[PATCH bpf-next v3 1/4] bpf: add plumbing for BPF_LWT_ENCAP_IP in bpf_lwt_push_encap

[Peter Oskolkov <posk@google.com>]

This patch adds all needed plumbing in preparation to allowing
bpf programs to do IP encapping via bpf_lwt_push_encap. Actual
implementation is added in the next patch in the patchset.

Of note:
- bpf_lwt_push_encap can now be called from BPF_PROG_TYPE_LWT_XMIT
  prog types in addition to BPF_PROG_TYPE_LWT_IN;
- as route lookups are different for ingress vs egress, the single
  external bpf_lwt_push_encap BPF helper is routed internally to
  either bpf_lwt_in_push_encap or bpf_lwt_xmit_push_encap BPF_CALLs,
  depending on prog type.

Signed-off-by: Peter Oskolkov <posk@google.com>
---
 include/uapi/linux/bpf.h | 23 ++++++++++++++++++--
 net/core/filter.c        | 46 +++++++++++++++++++++++++++++++++++-----
 2 files changed, 62 insertions(+), 7 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 60b99b730a41..c4fee8b45762 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2015,6 +2015,16 @@ union bpf_attr {
  *			Only works if *skb* contains an IPv6 packet. Insert a
  *			Segment Routing Header (**struct ipv6_sr_hdr**) inside
  *			the IPv6 header.
+ *		**BPF_LWT_ENCAP_IP**
+ *			IP encapsulation (GRE/GUE/IPIP/etc). The outer header
+ *			must be IPv4 or IPv6, followed by zero or more
+ *			additional headers, up to LWT_BPF_MAX_HEADROOM total
+ *			bytes in all prepended headers.
+ *
+ *		BPF_LWT_ENCAP_SEG6*** types can be called by bpf programs of
+ *		type BPF_PROG_TYPE_LWT_IN; BPF_LWT_ENCAP_IP type can be called
+ *		by bpf programs of types BPF_PROG_TYPE_LWT_IN and
+ *		BPF_PROG_TYPE_LWT_XMIT.
  *
  * 		A call to this helper is susceptible to change the underlaying
  * 		packet buffer. Therefore, at load time, all checks on pointers
@@ -2495,7 +2505,8 @@ enum bpf_hdr_start_off {
 /* Encapsulation type for BPF_FUNC_lwt_push_encap helper. */
 enum bpf_lwt_encap_mode {
 	BPF_LWT_ENCAP_SEG6,
-	BPF_LWT_ENCAP_SEG6_INLINE
+	BPF_LWT_ENCAP_SEG6_INLINE,
+	BPF_LWT_ENCAP_IP,
 };
 
 #define __bpf_md_ptr(type, name)	\
@@ -2583,7 +2594,15 @@ enum bpf_ret_code {
 	BPF_DROP = 2,
 	/* 3-6 reserved */
 	BPF_REDIRECT = 7,
-	/* >127 are reserved for prog type specific return codes */
+	/* >127 are reserved for prog type specific return codes.
+	 *
+	 * BPF_LWT_REROUTE: used by BPF_PROG_TYPE_LWT_IN and
+	 *    BPF_PROG_TYPE_LWT_XMIT to indicate that skb's dst
+	 *    has changed and appropriate dst_input() or dst_output()
+	 *    action has to be taken (this is an L3 redirect, as
+	 *    opposed to L2 redirect represented by BPF_REDIRECT above).
+	 */
+	BPF_LWT_REROUTE = 128,
 };
 
 struct bpf_sock {
diff --git a/net/core/filter.c b/net/core/filter.c
index 8e587dd1da20..fd3ae092d3d7 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4793,7 +4793,13 @@ static int bpf_push_seg6_encap(struct sk_buff *skb, u32 type, void *hdr, u32 len
 }
 #endif /* CONFIG_IPV6_SEG6_BPF */
 
-BPF_CALL_4(bpf_lwt_push_encap, struct sk_buff *, skb, u32, type, void *, hdr,
+static int bpf_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len,
+			     bool ingress)
+{
+	return -EINVAL;  /* Implemented in the next patch. */
+}
+
+BPF_CALL_4(bpf_lwt_in_push_encap, struct sk_buff *, skb, u32, type, void *, hdr,
 	   u32, len)
 {
 	switch (type) {
@@ -4801,14 +4807,41 @@ BPF_CALL_4(bpf_lwt_push_encap, struct sk_buff *, skb, u32, type, void *, hdr,
 	case BPF_LWT_ENCAP_SEG6:
 	case BPF_LWT_ENCAP_SEG6_INLINE:
 		return bpf_push_seg6_encap(skb, type, hdr, len);
+#endif
+#if IS_ENABLED(CONFIG_LWTUNNEL_BPF)
+	case BPF_LWT_ENCAP_IP:
+		return bpf_push_ip_encap(skb, hdr, len, true /* ingress */);
 #endif
 	default:
 		return -EINVAL;
 	}
 }
 
-static const struct bpf_func_proto bpf_lwt_push_encap_proto = {
-	.func		= bpf_lwt_push_encap,
+BPF_CALL_4(bpf_lwt_xmit_push_encap, struct sk_buff *, skb, u32, type,
+	   void *, hdr, u32, len)
+{
+	switch (type) {
+#if IS_ENABLED(CONFIG_LWTUNNEL_BPF)
+	case BPF_LWT_ENCAP_IP:
+		return bpf_push_ip_encap(skb, hdr, len, false /* egress */);
+#endif
+	default:
+		return -EINVAL;
+	}
+}
+
+static const struct bpf_func_proto bpf_lwt_in_push_encap_proto = {
+	.func		= bpf_lwt_in_push_encap,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_ANYTHING,
+	.arg3_type	= ARG_PTR_TO_MEM,
+	.arg4_type	= ARG_CONST_SIZE
+};
+
+static const struct bpf_func_proto bpf_lwt_xmit_push_encap_proto = {
+	.func		= bpf_lwt_xmit_push_encap,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
 	.arg1_type	= ARG_PTR_TO_CTX,
@@ -5274,7 +5307,8 @@ bool bpf_helper_changes_pkt_data(void *func)
 	    func == bpf_lwt_seg6_adjust_srh ||
 	    func == bpf_lwt_seg6_action ||
 #endif
-	    func == bpf_lwt_push_encap)
+	    func == bpf_lwt_in_push_encap ||
+	    func == bpf_lwt_xmit_push_encap)
 		return true;
 
 	return false;
@@ -5652,7 +5686,7 @@ lwt_in_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
 	switch (func_id) {
 	case BPF_FUNC_lwt_push_encap:
-		return &bpf_lwt_push_encap_proto;
+		return &bpf_lwt_in_push_encap_proto;
 	default:
 		return lwt_out_func_proto(func_id, prog);
 	}
@@ -5688,6 +5722,8 @@ lwt_xmit_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_l4_csum_replace_proto;
 	case BPF_FUNC_set_hash_invalid:
 		return &bpf_set_hash_invalid_proto;
+	case BPF_FUNC_lwt_push_encap:
+		return &bpf_lwt_xmit_push_encap_proto;
 	default:
 		return lwt_out_func_proto(func_id, prog);
 	}
-- 
2.20.1.495.gaa96b0ce6b-goog