Re: BPF redirect API design issue for BPF-prog MTU feedback?

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Jesper Dangaard Brouer <brouer@redhat.com> writes:

> On Thu, 17 Sep 2020 12:11:33 -0700
> Saeed Mahameed <saeed@kernel.org> wrote:
>
>> On Thu, 2020-09-17 at 05:54 -0700, Maciej Żenczykowski wrote:
>> > On Thu, Sep 17, 2020 at 5:39 AM Jesper Dangaard Brouer
>> > <brouer@redhat.com> wrote:  
>> > > 
>> > > As you likely know[1] I'm looking into moving the MTU check (for
>> > > TC-BPF) in __bpf_skb_max_len() when e.g. called by
>> > > bpf_skb_adjust_room(), because when redirecting packets to
>> > > another netdev it is not correct to limit the MTU based on the
>> > > incoming netdev.
>> > > 
>> > > I was looking at doing the MTU check in bpf_redirect() helper,
>> > > because at this point we know the redirect to netdev, and
>> > > returning an indication/error that MTU was exceed, would allow
>> > > the BPF-prog logic to react, e.g. sending ICMP (instead of packet
>> > > getting silently dropped). 
>> > > BUT this is not possible because bpf_redirect(index, flags) helper
>> > > don't provide the packet context-object (so I cannot lookup the
>> > > packet length).
>> > > 
>> > > Seeking input:
>> > > 
>> > > Should/can we change the bpf_redirect API or create a new helper
>> > > with packet-context?
>> > > 
>> > >  Note: We have the same need for the packet context for XDP when
>> > >  redirecting the new multi-buffer packets, as not all destination
>> > >  netdev will support these new multi-buffer packets.
>> > > 
>> > > I can of-cause do the MTU checks on kernel-side in
>> > > skb_do_redirect, but then how do people debug this? as packet
>> > > will basically be silently dropped.
>> > > 
>> > > 
>> > > 
>> > > (Looking at how does BPF-prog logic handle MTU today)
>> > > 
>> > > How do bpf_skb_adjust_room() report that the MTU was exceeded?
>> > > Unfortunately it uses a common return code -ENOTSUPP which used
>> > > for multiple cases (include MTU exceeded). Thus, the BPF-prog
>> > > logic cannot use this reliably to know if this is a MTU exceeded
>> > > event. (Looked BPF-prog code and they all simply exit with
>> > > TC_ACT_SHOT for all error codes, cloudflare have the most
>> > > advanced handling with metrics->errors_total_encap_adjust_failed++).
>> > > 
>> > > 
>> > > [1] 
>> > > https://lore.kernel.org/bpf/159921182827.1260200.9699352760916903781.stgit@firesoul/
>> > > --
>> > > Best regards,
>> > >   Jesper Dangaard Brouer
>> > >   MSc.CS, Principal Kernel Engineer at Red Hat
>> > >   LinkedIn: http://www.linkedin.com/in/brouer
>> > >   
>> > 
>> > (a) the current state of the world seems very hard to use correctly,
>> > so adding new apis, or even changing existing ones seems ok to me.
>> > especially if this just means changing what error code they return
>> > 
>> > (b) another complexity with bpf_redirect() is you can call it, it
>> > can succeed, but then you can not return TC_ACT_REDIRECT from the
>> > bpf program, which effectively makes the earlier *successful*
>> > bpf_redirect() call an utter no-op.
>> > 
>> > (bpf_redirect() just determines what a future return TC_ACT_REDIRECT
>> > will do)
>> > 
>> > so if you bpf_redirect to interface with larger mtu, then increase
>> > packet size,  
>> 
>> why would you redirect then touch the packet afterwards ? 
>> if you have a bad program, then it is a user issue.
>> 
>> > then return TC_ACT_OK, then you potentially end up with excessively
>> > large packet egressing through original interface (with small mtu).
>> > 
>
> This is a good point.  As bpf_skb_adjust_room() can just be run after
> bpf_redirect() call, then a MTU check in bpf_redirect() actually
> doesn't make much sense.  As clever/bad BPF program can then avoid the
> MTU check anyhow.  This basically means that we have to do the MTU
> check (again) on kernel side anyhow to catch such clever/bad BPF
> programs.  (And I don't like wasting cycles on doing the same check two
> times).
>
> If we do the MTU check on the kernel side, then there are no feedback
> to the program, and how are end-users going to debug this?

The same way any other MTU-related error is seen? Isn't there a counter
or something? Presumably (since this is in the skb path) it would also
be caught by drop_monitor?

-Toke