[PATCH bpf-next v5 2/5] bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap

[Peter Oskolkov <posk@google.com>]

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@google.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);
+
+	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);
-- 
2.20.1.495.gaa96b0ce6b-goog