Re: [PATCH v1 bpf-next 3/8] bpf: tcp: Support bpf_skb_load_bytes() for BPF_SOCK_OPS_RCVLOWAT_CB.

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

On 05/08, 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_RCVLOWAT_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_RCVLOWAT_CB.
> 
> Two 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().
> 
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
>  net/core/filter.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 5fa9189eb772..94d07a15b2ab 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -7718,6 +7718,38 @@ 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_RCVLOWAT_CB) {
> +		err = -EOPNOTSUPP;
> +		goto err_clear;
> +	}
> +
> +	if (!bpf_sock->skb) {
> +		err = -EPERM;
> +		goto err_clear;
> +	}
> +
> +	return ____bpf_skb_load_bytes(bpf_sock->skb, offset, to, len);

nit: wondering whether it's better to use __bpf_skb_load_bytes (which
wraps BPF_CALL_4's ____bpf_skb_load_bytes) here? Seems a bit less
magic and used already by dynptr?