unix: Convert unix_stream_sendpage() to use MSG_SPLICE_PAGES

Convert unix_stream_sendpage() to use sendmsg() with MSG_SPLICE_PAGES
rather than directly splicing in the pages itself.

This allows ->sendpage() to be replaced by something that can handle
multiple multipage folios in a single transaction.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Kuniyuki Iwashima <kuniyu@amazon.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -1839,24 +1839,6 @@ static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
 	}
 }
 
-static int maybe_init_creds(struct scm_cookie *scm,
-			    struct socket *socket,
-			    const struct sock *other)
-{
-	int err;
-	struct msghdr msg = { .msg_controllen = 0 };
-
-	err = scm_send(socket, &msg, scm, false);
-	if (err)
-		return err;
-
-	if (unix_passcred_enabled(socket, other)) {
-		scm->pid = get_pid(task_tgid(current));
-		current_uid_gid(&scm->creds.uid, &scm->creds.gid);
-	}
-	return err;
-}
-
 static bool unix_skb_scm_eq(struct sk_buff *skb,
 			    struct scm_cookie *scm)
 {
@@ -2292,117 +2274,15 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 static ssize_t unix_stream_sendpage(struct socket *socket, struct page *page,
 				    int offset, size_t size, int flags)
 {
-	int err;
-	bool send_sigpipe = false;
-	bool init_scm = true;
-	struct scm_cookie scm;
-	struct sock *other, *sk = socket->sk;
-	struct sk_buff *skb, *newskb = NULL, *tail = NULL;
-
-	if (flags & MSG_OOB)
-		return -EOPNOTSUPP;
+	struct bio_vec bvec;
+	struct msghdr msg = { .msg_flags = flags | MSG_SPLICE_PAGES };
 
-	other = unix_peer(sk);
-	if (!other || sk->sk_state != TCP_ESTABLISHED)
-		return -ENOTCONN;
-
-	if (false) {
-alloc_skb:
-		unix_state_unlock(other);
-		mutex_unlock(&unix_sk(other)->iolock);
-		newskb = sock_alloc_send_pskb(sk, 0, 0, flags & MSG_DONTWAIT,
-					      &err, 0);
-		if (!newskb)
-			goto err;
-	}
-
-	/* we must acquire iolock as we modify already present
-	 * skbs in the sk_receive_queue and mess with skb->len
-	 */
-	err = mutex_lock_interruptible(&unix_sk(other)->iolock);
-	if (err) {
-		err = flags & MSG_DONTWAIT ? -EAGAIN : -ERESTARTSYS;
-		goto err;
-	}
-
-	if (sk->sk_shutdown & SEND_SHUTDOWN) {
-		err = -EPIPE;
-		send_sigpipe = true;
-		goto err_unlock;
-	}
-
-	unix_state_lock(other);
+	if (flags & MSG_SENDPAGE_NOTLAST)
+		msg.msg_flags |= MSG_MORE;
 
-	if (sock_flag(other, SOCK_DEAD) ||
-	    other->sk_shutdown & RCV_SHUTDOWN) {
-		err = -EPIPE;
-		send_sigpipe = true;
-		goto err_state_unlock;
-	}
-
-	if (init_scm) {
-		err = maybe_init_creds(&scm, socket, other);
-		if (err)
-			goto err_state_unlock;
-		init_scm = false;
-	}
-
-	skb = skb_peek_tail(&other->sk_receive_queue);
-	if (tail && tail == skb) {
-		skb = newskb;
-	} else if (!skb || !unix_skb_scm_eq(skb, &scm)) {
-		if (newskb) {
-			skb = newskb;
-		} else {
-			tail = skb;
-			goto alloc_skb;
-		}
-	} else if (newskb) {
-		/* this is fast path, we don't necessarily need to
-		 * call to kfree_skb even though with newskb == NULL
-		 * this - does no harm
-		 */
-		consume_skb(newskb);
-		newskb = NULL;
-	}
-
-	if (skb_append_pagefrags(skb, page, offset, size, MAX_SKB_FRAGS)) {
-		tail = skb;
-		goto alloc_skb;
-	}
-
-	skb->len += size;
-	skb->data_len += size;
-	skb->truesize += size;
-	refcount_add(size, &sk->sk_wmem_alloc);
-
-	if (newskb) {
-		err = unix_scm_to_skb(&scm, skb, false);
-		if (err)
-			goto err_state_unlock;
-		spin_lock(&other->sk_receive_queue.lock);
-		__skb_queue_tail(&other->sk_receive_queue, newskb);
-		spin_unlock(&other->sk_receive_queue.lock);
-	}
-
-	unix_state_unlock(other);
-	mutex_unlock(&unix_sk(other)->iolock);
-
-	other->sk_data_ready(other);
-	scm_destroy(&scm);
-	return size;
-
-err_state_unlock:
-	unix_state_unlock(other);
-err_unlock:
-	mutex_unlock(&unix_sk(other)->iolock);
-err:
-	kfree_skb(newskb);
-	if (send_sigpipe && !(flags & MSG_NOSIGNAL))
-		send_sig(SIGPIPE, current, 0);
-	if (!init_scm)
-		scm_destroy(&scm);
-	return err;
+	bvec_set_page(&bvec, page, size, offset);
+	iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, size);
+	return unix_stream_sendmsg(socket, &msg, size);
 }
 
 static int unix_seqpacket_sendmsg(struct socket *sock, struct msghdr *msg,