Re: [PATCH bpf-next v11 11/12] bpf: support selective sampling for bpf timestamping

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

Jason Xing wrote:
> Add the bpf_sock_ops_enable_tx_tstamp kfunc to allow BPF programs to
> selectively enable TX timestamping on a skb during tcp_sendmsg().
> 
> For example, BPF program will limit tracking X numbers of packets
> and then will stop there instead of tracing all the sendmsgs of
> matched flow all along. It would be helpful for users who cannot
> afford to calculate latencies from every sendmsg call probably
> due to the performance or storage space consideration.
> 
> Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
> ---
>  kernel/bpf/btf.c  |  1 +
>  net/core/filter.c | 33 ++++++++++++++++++++++++++++++++-
>  2 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 9433b6467bbe..740210f883dc 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -8522,6 +8522,7 @@ static int bpf_prog_type_to_kfunc_hook(enum bpf_prog_type prog_type)
>  	case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
>  	case BPF_PROG_TYPE_CGROUP_SOCKOPT:
>  	case BPF_PROG_TYPE_CGROUP_SYSCTL:
> +	case BPF_PROG_TYPE_SOCK_OPS:
>  		return BTF_KFUNC_HOOK_CGROUP;
>  	case BPF_PROG_TYPE_SCHED_ACT:
>  		return BTF_KFUNC_HOOK_SCHED_ACT;
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7f56d0bbeb00..3b4c1e7b1470 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -12102,6 +12102,27 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
>  #endif
>  }
>  
> +__bpf_kfunc int bpf_sock_ops_enable_tx_tstamp(struct bpf_sock_ops_kern *skops,
> +					      u64 flags)
> +{
> +	struct sk_buff *skb;
> +	struct sock *sk;
> +
> +	if (skops->op != BPF_SOCK_OPS_TS_SND_CB)
> +		return -EOPNOTSUPP;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	skb = skops->skb;
> +	sk = skops->sk;

nit: not used

> +	skb_shinfo(skb)->tx_flags |= SKBTX_BPF;
> +	TCP_SKB_CB(skb)->txstamp_ack |= TSTAMP_ACK_BPF;
> +	skb_shinfo(skb)->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;

Can this overwrite the seqno previously calculated by tcp_tx_timestamp?

I suppose that that is safe as long as both calculate the same value.
But good to have explicit.

> +
> +	return 0;
> +}
> +