Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx
From: Jesper Dangaard Brouer <hidden>
Date: 2020-11-20 08:26:56
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bpf
On Wed, 18 Nov 2020 16:29:35 +0100 Jesper Dangaard Brouer [off-list ref] 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()). Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g. zero) then keep existing behaviour intact.
Carlo pointed out (in slack) that the logic is not correctly implemented in this patch. I will send a V7.
quoted hunk ↗ jump to hunk
Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status") Reported-by: Carlo Carraro <redacted> Signed-off-by: Jesper Dangaard Brouer <redacted> --- net/core/filter.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)diff --git a/net/core/filter.c b/net/core/filter.c index 1ee97fdeea64..ae1fe8e6069a 100644 --- a/net/core/filter.c +++ b/net/core/filter.c@@ -5567,10 +5567,20 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb, if (!rc) { struct net_device *dev; + u32 mtu; dev = dev_get_by_index_rcu(net, params->ifindex); - if (!is_skb_forwardable(dev, skb)) + mtu = dev->mtu; + + /* Using tot_len for L3 MTU check if provided by user. Notice at + * this TC cls_bpf level skb->len contains L2 size, but + * is_skb_forwardable takes that into account. + */ + 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; + } } return rc;
-- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer