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Date:   Tue, 19 Feb 2019 23:53:04 +0000
From:   Lawrence Brakmo <brakmo@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        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 2/19/19, 2:52 AM, "netdev-owner@...r.kernel.org on behalf of Daniel Borkmann" <netdev-owner@...r.kernel.org on behalf of daniel@...earbox.net> wrote:

    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

Thanks!
    
    > + *	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

Thanks!
    
    > +		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?

Cgroup skb bpf programs do not have write access to packet data. I originally was doing what you propose, by adding write support and changing the ecn value in the bpf program, but Alexei felt that could create problems. Hence this approach.
    
    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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