Re: [PATCH v3 bpf-next 2/9] bpf: tcp: Add a new per-socket flag and kfunc for bpf_tcp_ops.

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 10/05, Kuniyuki Iwashima wrote:
> The legacy SOCK_OPS guards some hooks with a per-socket flag,
> tp->bpf_sock_ops_cb_flags.
> 
> In contrast, bpf_tcp_ops was initially designed without per-socket
> flags so that users can simply define only the callbacks they need.
> 
> However, it turned out that even attaching a bpf_tcp_ops with a NULL
> callback incurs measurable overhead in the fast path. [0]
> 
> We could reuse tp->bpf_sock_ops_cb_flags, but it is u8 and has only
> 1 bit left for a new opt-in callback, while not all of the 7 existing
> opt-in hooks are in the fast path and really need a flag guard.
> 
> Moreover, tp->bpf_sock_ops_cb_flags has two bpf helper functions to
> update it, both of which require sock_owned_by_me(sk):
> 
>   * bpf_sock_ops_cb_flags_set()
>   * bpf_setsockopt(TCP_BPF_SOCK_OPS_CB_FLAGS)

Can we return -EINVAL from these when used with sock_ops?
If we keep one flag we can presumably put both tcp_call_bpf_Xarg and
bpf_tcp_ops_call under the same if conditional?