Re: [PATCH v2 bpf-next 6/8] bpf: mptcp: Don't support BPF_SOCK_OPS_RCVQ_CB_FLAG.

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Sep 23, 2026 at 9:35 PM UTC, Kuniyuki Iwashima wrote:
> The next patch exposes a new kfunc calling __tcp_set_rcvlowat()
> to bpf_tcp_ops.
>
> MPTCP has its own sock->ops->set_rcvlowat() / mptcp_set_rcvlowat(),
> so we should not allow calling __tcp_set_rcvlowat() on MPTCP
> subflows.
>
> Let's disable BPF_SOCK_OPS_RCVQ_CB_FLAG for MPTCP for now.
>
> If needed in the future, bpf_tcp_ops_set_rcvlowat() could be
> extended to properly support MPTCP.
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
>  include/net/tcp.h      | 15 +++++++++++++++
>  net/core/filter.c      | 10 ++++++----
>  net/ipv4/bpf_tcp_ops.c |  5 ++++-
>  3 files changed, 25 insertions(+), 5 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 07426e8641b7..d3cf655da9ec 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -2932,6 +2932,16 @@ static inline int tcp_call_bpf_3arg(struct sock *sk, int op, u32 arg1, u32 arg2,
>  	return tcp_call_bpf(sk, op, 3, args);
>  }
>  
> +static inline int tcp_set_sock_ops_cb_flags(struct sock *sk, int val)
> +{
> +	if (sk_is_mptcp(sk) &&
> +	    (val & BPF_SOCK_OPS_RCVQ_CB_FLAG))
> +		return -EOPNOTSUPP;
> +
> +	tcp_sk(sk)->bpf_sock_ops_cb_flags = val;
> +	return 0;
> +}
> +
>  static inline void tcp_clear_sock_ops_cb_flags(struct sock *sk)
>  {
>  	tcp_sk(sk)->bpf_sock_ops_cb_flags = 0;
> @@ -2954,6 +2964,11 @@ static inline int tcp_call_bpf_3arg(struct sock *sk, int op, u32 arg1, u32 arg2,
>  	return -EPERM;
>  }
>  
> +static inline int tcp_set_sock_ops_cb_flags(struct sock *sk, int val)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
>  static inline void tcp_clear_sock_ops_cb_flags(struct sock *sk)
>  {
>  }
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 5feb99884682..f29c061bb066 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5588,8 +5588,7 @@ static int bpf_sol_tcp_setsockopt(struct sock *sk, int optname,
>  	case TCP_BPF_SOCK_OPS_CB_FLAGS:
>  		if (val & ~(BPF_SOCK_OPS_ALL_CB_FLAGS))
>  			return -EINVAL;
> -		tp->bpf_sock_ops_cb_flags = val;
> -		break;
> +		return tcp_set_sock_ops_cb_flags(sk, val);
>  	default:
>  		return -EINVAL;
>  	}
> @@ -6178,8 +6177,9 @@ static const struct bpf_func_proto bpf_sock_ops_getsockopt_proto = {
>  BPF_CALL_2(bpf_sock_ops_cb_flags_set, struct bpf_sock_ops_kern *, bpf_sock,
>  	   int, argval)
>  {
> -	struct sock *sk = bpf_sock->sk;
>  	int val = argval & BPF_SOCK_OPS_ALL_CB_FLAGS;
> +	struct sock *sk = bpf_sock->sk;
> +	int err;
>  
>  	if (!is_locked_tcp_sock_ops(bpf_sock))
>  		return -EOPNOTSUPP;
> @@ -6187,7 +6187,9 @@ BPF_CALL_2(bpf_sock_ops_cb_flags_set, struct bpf_sock_ops_kern *, bpf_sock,
>  	if (!IS_ENABLED(CONFIG_INET) || !sk_fullsock(sk))
>  		return -EINVAL;
>  
> -	tcp_sk(sk)->bpf_sock_ops_cb_flags = val;
> +	err = tcp_set_sock_ops_cb_flags(sk, val);
> +	if (err)
> +		return err;

Afaict the tcp_set_sock_ops_cb_flags is inconsistent with the previous return values,
in that it returns an errno instead of the flags it couldn't set (it also doesn't
set the flags that we can set for MPTCP when BPF_SOCK_OPS_RCVQ_CB_FLAG is set, which
is the convention of its callers). Would changing the error path to

	tp->bpf_sock_ops_cb_flags = val & BPF_SOCK_OPS_RCVQ_CB_FLAG;
	return (val & (~(BPF_SOCK_OPS_ALL_CB_FLAGS ^ BPF_SOCK_OPS_RCVQ_CB_FLAG);

work?


>  
>  	return argval & (~BPF_SOCK_OPS_ALL_CB_FLAGS);
>  }
> diff --git a/net/ipv4/bpf_tcp_ops.c b/net/ipv4/bpf_tcp_ops.c
> index 4b48711d92a2..b0cade34cce6 100644
> --- a/net/ipv4/bpf_tcp_ops.c
> +++ b/net/ipv4/bpf_tcp_ops.c
> @@ -223,8 +223,11 @@ const struct bpf_func_proto bpf_tcp_ops_get_retval_proto = {
>  BPF_CALL_2(bpf_tcp_ops_cb_flags_set, struct sock *, sk, int, argval)
>  {
>  	int val = argval & BPF_SOCK_OPS_ALL_CB_FLAGS;
> +	int err;
>  
> -	tcp_sk(sk)->bpf_sock_ops_cb_flags = val;
> +	err = tcp_set_sock_ops_cb_flags(sk, val);
> +	if (err)
> +		return err;
>  
>  	return argval & ~BPF_SOCK_OPS_ALL_CB_FLAGS;
>  }