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Date:   Thu, 7 May 2020 23:05:55 +0200
From:   Daniel Borkmann <>
To:     Maciej ┼╗enczykowski <>
Cc:     Alexei Starovoitov <>,
        Linux Network Development Mailing List 
        Linux Kernel Mailing List <>,
        BPF Mailing List <>,
        "David S . Miller" <>,
        Jakub Kicinski <>
Subject: Re: [PATCH v3] net: bpf: permit redirect from ingress L3 to egress L2
 devices at near max mtu

On 5/7/20 6:46 PM, Maciej ┼╗enczykowski wrote:
> (a) not clear why the max is SKB_MAX_ALLOC in the first place (this is
> PAGE_SIZE << 2, ie. 16K on x86), while lo mtu is 64k

Agreed, tbh, it's not clear to me either atm. :) The SKB_MAX_ALLOC constant itself
should be replaced with something more appropriate. Also as a small side note,
the !skb->dev check should be removed since in tc ingress/egress the skb->dev
is never NULL. (See also tc_cls_act_convert_ctx_access() on struct __sk_buff,
ifindex conversion.)

> (b) hmm, if we're not redirecting, then exceeding the ingress device's
> mtu doesn't seem to be a problem.
> Indeed AFAIK this can already happen, some devices will round mtu up
> when they configure the device mru buffers.
> (ie. you configure L3 mtu 1500, they treat that as L2 1536 or 1532 [-4
> fcs], simply because 3 * 512 is a nice amount of buffers, or they'll
> accept not only 1514 L2, but also 1518 L2 or even 1522 L2 for VLAN and
> Q-IN-Q -- even if the packets aren't actually VLAN'ed, so your non
> VLAN mru might be 1504 or 1508)
> Indeed my corp dell workstation with some standard 1 gigabit
> motherboard nic has a standard default mtu of 1500, and I've seen it
> receive L3 mtu 1520 packets (apparently due to misconfiguration in our
> hardware [cisco? juniper?] ipv4->ipv6 translator which can take 1500
> mtu ipv4 packets and convert them to 1520 mtu ipv6 packets without
> fragmenting or generating icmp too big errors).  While it's obviously
> wrong, it does just work (the network paths themselves are also
> obviously 1520 clean).

Right, agree on tc ingress side when skb goes further up the stack.

> (c) If we are redirecting we'll eventually (after bpf program returns)
> hit dev_queue_xmit(), and shouldn't that be what returns an error?

You mean whether the check should be inside __dev_queue_xmit() itself
instead? Maybe. From a cursory glance the MTU checks are spread in upper
layer protos. As mentioned, we would need to have coverage for BPF progs
attached on the tc ingress and egress (sch_handle_{ingress,egress}())
hook where for each case an skb can be just TC_ACT_OK'ed (up to stack or
down to driver), redirected via ____dev_forward_skb() or dev_queue_xmit().
The ____dev_forward_skb() has us covered and for TC_ACT_OK on tc ingress,
we'd only need a generic upper cap. So for the rest of the cases, we'd
need to have some sort of sanity check somewhere.

> btw. is_skb_forwardable() actually tests
> - device is up && (packet is gso || skb->len < dev->mtu +
> dev->hard_header_len + VLAN_HLEN);
> which is also wrong and in 2 ways, cause VLAN_HLEN makes no sense on
> non ethernet, and the __bpf_skb_max_len function doesn't account for
> VLAN...  (which possibly has implications if you try to redirect to a
> vlan interface)

Yeah, it probably would for QinQ which is probably why noone was running
into it so far. At least the skb_vlan_push() would first store the tag
via __vlan_hwaccel_put_tag() in the skb itself.

> I think having an mtu check is useful, but I think the mtu should be
> selectable by the bpf program.  Because it might not even be device
> mtu at all, it might be path mtu which we should be testing against.
> It should also be checked for gso frames, since the max post
> segmentation size should be enforced.
> I agree we should expose dev->mtu (and dev->hard_header_len and hatype)
> I'll mull this over a bit more, but I'm not convinced this patch isn't ok as is.
> There just is probably more we should do.

Ok, makes sense. Thanks for looking into it!

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