Subject: Re: [PATCH v1 bpf-next 7/8] bpf: tcp: Add SOCK_OPS rcvlowat hook.

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 8 May 2026 08:28:10 -0700

On 05/08, Kuniyuki Iwashima wrote:
> Now, it is time to add the new hooks for BPF_SOCK_OPS_RCVLOWAT_CB.
> 
> Let's invoke the BPF SOCK_OPS prog when
> 
>   1. TCP stack enqueues skb to sk->sk_receive_queue
>      -> tcp_queue_rcv(), tcp_ofo_queue(), and tcp_fastopen_add_skb()
> 
>   2. TCP recvmsg() completes
>      -> __tcp_cleanup_rbuf()
> 
> This will allow the BPF prog to parse each skb and dynamically
> adjust sk->sk_rcvlowat to suppress unnecessary EPOLLIN wakeups
> until sufficient data (e.g., a full RPC frame) is available
> in the receive queue.
> 
> Note that the direct access to bpf_sock_ops.data is intentionally
> disabled by passing 0 as end_offset.
> 
> Instead, the BPF prog is supposed to use bpf_skb_load_bytes()
> with bpf_sock_ops because payload is not in the linear area
> with TCP header/data split on and skb may contain a RPC
> descriptor in skb frag.  This also simplifies the BPF prog.
> 
> Signed-off-by: Kuniyuki Iwashima <[email-masked]>

I was reading the series expecting to find some skb_queue_walk-like
implementation, but since it's a cgroup hook we obviously don't
need to do that.. So at this point BPF_SOCK_OPS_RCVLOWAT_CB_FLAG
is basically a "rx queue skb" hook, right? So should we make
the name more generic? There is really nothing lowat-specific
here besides your new kfunc to read the payload?

> ---
>  include/net/tcp.h       | 14 ++++++++++++++
>  net/ipv4/tcp.c          |  2 ++
>  net/ipv4/tcp_fastopen.c |  2 ++
>  net/ipv4/tcp_input.c    | 10 ++++++++++
>  4 files changed, 28 insertions(+)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 4e9e634e276b..003e46c9b500 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -737,6 +737,20 @@ static inline struct request_sock *cookie_bpf_check(struct net *net, struct sock
>  }
>  #endif
>  
> +#ifdef CONFIG_CGROUP_BPF
> +void bpf_skops_rcvlowat(struct sock *sk, struct sk_buff *skb);
> +
> +static inline void tcp_bpf_rcvlowat(struct sock *sk, struct sk_buff *skb)
> +{
> +	if (BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk), BPF_SOCK_OPS_RCVLOWAT_CB_FLAG))
> +		bpf_skops_rcvlowat(sk, skb);
> +}
> +#else
> +static inline void tcp_bpf_rcvlowat(struct sock *sk, struct sk_buff *skb)
> +{
> +}
> +#endif
> +
>  /* From net/ipv6/syncookies.c */
>  int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th);
>  struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb);
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 1d9e52fc454f..80144b97a87a 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1602,6 +1602,8 @@ void __tcp_cleanup_rbuf(struct sock *sk, int copied)
>  		tcp_mstamp_refresh(tp);
>  		tcp_send_ack(sk);
>  	}
> +
> +	tcp_bpf_rcvlowat(sk, NULL);
>  }
>  
>  void tcp_cleanup_rbuf(struct sock *sk, int copied)
> diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
> index 471c78be5513..91bf421fc5b6 100644
> --- a/net/ipv4/tcp_fastopen.c
> +++ b/net/ipv4/tcp_fastopen.c
> @@ -281,6 +281,8 @@ void tcp_fastopen_add_skb(struct sock *sk, struct sk_buff *skb)
>  	TCP_SKB_CB(skb)->seq++;
>  	TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_SYN;
>  
> +	tcp_bpf_rcvlowat(sk, skb);
> +

I'm also not sure about the particular placement of some of these..
For example here, why do it before updating tp? Why not after?

(and same for tcp_ofo_queue)