+/* 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 allif (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