Re: [PATCH net-next v3 10/14] net-timestamp: add basic support with tskey offset

[Martin KaFai Lau <martin.lau@linux.dev>]

On 10/28/24 4:05 AM, Jason Xing wrote:
> +/* Used to track the tskey for bpf extension
> + *
> + * @sk_tskey: bpf extension can use it only when no application uses.
> + *            Application can use it directly regardless of bpf extension.
> + *
> + * There are three strategies:
> + * 1) If we've already set through setsockopt() and here we're going to set
> + *    OPT_ID for bpf use, we will not re-initialize the @sk_tskey and will
> + *    keep the record of delta between the current "key" and previous key.
> + * 2) If we've already set through bpf_setsockopt() and here we're going to
> + *    set for application use, we will record the delta first and then
> + *    override/initialize the @sk_tskey.
> + * 3) other cases, which means only either of them takes effect, so initialize
> + *    everything simplely.
> + */
> +static long int sock_calculate_tskey_offset(struct sock *sk, int val, int bpf_type)
> +{
> +	u32 tskey;
> +
> +	if (sk_is_tcp(sk)) {
> +		if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
> +			return -EINVAL;
> +
> +		if (val & SOF_TIMESTAMPING_OPT_ID_TCP)
> +			tskey = tcp_sk(sk)->write_seq;
> +		else
> +			tskey = tcp_sk(sk)->snd_una;
> +	} else {
> +		tskey = 0;
> +	}
> +
> +	if (bpf_type && (sk->sk_tsflags & SOF_TIMESTAMPING_OPT_ID)) {
> +		sk->sk_tskey_bpf_offset = tskey - atomic_read(&sk->sk_tskey);
> +		return 0;
> +	} else if (!bpf_type && (sk->sk_tsflags_bpf & SOF_TIMESTAMPING_OPT_ID)) {
> +		sk->sk_tskey_bpf_offset = atomic_read(&sk->sk_tskey) - tskey;
> +	} else {
> +		sk->sk_tskey_bpf_offset = 0;
> +	}
> +
> +	return tskey;
> +}

Before diving into this route, the bpf prog can peek into the tcp seq no in the 
skb. It can also look at the sk->sk_tskey for UDP socket. Can you explain why 
those are not enough information for the bpf prog?