From: Jesper Dangaard Brouer <brouer@redhat.com>
To: John Fastabend <john.fastabend@gmail.com>
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org,
Daniel Borkmann <borkmann@iogearbox.net>,
Alexei Starovoitov <alexei.starovoitov@gmail.com>,
maze@google.com, lmb@cloudflare.com, shaun@tigera.io,
Lorenzo Bianconi <lorenzo@kernel.org>,
marek@cloudflare.com, Jakub Kicinski <kuba@kernel.org>,
eyal.birger@gmail.com, brouer@redhat.com
Subject: Re: [PATCH bpf-next V5 3/5] bpf: add BPF-helper for MTU checking
Date: Mon, 2 Nov 2020 21:10:34 +0100 [thread overview]
Message-ID: <20201102211034.563ef994@carbon> (raw)
In-Reply-To: <5fa04a3c7c173_1ecdb20821@john-XPS-13-9370.notmuch>
On Mon, 02 Nov 2020 10:04:44 -0800
John Fastabend <john.fastabend@gmail.com> wrote:
> > > > +
> > > > + /* Same relax as xdp_ok_fwd_dev() and is_skb_forwardable() */
> > > > + if (flags & BPF_MTU_CHK_RELAX)
> > > > + mtu += VLAN_HLEN;
> > >
> > > I'm trying to think about the use case where this might be used?
> > > Compared to just adjusting MTU in BPF program side as needed for
> > > packet encapsulation/headers/etc.
> >
> > As I wrote above, this were added because the kernels own forwarding
> > have this relaxation in it's checks (in is_skb_forwardable()). I even
> > tried to dig through the history, introduced in [1] and copy-pasted
> > in[2]. And this seems to be a workaround, that have become standard,
> > that still have practical implications.
> >
> > My practical experiments showed, that e.g. ixgbe driver with MTU=1500
> > (L3-size) will allow and fully send packets with 1504 (L3-size). But
> > i40e will not, and drops the packet in hardware/firmware step. So,
> > what is the correct action, strict or relaxed?
> >
> > My own conclusion is that we should inverse the flag. Meaning to
> > default add this VLAN_HLEN (4 bytes) relaxation, and have a flag to do
> > more strict check, e.g. BPF_MTU_CHK_STRICT. As for historical reasons
> > we must act like kernels version of MTU check. Unless you object, I will
> > do this in V6.
>
> I'm fine with it either way as long as its documented in the helper
> description so I have a chance of remembering this discussion in 6 months.
> But, if you make it default won't this break for XDP cases? I assume the
> XDP use case doesn't include the VLAN 4-bytes. Would you need to prevent
> the flag from being used from XDP?
XDP actually do include the VLAN_HLEN 4-bytes, see xdp_ok_fwd_dev(). I
was so certain that you John added this code, but looking through git
blame it pointed back to myself. Going 5 levels git history deep and
3+ years, does seem like I move/reused some of Johns code containing
VLAN_HLEN in the MTU check, introduced for xdp-generic (6103aa96ec077)
which I acked. Thus, I guess I cannot push this away and have to take
blame myself ;-)
I conclude that we default need to include this VLAN_HLEN, else the XDP
bpf_check_mtu could say deny, while it would have passed the check in
xdp_ok_fwd_dev(). As i40e will drop 1504 this at HW/FW level, I still
see a need for a BPF_MTU_CHK_STRICT flag for programs that want to
catch this.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
next prev parent reply other threads:[~2020-11-02 20:12 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-30 16:50 [PATCH bpf-next V5 0/5] Subj: bpf: New approach for BPF MTU handling Jesper Dangaard Brouer
2020-10-30 16:50 ` [PATCH bpf-next V5 1/5] bpf: Remove MTU check in __bpf_skb_max_len Jesper Dangaard Brouer
2020-10-30 16:50 ` [PATCH bpf-next V5 2/5] bpf: bpf_fib_lookup return MTU value as output when looked up Jesper Dangaard Brouer
2020-10-30 19:40 ` John Fastabend
2020-11-02 9:28 ` Jesper Dangaard Brouer
2020-11-02 15:59 ` David Ahern
2020-11-02 16:18 ` John Fastabend
2020-10-31 15:52 ` David Ahern
2020-10-30 16:51 ` [PATCH bpf-next V5 3/5] bpf: add BPF-helper for MTU checking Jesper Dangaard Brouer
2020-10-30 20:23 ` John Fastabend
2020-11-02 11:15 ` Jesper Dangaard Brouer
2020-11-02 18:04 ` John Fastabend
2020-11-02 20:10 ` Jesper Dangaard Brouer [this message]
2020-11-12 12:58 ` Jesper Dangaard Brouer
2020-10-30 16:51 ` [PATCH bpf-next V5 4/5] bpf: drop MTU check when doing TC-BPF redirect to ingress Jesper Dangaard Brouer
2020-10-30 20:36 ` John Fastabend
2020-11-02 12:46 ` Jesper Dangaard Brouer
2020-11-02 16:23 ` John Fastabend
2020-10-30 16:51 ` [PATCH bpf-next V5 5/5] bpf: make it possible to identify BPF redirected SKBs Jesper Dangaard Brouer
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