Subject: Re: [PATCH net-next v2 1/2] net_tstamp: add SCM_TS_OPT_ID to provide OPT_ID in control message

From: Simon Horman <[email-masked]>
Date: Mon, 2 Sep 2024 19:38:33 +0100

On Mon, Sep 02, 2024 at 06:09:35AM -0700, Vadim Fedorenko wrote:
> SOF_TIMESTAMPING_OPT_ID socket option flag gives a way to correlate TX
> timestamps and packets sent via socket. Unfortunately, there is no way
> to reliably predict socket timestamp ID value in case of error returned
> by sendmsg. For UDP sockets it's impossible because of lockless
> nature of UDP transmit, several threads may send packets in parallel. In
> case of RAW sockets MSG_MORE option makes things complicated. More
> details are in the conversation [1].
> This patch adds new control message type to give user-space
> software an opportunity to control the mapping between packets and
> values by providing ID with each sendmsg. This works fine for UDP
> sockets only, and explicit check is added to control message parser.
> 
> [1] https://lore.kernel.org/netdev/[email-masked]/
> 
> Signed-off-by: Vadim Fedorenko <[email-masked]>

...

> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c

...

> @@ -1543,10 +1546,15 @@ static int __ip6_append_data(struct sock *sk,
>  			flags &= ~MSG_SPLICE_PAGES;
>  	}
>  
> -	hold_tskey = cork->tx_flags & SKBTX_ANY_TSTAMP &&
> -		     READ_ONCE(sk->sk_tsflags) & SOF_TIMESTAMPING_OPT_ID;
> -	if (hold_tskey)
> -		tskey = atomic_inc_return(&sk->sk_tskey) - 1;
> +	if (cork->tx_flags & SKBTX_ANY_TSTAMP &&
> +	    READ_ONCE(sk->sk_tsflags) & SOF_TIMESTAMPING_OPT_ID) {
> +		if (cork->flags & IPCORK_TS_OPT_ID) {
> +			tskey = cork->ts_opt_id;
> +		} else {
> +			tskey = atomic_inc_return(&sk->sk_tskey) - 1;
> +			hold_tskey = true;

Hi Vadim,

I think that hold_tskey also needs to be assigned a value in
the cases where wither of the if conditions above are false.

Flagged by Smatch.

> +		}
> +	}
>  
>  	/*
>  	 * Let's try using as much space as possible.

-- 
pw-bot: cr