Re: [PATCH net-next v5 1/2] net-timestamp: introduce SOF_TIMESTAMPING_OPT_RX_FILTER flag

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> introduce a new flag SOF_TIMESTAMPING_OPT_RX_FILTER in the receive
> path. User can set it with SOF_TIMESTAMPING_SOFTWARE to filter
> out rx software timestamp report, especially after a process turns on
> netstamp_needed_key which can time stamp every incoming skb.
> 
> Previously, we found out if an application starts first which turns on
> netstamp_needed_key, then another one only passing SOF_TIMESTAMPING_SOFTWARE
> could also get rx timestamp. Now we handle this case by introducing this
> new flag without breaking users.
> 
> Quoting Willem to explain why we need the flag:
> "why a process would want to request software timestamp reporting, but
> not receive software timestamp generation. The only use I see is when
> the application does request
> SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_TX_SOFTWARE."
> 
> Similarly, this new flag could also be used for hardware case where we
> can set it with SOF_TIMESTAMPING_RAW_HARDWARE, then we won't receive
> hardware receive timestamp.
> 
> Another thing about errqueue in this patch I have a few words to say:
> In this case, we need to handle the egress path carefully, or else
> reporting the tx timestamp will fail. Egress path and ingress path will
> finally call sock_recv_timestamp(). We have to distinguish them.
> Errqueue is a good indicator to reflect the flow direction.
> 
> Suggested-by: Willem de Bruijn <willemb@google.com>
> Signed-off-by: Jason Xing <kernelxing@tencent.com>

High level: where is the harm in receiving unsolicited timestamps?
A process can easily ignore them. I do wonder if the only use case is
an overly strict testcase. Was reminded of this as I tried to write
a more concise paragraph for the documentation.

Otherwise implementation looks fine, only the tiniest nit.

> @@ -946,11 +946,17 @@ void __sock_recv_timestamp(struct msghdr *msg, struct sock *sk,
>  
>  	memset(&tss, 0, sizeof(tss));
>  	tsflags = READ_ONCE(sk->sk_tsflags);
> -	if ((tsflags & SOF_TIMESTAMPING_SOFTWARE) &&
> +	if ((tsflags & SOF_TIMESTAMPING_SOFTWARE &&
> +	     (tsflags & SOF_TIMESTAMPING_RX_SOFTWARE ||
> +	     skb_is_err_queue(skb) ||
> +	     !(tsflags & SOF_TIMESTAMPING_OPT_RX_FILTER))) &&

Nit: these statements should all align on the inner brace, so indent
by one character.

>  	    ktime_to_timespec64_cond(skb->tstamp, tss.ts + 0))
>  		empty = 0;
>  	if (shhwtstamps &&
> -	    (tsflags & SOF_TIMESTAMPING_RAW_HARDWARE) &&
> +	    (tsflags & SOF_TIMESTAMPING_RAW_HARDWARE &&
> +	    (tsflags & SOF_TIMESTAMPING_RX_HARDWARE ||
> +	    skb_is_err_queue(skb) ||
> +	    !(tsflags & SOF_TIMESTAMPING_OPT_RX_FILTER))) &&
>  	    !skb_is_swtx_tstamp(skb, false_tstamp)) {
>  		if_index = 0;
>  		if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP_NETDEV)
> -- 
> 2.37.3
>