netdev - Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

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Date:   Fri, 20 Nov 2020 08:53:47 -0700
From:   David Ahern <dsahern@...il.com>
To:     Carlo Carraro <colrack@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, 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
Subject: Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check
 for SKB ctx

On 11/20/20 6:15 AM, Carlo Carraro wrote:
> I report here the issue with the previous patch.
> The code is now checking against params->tot_len but then it is still
> using is_skb_forwardable.
> Consider this case where I shrink the packet:
> skb->len == 1520
> dev->mtu == 1500
> params->tot_len == 1480
> So the incoming pkt has len 1520, and the out interface has mtu 1500.
> In this case fragmentation is not needed because params->tot_len < dev->mtu.
> However the code calls is_skb_forwardable and may return false because
> skb->len > dev->mtu, resulting in BPF_FIB_LKUP_RET_FRAG_NEEDED.
> What I propose is using params->tot_len only if provided, without
> falling back to use is_skb_forwardable when provided.
> Something like this:
> 
> if (params->tot_len > 0) {
>   if (params->tot_len > mtu)
>     rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
> } else if (!is_skb_forwardable(dev, skb)) {
>   rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
> }
> 
> However, doing so we are skipping more relaxed MTU checks inside
> is_skb_forwardable, so I'm not sure about this.
> Please comment


Daniel's just proposed patch changes this again (removes the
is_skb_forwardable check). Jesper: you might want to hold off until that
happens.