netdev - RE: [PATCH bpf-next V13 2/7] bpf: fix bpf_fib

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Message-ID: <6013a4869b7df_2683c20839@john-XPS-13-9370.notmuch>
Date:   Thu, 28 Jan 2021 22:00:38 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, bpf@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com,
        colrack@...il.com
Subject: RE: [PATCH bpf-next V13 2/7] bpf: fix bpf_fib_lookup helper MTU check
 for SKB ctx

Jesper Dangaard Brouer wrote:
> BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
> bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
> by adjusting fib_params 'tot_len' with the packet length plus the expected
> encap size. (Just like the bpf_check_mtu helper supports). He discovered
> that for SKB ctx the param->tot_len was not used, instead skb->len was used
> (via MTU check in is_skb_forwardable() that checks against netdev MTU).
> 
> Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.
> zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU
> check is done like XDP code-path, which checks against FIB-dst MTU.
> 
> V13:
> - Only do ifindex lookup one time, calling dev_get_by_index_rcu().
> 
> V10:
> - Use same method as XDP for 'tot_len' MTU check
> 
> Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
> Reported-by: Carlo Carraro <colrack@...il.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---

Acked-by: John Fastabend <john.fastabend@...il.com>