Re: [PATCH bpf-next v12 10/12] bpf: add BPF_SOCK_OPS_TS_SND_CB callback

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

Jason Xing wrote:
> This patch introduces a new callback in tcp_tx_timestamp() to correlate
> tcp_sendmsg timestamp with timestamps from other tx timestamping
> callbacks (e.g., SND/SW/ACK).
> 
> Without this patch, BPF program wouldn't know which timestamps belong
> to which flow because of no socket lock protection. This new callback
> is inserted in tcp_tx_timestamp() to address this issue because
> tcp_tx_timestamp() still owns the same socket lock with
> tcp_sendmsg_locked() in the meanwhile tcp_tx_timestamp() initializes
> the timestamping related fields for the skb, especially tskey. The
> tskey is the bridge to do the correlation.
> 
> For TCP, BPF program hooks the beginning of tcp_sendmsg_locked() and
> then stores the sendmsg timestamp at the bpf_sk_storage, correlating
> this timestamp with its tskey that are later used in other sending
> timestamping callbacks.
> 
> Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
> ---
>  include/uapi/linux/bpf.h       | 5 +++++
>  net/ipv4/tcp.c                 | 4 ++++
>  tools/include/uapi/linux/bpf.h | 5 +++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 9355d617767f..86fca729fbd8 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7052,6 +7052,11 @@ enum {
>  					 * when SK_BPF_CB_TX_TIMESTAMPING
>  					 * feature is on.
>  					 */
> +	BPF_SOCK_OPS_TS_SND_CB,		/* Called when every sendmsg syscall
> +					 * is triggered. It's used to correlate
> +					 * sendmsg timestamp with corresponding
> +					 * tskey.
> +					 */
>  };
>  
>  /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 12b9c4f9c151..4b9739cd3bc5 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -492,6 +492,10 @@ static void tcp_tx_timestamp(struct sock *sk, struct sockcm_cookie *sockc)
>  		if (tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK)
>  			shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
>  	}
> +
> +	if (cgroup_bpf_enabled(CGROUP_SOCK_OPS) &&
> +	    SK_BPF_CB_FLAG_TEST(sk, SK_BPF_CB_TX_TIMESTAMPING) && skb)
> +		bpf_skops_tx_timestamping(sk, skb, BPF_SOCK_OPS_TS_SND_CB);
>  }
>  
>  static bool tcp_stream_is_readable(struct sock *sk, int target)
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index d3e2988b3b4c..2739ee0154a0 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -7042,6 +7042,11 @@ enum {
>  					 * when SK_BPF_CB_TX_TIMESTAMPING
>  					 * feature is on.
>  					 */
> +	BPF_SOCK_OPS_TS_SND_CB,		/* Called when every sendmsg syscall
> +					 * is triggered. It's used to correlate
> +					 * sendmsg timestamp with corresponding
> +					 * tskey.
> +					 */

Feel free to decline this late in the review process, but a bit more
bikeshedding..

Can we spell out TSTAMP instead of TS in these definitions? Within
the context of this series it is self-explanatory, but when reading
kernel code the meaning of a two letter acronym is not that clear.

And instead of SND can we use SENDMSG or something like that?
SND here confused me as the software timestamp is SCM_TSTAMP_SND.

For instance:

    BPF_SOCK_OPS_TSTAMP_SENDMSG_CB,
    BPF_SOCK_OPS_TSTAMP_SCHED_CB,
    BPF_SOCK_OPS_TSTAMP_SND_SW_CB,
    BPF_SOCK_OPS_TSTAMP_SND_HW_CB,
   (BPF_SOCK_OPS_TSTAMP_TX_COMPLETION_CB,)
    BPF_SOCK_OPS_TSTAMP_ACK_CB,

(not sure what the OPT in OPT_CB added).