Subject: Re: [PATCH net-next v3 2/4] net_tstamp: add SCM_TS_OPT_ID for TCP sockets

From: Vadim Fedorenko <[email-masked]>
Date: Thu, 5 Sep 2024 15:58:25 +0100

On 04/09/2024 12:31, Vadim Fedorenko wrote:
> TCP sockets have different flow for providing timestamp OPT_ID value.
> Adjust the code to support SCM_TS_OPT_ID option for TCP sockets.
> 
> Signed-off-by: Vadim Fedorenko <[email-masked]>
> ---
>   net/ipv4/tcp.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 8a5680b4e786..5553a8aeee80 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -474,9 +474,10 @@ void tcp_init_sock(struct sock *sk)
>   }
>   EXPORT_SYMBOL(tcp_init_sock);
>   
> -static void tcp_tx_timestamp(struct sock *sk, u16 tsflags)
> +static void tcp_tx_timestamp(struct sock *sk, struct sockcm_cookie *sockc)
>   {
>   	struct sk_buff *skb = tcp_write_queue_tail(sk);
> +	u32 tsflags = sockc->tsflags;
>   
>   	if (tsflags && skb) {
>   		struct skb_shared_info *shinfo = skb_shinfo(skb);
> @@ -485,8 +486,12 @@ static void tcp_tx_timestamp(struct sock *sk, u16 tsflags)
>   		sock_tx_timestamp(sk, tsflags, &shinfo->tx_flags);
>   		if (tsflags & SOF_TIMESTAMPING_TX_ACK)
>   			tcb->txstamp_ack = 1;
> -		if (tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK)
> -			shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
> +		if (tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK) {
> +			if (tsflags & SOCKCM_FLAG_TS_OPT_ID)
> +				shinfo->tskey = sockc->ts_opt_id;
> +			else
> +				shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
> +		}
>   	}
>   }
>   
> @@ -1318,7 +1323,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>   
>   out:
>   	if (copied) {
> -		tcp_tx_timestamp(sk, sockc.tsflags);
> +		tcp_tx_timestamp(sk, &sockc);
>   		tcp_push(sk, flags, mss_now, tp->nonagle, size_goal);
>   	}
>   out_nopush:

Hi Willem,

Unfortunately, these changes are not enough to enable custom OPT_ID for
TCP sockets. There are some functions which rewrite shinfo->tskey in TCP
flow:

tcp_skb_collapse_tstamp()
tcp_fragment_tstamp()
tcp_gso_tstamp()

I believe the last one breaks tests, but the problem is that there is no
easy way to provide the flag of constant tskey to it. Only
shinfo::tx_flags are available at the caller side and we have already
discussed that we shouldn't use the last bit of this field.

So, how should we deal with the problem? Or is it better to postpone
support for TCP sockets in this case?

Thanks,
Vadim