[PATCH bpf-next V7 2/8] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

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 <redacted>
Signed-off-by: Jesper Dangaard Brouer <redacted>
---
 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;
 	}