On Mon, 02 Nov 2020 10:04:44 -0800
John Fastabend [off-list ref] wrote:

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+
+	/*  Same relax as xdp_ok_fwd_dev() and is_skb_forwardable() */
+	if (flags & BPF_MTU_CHK_RELAX)
+		mtu += VLAN_HLEN;    

I'm trying to think about the use case where this might be used?
Compared to just adjusting MTU in BPF program side as needed for
packet encapsulation/headers/etc.  

As I wrote above, this were added because the kernels own forwarding
have this relaxation in it's checks (in is_skb_forwardable()).  I even
tried to dig through the history, introduced in [1] and copy-pasted
in[2].  And this seems to be a workaround, that have become standard,
that still have practical implications.

My practical experiments showed, that e.g. ixgbe driver with MTU=1500
(L3-size) will allow and fully send packets with 1504 (L3-size). But
i40e will not, and drops the packet in hardware/firmware step.  So,
what is the correct action, strict or relaxed?

My own conclusion is that we should inverse the flag.  Meaning to
default add this VLAN_HLEN (4 bytes) relaxation, and have a flag to do
more strict check,  e.g. BPF_MTU_CHK_STRICT. As for historical reasons
we must act like kernels version of MTU check. Unless you object, I will
do this in V6.  

I'm fine with it either way as long as its documented in the helper
description so I have a chance of remembering this discussion in 6 months.
But, if you make it default won't this break for XDP cases? I assume the
XDP use case doesn't include the VLAN 4-bytes. Would you need to prevent
the flag from being used from XDP?

XDP actually do include the VLAN_HLEN 4-bytes, see xdp_ok_fwd_dev(). I
was so certain that you John added this code, but looking through git
blame it pointed back to myself.  Going 5 levels git history deep and
3+ years, does seem like I move/reused some of Johns code containing
VLAN_HLEN in the MTU check, introduced for xdp-generic (6103aa96ec077)
which I acked.  Thus, I guess I cannot push this away and have to take
blame myself ;-)

I conclude that we default need to include this VLAN_HLEN, else the XDP
bpf_check_mtu could say deny, while it would have passed the check in
xdp_ok_fwd_dev().  As i40e will drop 1504 this at HW/FW level, I still
see a need for a BPF_MTU_CHK_STRICT flag for programs that want to
catch this.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer