On Fri, Jul 17, 2020 at 1:52 PM Matt Sandy <mooseb...@gmail.com> wrote:
>
> I've been playing around with raw sockets and timestamps, but seem to
> be getting strange timestamp data back on the errqueue. Specifically,
> if I am creating a socket(AF_PACKET, SOCK_RAW, htons(ETH_P_IP)) and
> requesting SO_TIMESTAMPING_NEW with options 0x4DF. I am not modifying
> the flags with control messages.
>
> On both send and receive, I get the expected
> SOL_SOCKET/SO_TIMESTAMPING_NEW cmsg (in errqueue on send, in the
> message itself on receive), and it contains what appears to be valid
> timestamps in the ts[0] field. On send, however, I receive an
> additional cmsg with level = SOL_PACKET/PACKET_TX_TIMESTAMP, whose
> content is just the fixed value `char[16] { 42, 0, 0, 0, 4, <zeros>
> }`.
>
> Any ideas why I'd be getting the SOL_PACKET message on transmit, and
> why its payload is clearly not a valid timestamp? In case it matters,
> this is on an Intel I210 nic using the igb driver.
This is not a char[16], but a struct sock_extended_err.
The first four bytes correspond to __u32 ee_errno, where 42 is ENOMSG.
The fifth byte is __u8 ee_origin, where 4 corresponds to
SO_EE_ORIGIN_TIMESTAMPING.
This is metadata stored along with the skb by __skb_complete_tx_timestamp.
This helps demultiplex timestamps received from the error queue from
other messages.
Additionally, in the case of timestamps it may include additional
associated information:
serr->ee.ee_info = tstype;
serr->opt_stats = opt_stats;
serr->header.h4.iif = skb->dev ? skb->dev->ifindex : 0;
if (sk->sk_tsflags & SOF_TIMESTAMPING_OPT_ID) {
serr->ee.ee_data = skb_shinfo(skb)->tskey;
if (sk->sk_protocol == IPPROTO_TCP &&
sk->sk_type == SOCK_STREAM)
serr->ee.ee_data -= sk->sk_tskey;
}
The fact that the field after ee_origin is zero means that this is a
timestamp captured at device transmit (SCM_TSTAMP_SND), for instance.
The csmg_level and type themselves are chosen on recv errqueue in
packet_recvmsg:
if (flags & MSG_ERRQUEUE) {
err = sock_recv_errqueue(sk, msg, len,
SOL_PACKET, PACKET_TX_TIMESTAMP);
goto out;
}
