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Fri, 20 Nov 2020 07:53:48 -0800 (PST) Subject: Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx To: Carlo Carraro , Jesper Dangaard Brouer Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Daniel Borkmann , Alexei Starovoitov , maze@google.com, lmb@cloudflare.com, shaun@tigera.io, Lorenzo Bianconi , marek@cloudflare.com, John Fastabend , Jakub Kicinski , eyal.birger@gmail.com References: <160571331409.2801246.11527010115263068327.stgit@firesoul> <160571337537.2801246.15228178384451037535.stgit@firesoul> <20201120092638.14e09025@carbon> From: David Ahern Message-ID: Date: Fri, 20 Nov 2020 08:53:47 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.4.3 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org 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.