netdev - Re: Invalid transport_offset with AF

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Date:   Tue, 27 Nov 2018 15:32:57 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     maximmi@...lanox.com
Cc:     Network Development <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jason Wang <jasowang@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>, eranbe@...lanox.com,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: Invalid transport_offset with AF_PACKET socket

On Tue, Nov 27, 2018 at 2:58 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Tue, Nov 27, 2018 at 1:41 PM Maxim Mikityanskiy <maximmi@...lanox.com> wrote:
> >
> > Hi everyone,
> >
> > We are experiencing an issue with Mellanox mlx5 driver, and I tracked it down to
> > the packet_snd function in net/packet/af_packet.c.
> >
> > Brief description: when a socket is created by calling `socket(AF_PACKET,
> > SOCK_RAW, 0)`, the mlx5 driver receives an skb with wrong transport_offset,
> > which can confuse the driver and cause the transmit to fail (depending on the
> > configuration of the NIC).
> >
> > The flow is the following:
> >
> > 1. packet_snd is called.
> >
> > 2. dev->hard_header_len (which is 14) is assigned to reserve.
> >
> > 3. The value of the third parameter of the initial socket() call is assigned to
> > skb->protocol. In our case, it's 0.
> >
> > 4. skb_probe_transport_header is called with offset_hint == reserve (which is
> > 14).
> >
> > 5. __skb_flow_dissect fails, because skb->protocol is 0.
> >
> > 6. skb_probe_transport_header happily sets transport_header to 14.
> >
> > I find this behavior (defaulting to 14) strange, because network_header is also
> > set to 14, and the transport_header value is just wrong. Moreover, there are two
> > more calls to skb_probe_transport_header in this file with offset_hint == 0,
> > which looks more reasonable (if we can't find the transport header, we indicate
> > that there is none, instead of pointing to the network header).
>
> That is not what offset_hint 0 does. It also sets the transport header
> to the same as the network header.

Actually, what you observe may be due to commit  b84bbaf7a6c8cc
("packet: in packet_snd start writing at link layer allocation"). This
updated skb_set_network_header, but not the fall-back value for
skb_probe_transport_header. Let me take a closer look.