Subject: Re: [PATCH v3 bpf-next 03/11] bpf: tcp: Support bpf_skb_load_bytes() for BPF_SOCK_OPS_RCVQ_CB.

From: Martin KaFai Lau <[email-masked]>
Date: Tue, 26 May 2026 13:34:03 -0700

On Sat, May 23, 2026 at 08:29:32AM +0000, Kuniyuki Iwashima wrote:
> When a TCP skb is queued to sk->sk_receive_queue, BPF SOCK_OPS
> prog can be called with BPF_SOCK_OPS_RCVQ_CB.
> 
> In this hook, we want to parse the RPC descriptor in the skb
> and adjust sk->sk_rcvlowat based on the RPC frame size.
> 
> However, we cannot access payload via bpf_sock_ops.data on
> modern NICs with TCP header/data split on as the payload is
> not placed in the linear area.
> 
> Let's support bpf_skb_load_bytes() for BPF_SOCK_OPS_RCVQ_CB.
> 
> Three notes:
> 
>   1) bpf_sock_ops_kern.skb will be NULL when the BPF prog is
>       invoked from recvmsg().
> 
>   2) Access to bpf_sock_ops.data will be disabled by passing
>       0 end_offset to bpf_skops_init_skb().
> 
>   3) ____bpf_skb_load_bytes() is called directly instead of
>      __bpf_skb_load_bytes() to allow compilers to inline it
>      instead of generating a tail-call.

Some observations below.

> 
> Signed-off-by: Kuniyuki Iwashima <[email-masked]>
> ---
> v2: Explain why using ____ version instead of __
> ---
>  net/core/filter.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 4a50fe2cd863..fa8a7c7d86eb 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -7760,6 +7760,38 @@ static const struct bpf_func_proto bpf_sk_assign_proto = {
>  	.arg3_type	= ARG_ANYTHING,
>  };
>  
> +BPF_CALL_4(bpf_sock_ops_skb_load_bytes, struct bpf_sock_ops_kern *, bpf_sock,
> +	   u32, offset, void *, to, u32, len)
> +{
> +	int err;
> +
> +	if (bpf_sock->op != BPF_SOCK_OPS_RCVQ_CB) {

bpf_dynptr_from_skb() and bpf_dynptr_slice() kfunc could also be considered.
One less bpf_sock->op check in filter.c to maintain and could also avoid
a data copy. There is a bpf_cast_to_kern_ctx() to get to a trusted
skops_kern pointer but this will need changes in verifier.c to get to
skops_kern->skb (e.g. in type_is_trusted_or_null) and this is the tradeoff.

If this new rcvq callback is added to the 'bpf_tcp_ops' proposal [1],
all this will go away. 'struct sk_buff *skb' can be directly passed to an
ops of the 'bpf_tcp_ops'. Supporting '*skb' in a struct_ops has already
been done in the bpf_qdisc.

[1]: https://lore.kernel.org/bpf/[email-masked]/