> +/* Compute SNE for a specific packet (by seq). */
> +static int compute_packet_sne(struct sock *sk, struct tcp_authopt_info *info,
> + u32 seq, bool input, __be32 *sne)
> +{
> + u32 rcv_nxt, snd_nxt;
> +
> + // We can't use normal SNE computation before reaching TCP_ESTABLISHED
> + // For TCP_SYN_SENT the dst_isn field is initialized only after we
> + // validate the remote SYN/ACK
> + // For TCP_NEW_SYN_RECV there is no tcp_authopt_info at all
> + if (sk->sk_state == TCP_SYN_SENT ||
> + sk->sk_state == TCP_NEW_SYN_RECV ||
> + sk->sk_state == TCP_LISTEN)
> + return 0;
> +
In case of TCP_NEW_SYN_RECV, if our SYNACK had sequence number
0xffffffff, we will receive an ACK sequence number of 0, which
should have sne = 1.
In a somewhat similar corner case, when we receive a SYNACK to
our SYN in tcp_rcv_synsent_state_process, if the SYNACK has
sequence number 0xffffffff, we set tp->rcv_nxt to 0, and we
should set sne to 1.
There may be more similar corner cases related to a wraparound
during the handshake.
Since as you pointed out all we need is "recent" valid <sne, seq>
pairs as reference, rather than relying on rcv_sne being paired
with tp->rcv_nxt (and similarly for snd_sne and tp->snd_nxt),
would it be easier to maintain reference <sne, seq> pairs for send
and receive in tcp_authopt_info, appropriately handle the different
handshake cases and initialize the pairs, and only then track them
in tcp_rcv_nxt_update and tcp_rcv_snd_update?
> static void tcp_rcv_nxt_update(struct tcp_sock *tp, u32 seq)
> {
> u32 delta = seq - tp->rcv_nxt;
>
> sock_owned_by_me((struct sock *)tp);
> + tcp_authopt_update_rcv_sne(tp, seq);
> tp->bytes_received += delta;
> WRITE_ONCE(tp->rcv_nxt, seq);
> }
>
Since rcv_sne and tp->rcv_nxt are not updated atomically, could
there ever be a case where a reader might use the new sne with
the old rcv_nxt?
Francesco
