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.
Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
Reported-by: Carlo Carraro <colrack@xxxxxxxxx>
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
net/core/filter.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/net/core/filter.c b/net/core/filter.c
index 1ee97fdeea64..84d77c425fbe 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5565,11 +5565,21 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
#endif
}
- if (!rc) {
+ if (rc == BPF_FIB_LKUP_RET_SUCCESS) {
struct net_device *dev;
+ u32 mtu;
dev = dev_get_by_index_rcu(net, params->ifindex);
- if (!is_skb_forwardable(dev, skb))
+ mtu = READ_ONCE(dev->mtu);
+
+ /* Using tot_len for (L3) MTU check if provided by user */
+ if (params->tot_len && params->tot_len > mtu)
+ rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
+
+ /* Notice at this TC cls_bpf level skb->len contains L2 size,
+ * but is_skb_forwardable takes that into account
+ */
+ if (params->tot_len == 0 && !is_skb_forwardable(dev, skb))
rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
}
