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From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Daniel Borkmann <borkmann@iogearbox.net>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	BPF-dev-list <bpf@vger.kernel.org>
Cc: brouer@redhat.com,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Maciej Zenczykowski <maze@google.com>,
	Lorenzo Bianconi <lorenzo.bianconi@redhat.com>,
	Lorenz Bauer <lmb@cloudflare.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>
Subject: BPF redirect API design issue for BPF-prog MTU feedback?
Date: Thu, 17 Sep 2020 14:38:46 +0200
Message-ID: <20200917143846.37ce43a0@carbon> (raw)


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


             reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-17 12:38 Jesper Dangaard Brouer [this message]
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
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

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