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

From: Martin Lau <kafai@fb.com>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Lawrence Brakmo <brakmo@fb.com>, netdev <netdev@vger.kernel.org>,
	"Alexei Starovoitov" <ast@fb.com>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v2 bpf-next 4/9] bpf: add bpf helper bpf_skb_ecn_set_ce
Date: Sat, 23 Feb 2019 07:30:35 +0000	[thread overview]
Message-ID: <20190223073031.utnow4seviqyfqta@kafai-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <fdcd5fbb-0f51-18f9-a13a-52e8009d082e@iogearbox.net>

On Sat, Feb 23, 2019 at 02:14:26AM +0100, Daniel Borkmann wrote:
> 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@fb.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)?
INET_ECN_set_ce(skb) is also called from a few net/sched/sch_*.c
but I don't see how they ensure if a skb is writable.

May be I have missed something there that can also be borrowed and
reused here?

Thanks,
Martin

> 
> > +}
> > +
> > +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