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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 5/8/26 3:33 PM, 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 <kuniyu@google.com>
> ---
>   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);
>   }
>   

tcp_read_skb (process frame 1 and __skb_unlink)
└─ sk_psock_verdict_recv
     └─ sk_psock_verdict_apply
         └─ tcp_eat_skb
             └─ tcp_cleanup_rbuf
                 └─ __tcp_cleanup_rbuf
                     └─ BPF RCVLOWAT_CB
                         └─ bpf_sock_ops_tcp_set_rcvlowat (wakeup=true)
                             └─ tcp_data_ready
                                 └─ sk_psock_verdict_data_ready
                                     └─ tcp_read_skb (frame 2)
                                         └─ ... → tcp_read_skb (frame 3) ...

For strparser it use read_sock instead of read_skb and it will become 
more complicated...

I think this will cause stack overflow with amounts of skbs in receive 
queue or infinite call(not tested) for sockmap/kTLS/strparser.