:10 +0200
> Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
>> We don't currently allow lookups into a devmap from eBPF, because the map
>> lookup returns a pointer directly to the dev->ifindex, which shouldn't be
>> modifiable from eBPF.
>>
>> However, being able to do lookups in devmaps is useful to know (e.g.)
>> whether forwarding to a specific interface is enabled. Currently, programs
>> work around this by keeping a shadow map of another type which indicates
>> whether a map index is valid.
>>
>> To allow lookups, simply copy the ifindex into a scratch variable and
>> return a pointer to this. If an eBPF program does modify it, this doesn't
>> matter since it will be overridden on the next lookup anyway. While this
>> does add a write to every lookup, the overhead of this is negligible
>> because the cache line is hot when both the write and the subsequent
>> read happens.
>
> When we choose the return value, here the ifindex, then this basically
> becomes UABI, right?
>
> Can we somehow use BTF to help us to make this extensible?
>
> As Toke mention in the cover letter, we really want to know if the
> chosen egress have actually enabled/allocated resources for XDP
> transmitting, but as we currently don't have in-kernel way to query
> thus (thus, we cannot expose such info).

Would it be better to add a helper like bpf_map_element_present(), which
just returns a boolean value indicating whether the entry is NULL or not?

This would solve this problem (and my xskmap problem).
-- 
Jonathan