netdev - Re: [PATCH v2 bpf-next 4/9] bpf: add bpf helper bpf_skb_ecn_set

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Message-ID: <fdcd5fbb-0f51-18f9-a13a-52e8009d082e@iogearbox.net>
Date:   Sat, 23 Feb 2019 02:14:26 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     brakmo <brakmo@...com>, netdev <netdev@...r.kernel.org>
Cc:     Martin Lau <kafai@...com>, Alexei Starovoitov <ast@...com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 4/9] bpf: add bpf helper bpf_skb_ecn_set_ce

On 02/23/2019 02:06 AM, brakmo wrote:
> This patch adds a new bpf helper BPF_FUNC_skb_ecn_set_ce
> "int bpf_skb_ecn_set_ce(struct sk_buff *skb)". It is added to
> BPF_PROG_TYPE_CGROUP_SKB typed bpf_prog which currently can
> be attached to the ingress and egress path. The helper is needed
> because his type of bpf_prog cannot modify the skb directly.
> 
> This helper is used to set the ECN field of ECN capable IP packets to ce
> (congestion encountered) in the IPv6 or IPv4 header of the skb. It can be
> used by a bpf_prog to manage egress or ingress network bandwdith limit
> per cgroupv2 by inducing an ECN response in the TCP sender.
> This works best when using DCTCP.
> 
> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> ---
>  include/uapi/linux/bpf.h | 10 +++++++++-
>  net/core/filter.c        | 14 ++++++++++++++
>  2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 95b5058fa945..fc646f3eaf9b 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2365,6 +2365,13 @@ union bpf_attr {
>   *		Make a tcp_sock enter CWR state.
>   *	Return
>   *		0 on success, or a negative error in case of failure.
> + *
> + * int bpf_skb_ecn_set_ce(struct sk_buf *skb)
> + *	Description
> + *		Sets ECN of IP header to ce (congestion encountered) if
> + *		current value is ect (ECN capable). Works with IPv6 and IPv4.
> + *	Return
> + *		1 if set, 0 if not set.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -2464,7 +2471,8 @@ union bpf_attr {
>  	FN(spin_unlock),		\
>  	FN(sk_fullsock),		\
>  	FN(tcp_sock),			\
> -	FN(tcp_enter_cwr),
> +	FN(tcp_enter_cwr),		\
> +	FN(skb_ecn_set_ce),
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>   * function eBPF program intends to call
> diff --git a/net/core/filter.c b/net/core/filter.c
> index ca57ef25279c..955369c6ed30 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5444,6 +5444,18 @@ static const struct bpf_func_proto bpf_tcp_enter_cwr_proto = {
>  	.ret_type    = RET_INTEGER,
>  	.arg1_type    = ARG_PTR_TO_TCP_SOCK,
>  };
> +
> +BPF_CALL_1(bpf_skb_ecn_set_ce, struct sk_buff *, skb)
> +{
> +	return INET_ECN_set_ce(skb);

Hm, but as mentioned last time, don't we have to ensure here that skb
is writable (aka skb->data private to us before writing into it)?

> +}
> +
> +static const struct bpf_func_proto bpf_skb_ecn_set_ce_proto = {
> +	.func		= bpf_skb_ecn_set_ce,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_CTX,
> +};
>  #endif /* CONFIG_INET */
>  
>  bool bpf_helper_changes_pkt_data(void *func)
> @@ -5610,6 +5622,8 @@ cg_skb_func_proto(enum bpf_func_id func_id, struct bpf_prog *prog)
>  		} else {
>  			return NULL;
>  		}
> +	case BPF_FUNC_skb_ecn_set_ce:
> +		return &bpf_skb_ecn_set_ce_proto;
>  #endif
>  	default:
>  		return sk_filter_func_proto(func_id, prog);
> 

Thanks,
Daniel