On Tue, Nov 27, 2018 at 2:58 PM Willem de Bruijn
[off-list ref] wrote:

On Tue, Nov 27, 2018 at 1:41 PM Maxim Mikityanskiy [off-list ref] wrote:

quoted

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.