netdev - Re: [PATCH bpf-next v5 2/5] bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push

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Date:   Fri, 1 Feb 2019 00:43:36 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Peter Oskolkov <posk@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org
Cc:     Peter Oskolkov <posk.devel@...il.com>,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH bpf-next v5 2/5] bpf: implement BPF_LWT_ENCAP_IP mode in
 bpf_lwt_push_encap

On 01/31/2019 12:51 AM, Peter Oskolkov wrote:
> This patch implements BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap
> BPF helper. It enables BPF programs (specifically, BPF_PROG_TYPE_LWT_IN
> and BPF_PROG_TYPE_LWT_XMIT prog types) to add IP encapsulation headers
> to packets (e.g. IP/GRE, GUE, IPIP).
> 
> This is useful when thousands of different short-lived flows should be
> encapped, each with different and dynamically determined destination.
> Although lwtunnels can be used in some of these scenarios, the ability
> to dynamically generate encap headers adds more flexibility, e.g.
> when routing depends on the state of the host (reflected in global bpf
> maps).
> 
> Signed-off-by: Peter Oskolkov <posk@...gle.com>
> ---
>  include/net/lwtunnel.h |  3 +++
>  net/core/filter.c      |  3 ++-
>  net/core/lwt_bpf.c     | 59 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 64 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/lwtunnel.h b/include/net/lwtunnel.h
> index 33fd9ba7e0e5..f0973eca8036 100644
> --- a/include/net/lwtunnel.h
> +++ b/include/net/lwtunnel.h
> @@ -126,6 +126,8 @@ int lwtunnel_cmp_encap(struct lwtunnel_state *a, struct lwtunnel_state *b);
>  int lwtunnel_output(struct net *net, struct sock *sk, struct sk_buff *skb);
>  int lwtunnel_input(struct sk_buff *skb);
>  int lwtunnel_xmit(struct sk_buff *skb);
> +int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len,
> +			  bool ingress);
>  
>  static inline void lwtunnel_set_redirect(struct dst_entry *dst)
>  {
> @@ -138,6 +140,7 @@ static inline void lwtunnel_set_redirect(struct dst_entry *dst)
>  		dst->input = lwtunnel_input;
>  	}
>  }
> +
>  #else
>  
>  static inline void lwtstate_free(struct lwtunnel_state *lws)
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 27d3fbe4b77b..de6bd4b4e0a3 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -73,6 +73,7 @@
>  #include <linux/seg6_local.h>
>  #include <net/seg6.h>
>  #include <net/seg6_local.h>
> +#include <net/lwtunnel.h>
>  
>  /**
>   *	sk_filter_trim_cap - run a packet through a socket filter
> @@ -4804,7 +4805,7 @@ static int bpf_push_seg6_encap(struct sk_buff *skb, u32 type, void *hdr, u32 len
>  static int bpf_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len,
>  			     bool ingress)
>  {
> -	return -EINVAL;  /* Implemented in the next patch. */
> +	return bpf_lwt_push_ip_encap(skb, hdr, len, ingress);
>  }
>  
>  BPF_CALL_4(bpf_lwt_in_push_encap, struct sk_buff *, skb, u32, type, void *, hdr,
> diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
> index a648568c5e8f..6a6e9acab73d 100644
> --- a/net/core/lwt_bpf.c
> +++ b/net/core/lwt_bpf.c
> @@ -390,6 +390,65 @@ static const struct lwtunnel_encap_ops bpf_encap_ops = {
>  	.owner		= THIS_MODULE,
>  };
>  
> +int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
> +{
> +	struct iphdr *iph;
> +	bool ipv4;
> +	int err;
> +
> +	if (unlikely(len < sizeof(struct iphdr) || len > LWT_BPF_MAX_HEADROOM))
> +		return -EINVAL;
> +
> +	/* validate protocol and length */
> +	iph = (struct iphdr *)hdr;
> +	if (iph->version == 4) {
> +		ipv4 = true;
> +		if (unlikely(len < iph->ihl * 4))
> +			return -EINVAL;
> +	} else if (iph->version == 6) {
> +		ipv4 = false;
> +		if (unlikely(len < sizeof(struct ipv6hdr)))
> +			return -EINVAL;
> +	} else {
> +		return -EINVAL;
> +	}
> +
> +	if (ingress)
> +		err = skb_cow_head(skb, len + skb->mac_len);
> +	else
> +		err = skb_cow_head(skb,
> +				   len + LL_RESERVED_SPACE(skb_dst(skb)->dev));
> +	if (unlikely(err))
> +		return err;
> +
> +	/* push the encap headers and fix pointers */
> +	skb_reset_inner_headers(skb);
> +	skb->encapsulation = 1;
> +	skb_push(skb, len);
> +	if (ingress)
> +		skb_postpush_rcsum(skb, iph, len);
> +	skb_reset_network_header(skb);
> +	memcpy(skb_network_header(skb), hdr, len);
> +	bpf_compute_data_pointers(skb);

Does this work transparently with GSO as well or would we need to
update shared info for this (like in nat64 case, for example)?

> +	if (ipv4) {
> +		skb->protocol = htons(ETH_P_IP);
> +		iph = ip_hdr(skb);
> +		if (iph->ihl * 4 < len)
> +			skb_set_transport_header(skb, iph->ihl * 4);
> +
> +		if (!iph->check)
> +			iph->check = ip_fast_csum((unsigned char *)iph,
> +						  iph->ihl);
> +	} else {
> +		skb->protocol = htons(ETH_P_IPV6);
> +		if (sizeof(struct ipv6hdr) < len)
> +			skb_set_transport_header(skb, sizeof(struct ipv6hdr));
> +	}
> +
> +	return 0;
> +}
> +
>  static int __init bpf_lwt_init(void)
>  {
>  	return lwtunnel_encap_add_ops(&bpf_encap_ops, LWTUNNEL_ENCAP_BPF);
>