On Tue, Nov 27, 2018 at 2:58 PM Willem de Bruijn
<willemdebruijn.ker...@gmail.com> wrote:
>
> On Tue, Nov 27, 2018 at 1:41 PM Maxim Mikityanskiy <maxi...@mellanox.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.
