Re: [PATCH bpf-next V5 3/5] bpf: add BPF-helper for MTU checking

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Mon, 2 Nov 2020 21:10:34 +0100
Jesper Dangaard Brouer <brouer@redhat.com> wrote:

> On Mon, 02 Nov 2020 10:04:44 -0800
> John Fastabend <john.fastabend@gmail.com> wrote:
> 
> > > > > +
> > > > > +	/*  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.

Disagreeing with myself... I want to keep the BPF_MTU_CHK_RELAX, and
let MTU check use the actual MTU value (adjusted to L2 of-cause).

With the argument, that because some drivers with MTU 1500 will
actually drop frame with MTU 1504 bytes (+14 eth_hdr) frames, it is
wrong to "approve" this MTU size in the check.  A BPF program will know
it is playing with VLAN headers and can choose to violate the MTU check
with 4 bytes.  While BPF programs using other types of encap headers
will get confused that MTU check gives them 4 bytes more, which if used
will get dropped on a subset of drivers.

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