From: "Jonathan Lemon" <jonathan.lemon@gmail.com>
To: "Jesper Dangaard Brouer" <brouer@redhat.com>
Cc: "Toke Høiland-Jørgensen" <toke@redhat.com>,
"David Miller" <davem@davemloft.net>,
netdev@vger.kernel.org, "Daniel Borkmann" <daniel@iogearbox.net>,
"Alexei Starovoitov" <ast@kernel.org>,
"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 12:41:20 -0700 [thread overview]
Message-ID: <A5CA54A9-34A0-4583-84E8-0530BAEE215B@gmail.com> (raw)
In-Reply-To: <20190604183559.10db09d2@carbon>
On 4 Jun 2019, at 9:35, Jesper Dangaard Brouer wrote:
> 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?
>
> 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
next prev parent reply other threads:[~2019-06-04 19:41 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
2019-06-04 19:41 ` Jonathan Lemon [this message]
2019-06-04 20:00 ` Toke Høiland-Jørgensen
2019-06-04 20:22 ` Toke Høiland-Jørgensen
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