tcp: Move skb_steal_sock() to request_sock.h

We will support arbitrary SYN Cookie with BPF.

If BPF prog validates ACK and kfunc allocates a reqsk, it will
be carried to TCP stack as skb->sk with req->syncookie 1.

In skb_steal_sock(), we need to check inet_reqsk(sk)->syncookie
to see if the reqsk is created by kfunc.  However, inet_reqsk()
is not available in sock.h.

Let's move skb_steal_sock() to request_sock.h.

While at it, we refactor skb_steal_sock() so it returns early if
skb->sk is NULL to minimise the following patch.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240115205514.68364-3-kuniyu@amazon.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/request_sock.h

@@ -83,6 +83,34 @@ static inline struct sock *req_to_sk(struct request_sock *req)
 	return (struct sock *)req;
 }
 
+/**
+ * skb_steal_sock - steal a socket from an sk_buff
+ * @skb: sk_buff to steal the socket from
+ * @refcounted: is set to true if the socket is reference-counted
+ * @prefetched: is set to true if the socket was assigned from bpf
+ */
+static inline struct sock *skb_steal_sock(struct sk_buff *skb,
+					  bool *refcounted, bool *prefetched)
+{
+	struct sock *sk = skb->sk;
+
+	if (!sk) {
+		*prefetched = false;
+		*refcounted = false;
+		return NULL;
+	}
+
+	*prefetched = skb_sk_is_prefetched(skb);
+	if (*prefetched)
+		*refcounted = sk_is_refcounted(sk);
+	else
+		*refcounted = true;
+
+	skb->destructor = NULL;
+	skb->sk = NULL;
+	return sk;
+}
+
 static inline struct request_sock *
 reqsk_alloc(const struct request_sock_ops *ops, struct sock *sk_listener,
 	    bool attach_listener)

include/net/sock.h

@@ -2814,31 +2814,6 @@ sk_is_refcounted(struct sock *sk)
 	return !sk_fullsock(sk) || !sock_flag(sk, SOCK_RCU_FREE);
 }
 
-/**
- * skb_steal_sock - steal a socket from an sk_buff
- * @skb: sk_buff to steal the socket from
- * @refcounted: is set to true if the socket is reference-counted
- * @prefetched: is set to true if the socket was assigned from bpf
- */
-static inline struct sock *
-skb_steal_sock(struct sk_buff *skb, bool *refcounted, bool *prefetched)
-{
-	if (skb->sk) {
-		struct sock *sk = skb->sk;
-
-		*refcounted = true;
-		*prefetched = skb_sk_is_prefetched(skb);
-		if (*prefetched)
-			*refcounted = sk_is_refcounted(sk);
-		skb->destructor = NULL;
-		skb->sk = NULL;
-		return sk;
-	}
-	*prefetched = false;
-	*refcounted = false;
-	return NULL;
-}
-
 /* Checks if this SKB belongs to an HW offloaded socket
  * and whether any SW fallbacks are required based on dev.
  * Check decrypted mark in case skb_orphan() cleared socket.