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Date:   Tue, 19 Feb 2019 11:52:34 +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>,
        Daniel Borkmann --cc=Kernel Team 
        <"daniel@...earbox.netKernel-team"@fb.com>
Subject: Re: [PATCH bpf-next 3/9] bpf: add bpf helper bpf_skb_set_ecn

On 02/19/2019 06:38 AM, brakmo wrote:
> This patch adds a new bpf helper BPF_FUNC_skb_set_ecn
> "int bpf_skb_set_Ecn(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. This type of
> bpf_prog cannot modify the skb directly.
> 
> This helper is used to set the ECN bits (2) of the IPv6 or IPv4
> header in skb. It can be used by a bpf_prog to manage egress
> network bandwdith limit per cgroupv2 by inducing an ECN
> response in the TCP sender (when the packet is ECN enabled).
> This works best when using DCTCP.
> 
> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> ---
>  include/uapi/linux/bpf.h | 10 +++++++++-
>  net/core/filter.c        | 29 +++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 9e9f4f1a0370..5daf404511f7 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
> + *
> + * int bpf_skb_set_ecn(struct sk_buf *skb, int val)

Nit: BPF_CALL_2() has u32 val

> + *	Description
> + *		Sets ECN bits (2) of IP header. Works with IPv6 and IPv4.
> + *		val should be one of 0, 1, 2, 3.
> + *	Return
> + *		-EINVAL on error (e.g. val > 3), 0 otherwise.
>   */
>  #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_set_ecn),
>  
>  /* 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 f51c4a781844..275acfb2117d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5438,6 +5438,33 @@ static const struct bpf_func_proto bpf_tcp_enter_cwr_proto = {
>  	.ret_type    = RET_INTEGER,
>  	.arg1_type    = ARG_PTR_TO_TCP_SOCK,
>  };
> +
> +BPF_CALL_2(bpf_skb_set_ecn, struct sk_buff *, skb, u32, val)
> +{
> +	struct ipv6hdr *ip6h = ipv6_hdr(skb);
> +
> +	if ((val & ~0x3) != 0)

Nit: INET_ECN_MASK

> +		return -EINVAL;
> +
> +	if (ip6h->version == 6) {
> +		ip6h->flow_lbl[0] = (ip6h->flow_lbl[0] & ~0x30) | (val << 4);
> +		return 0;
> +	} else if (ip6h->version == 4) {
> +		struct iphdr *ip4h = (struct iphdr *)ip6h;
> +
> +		ip4h->tos = (ip4h->tos & ~0x3) | val;
> +		return 0;
> +	}

Couldn't this be done as native BPF code via direct packet access instead?
Afaik, skb->data should most likely points to network header for the hooks
and skb->protocol should be one of ETH_P_IP{,V6}, no?

Aside from this, don't we also have cloned skbs here (in particular from
TCP side)?

Looking at cg_skb_verifier_ops ... it seems there also a bug in the current
code, namely that if we have a direct packet write, we don't make the skb
writable; at that point skb->data is not private. The cg_skb_is_valid_access()
allows to fetch PTR_TO_PACKET{,_END}, so we need a fix like the below for -bpf:

diff --git a/net/core/filter.c b/net/core/filter.c
index f7d0004fc160..34fe6da0a236 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5796,6 +5796,12 @@ static bool sk_filter_is_valid_access(int off, int size,
        return bpf_skb_is_valid_access(off, size, type, prog, info);
 }

+static int cg_skb_prologue(struct bpf_insn *insn_buf, bool direct_write,
+                          const struct bpf_prog *prog)
+{
+       return bpf_unclone_prologue(insn_buf, direct_write, prog, 0);
+}
+
 static bool cg_skb_is_valid_access(int off, int size,
                                   enum bpf_access_type type,
                                   const struct bpf_prog *prog,
@@ -7595,6 +7601,7 @@ const struct bpf_verifier_ops cg_skb_verifier_ops = {
        .get_func_proto         = cg_skb_func_proto,
        .is_valid_access        = cg_skb_is_valid_access,
        .convert_ctx_access     = bpf_convert_ctx_access,
+       .gen_prologue           = cg_skb_prologue,
 };

 const struct bpf_prog_ops cg_skb_prog_ops = {

> +	return -EINVAL;
> +}
> +
> +static const struct bpf_func_proto bpf_skb_set_ecn_proto = {
> +	.func		= bpf_skb_set_ecn,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_CTX,
> +	.arg2_type	= ARG_ANYTHING,
> +};
>  #endif /* CONFIG_INET */
>  
>  bool bpf_helper_changes_pkt_data(void *func)
> @@ -5599,6 +5626,8 @@ cg_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_tcp_sock_proto;
>  	case BPF_FUNC_tcp_enter_cwr:
>  		return &bpf_tcp_enter_cwr_proto;
> +	case BPF_FUNC_skb_set_ecn:
> +		return &bpf_skb_set_ecn_proto;
>  #endif
>  	default:
>  		return sk_filter_func_proto(func_id, prog);
> 

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