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

[Eyal Birger <eyal.birger@gmail.com>]

On Mon, Sep 21, 2020 at 7:30 PM Jesper Dangaard Brouer
<brouer@redhat.com> wrote:
>
> On Mon, 21 Sep 2020 17:08:17 +0200
> Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> > On 9/21/20 2:49 PM, Jesper Dangaard Brouer wrote:
> > > On Mon, 21 Sep 2020 11:37:18 +0100
> > > Lorenz Bauer <lmb@cloudflare.com> wrote:
> > >> On Sat, 19 Sep 2020 at 00:06, Maciej Żenczykowski <maze@google.com> wrote:
> > >>>
> > >>>> 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 you get rid of the check in bpf_redirect() you might as well get
> > >>> rid of *all* the checks for excessive mtu in all the helpers that
> > >>> adjust packet size one way or another way.  They *all* then become
> > >>> useless overhead.
> > >>>
> > >>> I don't like that.  There may be something the bpf program could do to
> > >>> react to the error condition (for example in my case, not modify
> > >>> things and just let the core stack deal with things - which will
> > >>> probably just generate packet too big icmp error).
> > >>>
> > >>> btw. right now our forwarding programs first adjust the packet size
> > >>> then call bpf_redirect() and almost immediately return what it
> > >>> returned.
> > >>>
> > >>> but this could I think easily be changed to reverse the ordering, so
> > >>> we wouldn't increase packet size before the core stack was informed we
> > >>> would be forwarding via a different interface.
> > >>
> > >> We do the same, except that we also use XDP_TX when appropriate. This
> > >> complicates the matter, because there is no helper call we could
> > >> return an error from.
> > >
> > > Do notice that my MTU work is focused on TC-BPF.  For XDP-redirect the
> > > MTU check is done in xdp_ok_fwd_dev() via __xdp_enqueue(), which also
> > > happens too late to give BPF-prog knowledge/feedback.  For XDP_TX I
> > > audited the drivers when I implemented xdp_buff.frame_sz, and they
> > > handled (or I added) handling against max HW MTU. E.g. mlx5 [1].
> > >
> > > [1] https://elixir.bootlin.com/linux/v5.9-rc6/source/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c#L267
> > >
> > >> My preference would be to have three helpers: get MTU for a device,
> > >> redirect ctx to a device (with MTU check), resize ctx (without MTU
> > >> check) but that doesn't work with XDP_TX. Your idea of doing checks
> > >> in redirect and adjust_room is pragmatic and seems easier to
> > >> implement.
> > >
> > > I do like this plan/proposal (with 3 helpers), but it is not possible
> > > with current API.  The main problem is the current bpf_redirect API
> > > doesn't provide the ctx, so we cannot do the check in the BPF-helper.
> > >
> > > Are you saying we should create a new bpf_redirect API (that incl packet ctx)?
> >
> > Sorry for jumping in late here... one thing that is not clear to me
> > is that if we are fully sure that skb is dropped by stack anyway due
> > to invalid MTU (redirect to ingress does this via dev_forward_skb(),
>
> Yes, TC-redirecting to *INGRESS* have a slightly relaxed MTU check via
> is_skb_forwardable() called via ____dev_forward_skb().  This MTU check
> seems redundant as netstack will do MTU checks anyhow.
>

I found the MTU check on redirect-to-ingress to be very unexpected.

We hit this when implementing NAT64 as a tc egress program which translates
the packet and redirects it to ingress from the same device.

It is beneficial to have the MTU of the device set to a limit fitting the
IPv4 MTU, so that the IP stack would fragment as needed on the IPv4->IPv6
path. But when translating the packet to IPv6, it can no longer be ingressed
from the same device because of the MTU check. Packets are silently dropped
without any hint.

So would definitely be nice if this check is removed, or a flag to avoid
it is supported in bpf_redirect().

Eyal.