Netdev Archive on lore.kernel.org
 help / color / Atom feed
From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Lorenz Bauer <lmb@cloudflare.com>
Cc: "Maciej Żenczykowski" <maze@google.com>,
	"Saeed Mahameed" <saeed@kernel.org>,
	"Daniel Borkmann" <borkmann@iogearbox.net>,
	"Alexei Starovoitov" <alexei.starovoitov@gmail.com>,
	BPF-dev-list <bpf@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"Lorenzo Bianconi" <lorenzo.bianconi@redhat.com>,
	"John Fastabend" <john.fastabend@gmail.com>,
	"Jakub Kicinski" <kuba@kernel.org>,
	"Shaun Crampton" <shaun@tigera.io>,
	"David Miller" <davem@davemloft.net>,
	"Marek Majkowski" <marek@cloudflare.com>,
	brouer@redhat.com
Subject: Re: BPF redirect API design issue for BPF-prog MTU feedback?
Date: Mon, 21 Sep 2020 14:49:53 +0200
Message-ID: <20200921144953.6456d47d@carbon> (raw)
In-Reply-To: <CACAyw9-v_o+gPUpC-R9SXsfzMywrdGsWV13Nk=tx2aS-fEBFYg@mail.gmail.com>

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)?

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


  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-17 12:38 Jesper Dangaard Brouer
2020-09-17 12:54 ` Maciej Żenczykowski
2020-09-17 19:11   ` Saeed Mahameed
2020-09-18 10:00     ` Jesper Dangaard Brouer
2020-09-18 10:34       ` Toke Høiland-Jørgensen
2020-09-18 23:06       ` Maciej Żenczykowski
2020-09-21 10:37         ` Lorenz Bauer
2020-09-21 12:49           ` Jesper Dangaard Brouer [this message]
2020-09-21 15:08             ` Daniel Borkmann
2020-09-21 16:21               ` Marek Zavodsky
2020-09-21 21:17                 ` Willem de Bruijn
2020-09-22  9:15                   ` Jesper Dangaard Brouer
2020-09-21 16:26               ` Jesper Dangaard Brouer
2020-09-22  6:56                 ` Eyal Birger
2020-09-21 18:04               ` John Fastabend
2020-10-06 11:45                 ` Jesper Dangaard Brouer
2020-09-21 10:42   ` Lorenz Bauer

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200921144953.6456d47d@carbon \
    --to=brouer@redhat.com \
    --cc=alexei.starovoitov@gmail.com \
    --cc=borkmann@iogearbox.net \
    --cc=bpf@vger.kernel.org \
    --cc=davem@davemloft.net \
    --cc=john.fastabend@gmail.com \
    --cc=kuba@kernel.org \
    --cc=lmb@cloudflare.com \
    --cc=lorenzo.bianconi@redhat.com \
    --cc=marek@cloudflare.com \
    --cc=maze@google.com \
    --cc=netdev@vger.kernel.org \
    --cc=saeed@kernel.org \
    --cc=shaun@tigera.io \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Netdev Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/netdev/0 netdev/git/0.git
	git clone --mirror https://lore.kernel.org/netdev/1 netdev/git/1.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 netdev netdev/ https://lore.kernel.org/netdev \
		netdev@vger.kernel.org
	public-inbox-index netdev

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.netdev


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git