Re: [PATCH bpf-next V15 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

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

On Mon, 8 Feb 2021 16:41:24 +0100
Daniel Borkmann <daniel@iogearbox.net> wrote:

> On 2/8/21 4:20 PM, Jesper Dangaard Brouer wrote:
> > On Mon, 8 Feb 2021 14:57:13 +0100
> > Jesper Dangaard Brouer <brouer@redhat.com> wrote:  
> >> On Fri, 5 Feb 2021 01:06:35 +0100
> >> Daniel Borkmann <daniel@iogearbox.net> wrote:  
> >>> On 2/2/21 5:26 PM, Jesper Dangaard Brouer 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() that checks against netdev MTU).
> >>>>
> >>>> Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.
> >>>> zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU
> >>>> check is done like XDP code-path, which checks against FIB-dst MTU.  
> [...]
> >>>> -	if (!rc) {
> >>>> -		struct net_device *dev;
> >>>> -
> >>>> -		dev = dev_get_by_index_rcu(net, params->ifindex);
> >>>> +	if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {
> >>>> +		/* When tot_len isn't provided by user,
> >>>> +		 * check skb against net_device MTU
> >>>> +		 */
> >>>>    		if (!is_skb_forwardable(dev, skb))
> >>>>    			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;  
> >>>
> >>> ... so using old cached dev from above will result in wrong MTU check &
> >>> subsequent passing of wrong params->mtu_result = dev->mtu this way.  
> >>
> >> Yes, you are right, params->ifindex have a chance to change in the calls.
> >> So, our attempt to save an ifindex lookup (dev_get_by_index_rcu) is not
> >> correct.
> >>  
> >>> So one
> >>> way to fix is that we would need to pass &dev to bpf_ipv{4,6}_fib_lookup().  
> >>
> >> Ok, I will try to code it up, and see how ugly it looks, but I'm no
> >> longer sure that it is worth saving this ifindex lookup, as it will
> >> only happen if BPF-prog didn't specify params->tot_len.  
> > 
> > I guess we can still do this as an "optimization", but the dev/ifindex
> > will very likely be another at this point.  
> 
> I would say for sake of progress, lets ship your series w/o this optimization so
> it can land, and we revisit this later on independent from here. 

I would really really like to make progress for this patchset.  I
unfortunately finished coding this up (and tested with selftests)
before I noticed this request (without optimizations).

I guess, I can revert my recent work by pulling in V12 of the patch.
I'll do it tomorrow, as I want to have time to run my tests before
re-sending patchset.

> Actually DavidA back then acked the old poc patch I posted, so maybe
> that's worth a revisit as well but needs more testing first.

Yes, we can always revisit this as an optimization.

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