From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: David Miller <davem@davemloft.net>,
netdev@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <ast@kernel.org>,
brouer@redhat.com, Jonathan Lemon <jonathan.lemon@gmail.com>,
Arnaldo Carvalho de Melo <acme@redhat.com>,
Jiri Olsa <jolsa@redhat.com>
Subject: Re: [PATCH net-next 2/2] devmap: Allow map lookups from eBPF
Date: Tue, 04 Jun 2019 20:42:49 +0200 [thread overview]
Message-ID: <87blzdfaza.fsf@toke.dk> (raw)
In-Reply-To: <20190604183559.10db09d2@carbon>
Jesper Dangaard Brouer <brouer@redhat.com> writes:
> On Tue, 04 Jun 2019 17:24:10 +0200
> Toke Høiland-Jørgensen <toke@redhat.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?
Well, we already have UABI on the insert side, where the value being
inserted has to be an ifindex. And we enforce value_size==4 when
creating the map. So IMO I'm just keeping to the already established
UAPI here.
That being said...
> Can we somehow use BTF to help us to make this extensible?
... this would not necessarily be a bad thing, it just needs to be done
on both the insert and lookup sides.
But I think this is a separate issue, which we need to solve anyway. And
I'm still not convinced that the map value is the right place to specify
what resources we want ;)
-Toke
next prev parent reply other threads:[~2019-06-04 18:42 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-04 15:24 [PATCH net-next 0/2] xdp: Allow lookup into devmaps before redirect Toke Høiland-Jørgensen
2019-06-04 15:24 ` [PATCH net-next 1/2] bpf_xdp_redirect_map: Add flag to return XDP_PASS on map lookup failure Toke Høiland-Jørgensen
2019-06-05 10:39 ` Jesper Dangaard Brouer
2019-06-05 15:09 ` Jonathan Lemon
2019-06-06 10:01 ` Toke Høiland-Jørgensen
2019-06-04 15:24 ` [PATCH net-next 2/2] devmap: Allow map lookups from eBPF Toke Høiland-Jørgensen
2019-06-04 16:35 ` Jesper Dangaard Brouer
2019-06-04 18:42 ` Toke Høiland-Jørgensen [this message]
2019-06-04 19:41 ` Jonathan Lemon
2019-06-04 20:00 ` Toke Høiland-Jørgensen
2019-06-04 20:22 ` Toke Høiland-Jørgensen
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