bpf: tcp: Add bpf_skops_parse_hdr()

The patch adds a function bpf_skops_parse_hdr().
It will call the bpf prog to parse the TCP header received at
a tcp_sock that has at least reached the ESTABLISHED state.

For the packets received during the 3WHS (SYN, SYNACK and ACK),
the received skb will be available to the bpf prog during the callback
in bpf_skops_established() introduced in the previous patch and
in the bpf_skops_write_hdr_opt() that will be added in the
next patch.

Calling bpf prog to parse header is controlled by two new flags in
tp->bpf_sock_ops_cb_flags:
BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG and
BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG.

When BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG is set,
the bpf prog will only be called when there is unknown
option in the TCP header.

When BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG is set,
the bpf prog will be called on all received TCP header.

This function is half implemented to highlight the changes in
TCP stack.  The actual codes preparing the bpf running context and
invoking the bpf prog will be added in the later patch with other
necessary bpf pieces.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/bpf/20200820190046.2885054-1-kafai@fb.com

include/uapi/linux/bpf.h

@@ -4173,8 +4173,10 @@ enum {
 	BPF_SOCK_OPS_RETRANS_CB_FLAG	= (1<<1),
 	BPF_SOCK_OPS_STATE_CB_FLAG	= (1<<2),
 	BPF_SOCK_OPS_RTT_CB_FLAG	= (1<<3),
+	BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG  = (1<<4),
+	BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG = (1<<5),
 /* Mask of all currently supported cb flags */
-	BPF_SOCK_OPS_ALL_CB_FLAGS       = 0xF,
+	BPF_SOCK_OPS_ALL_CB_FLAGS       = 0x3F,
 };
 
 /* List of known BPF sock_ops operators.

net/ipv4/tcp_input.c

@@ -139,6 +139,36 @@ EXPORT_SYMBOL_GPL(clean_acked_data_flush);
 #endif
 
 #ifdef CONFIG_CGROUP_BPF
+static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
+{
+	bool unknown_opt = tcp_sk(sk)->rx_opt.saw_unknown &&
+		BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
+				       BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG);
+	bool parse_all_opt = BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
+						    BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG);
+
+	if (likely(!unknown_opt && !parse_all_opt))
+		return;
+
+	/* The skb will be handled in the
+	 * bpf_skops_established() or
+	 * bpf_skops_write_hdr_opt().
+	 */
+	switch (sk->sk_state) {
+	case TCP_SYN_RECV:
+	case TCP_SYN_SENT:
+	case TCP_LISTEN:
+		return;
+	}
+
+	/* BPF prog will have access to the sk and skb.
+	 *
+	 * The bpf running context preparation and the actual bpf prog
+	 * calling will be implemented in a later PATCH together with
+	 * other bpf pieces.
+	 */
+}
+
 static void bpf_skops_established(struct sock *sk, int bpf_op,
 				  struct sk_buff *skb)
 {
@@ -155,6 +185,10 @@ static void bpf_skops_established(struct sock *sk, int bpf_op,
 	BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
 }
 #else
+static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
+{
+}
+
 static void bpf_skops_established(struct sock *sk, int bpf_op,
 				  struct sk_buff *skb)
 {
@@ -5623,6 +5657,8 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 		goto discard;
 	}
 
+	bpf_skops_parse_hdr(sk, skb);
+
 	return true;
 
 discard:

tools/include/uapi/linux/bpf.h

@@ -4173,8 +4173,10 @@ enum {
 	BPF_SOCK_OPS_RETRANS_CB_FLAG	= (1<<1),
 	BPF_SOCK_OPS_STATE_CB_FLAG	= (1<<2),
 	BPF_SOCK_OPS_RTT_CB_FLAG	= (1<<3),
+	BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG  = (1<<4),
+	BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG = (1<<5),
 /* Mask of all currently supported cb flags */
-	BPF_SOCK_OPS_ALL_CB_FLAGS       = 0xF,
+	BPF_SOCK_OPS_ALL_CB_FLAGS       = 0x3F,
 };
 
 /* List of known BPF sock_ops operators.