netdev - Re: [RFC net-next PATCH 1/5] samples/bpf: xdp_tx_iptunnel make use of map

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Date:   Fri, 19 May 2017 17:45:25 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     John Fastabend <john.r.fastabend@...el.com>, netdev@...r.kernel.org
Subject: Re: [RFC net-next PATCH 1/5] samples/bpf: xdp_tx_iptunnel make use
 of map_data[]

On 05/18/2017 05:41 PM, Jesper Dangaard Brouer wrote:
> There is no reason to use a compile time constant MAX_IPTNL_ENTRIES
> shared between the _user.c and _kern.c, when map_data[].def.max_entries
> can tell us dynamically what the max_entries were of the ELF map that
> the bpf loaded created.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Previous code was perhaps a bit more robust in the sense that the
order of the map wouldn't matter due to MAX_IPTNL_ENTRIES being
shared. Now you rely on it being in slot 1 (map_data[1].def.max_entries)
from "maps" section in ELF.

>   samples/bpf/xdp_tx_iptunnel_common.h |    2 --
>   samples/bpf/xdp_tx_iptunnel_kern.c   |    2 +-
>   samples/bpf/xdp_tx_iptunnel_user.c   |   14 +++++++++-----
>   3 files changed, 10 insertions(+), 8 deletions(-)

Not sure it's worth it given this actually adds more code and makes
it more fragile at the same time. Only point I could see is to demo
usage of map_data[1].def.max_entries for sample code.

Perhaps at the very minimum add a warning comment to xdp_tx_iptunnel_kern.c
that should the code be further extended with additional maps, that
ordering of struct bpf_map_def entries really matters here to not break
the _user.c part.

Other than that, this should be sent as stand-alone "cleanup" ...