skmsg: Return copied bytes in sk_msg_memcopy_from_iter

Previously sk_msg_memcopy_from_iter returns the copied bytes from the
last copy_from_iter{,_nocache} call upon success.

This commit changes it to return the total number of copied bytes on
success.

Signed-off-by: Levi Zim <rsworktech@outlook.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@kernel.org>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241130-tcp-bpf-sendmsg-v1-1-bae583d014f3@outlook.com

net/core/skmsg.c

@@ -369,8 +369,8 @@ int sk_msg_memcopy_from_iter(struct sock *sk, struct iov_iter *from,
 			     struct sk_msg *msg, u32 bytes)
 {
 	int ret = -ENOSPC, i = msg->sg.curr;
+	u32 copy, buf_size, copied = 0;
 	struct scatterlist *sge;
-	u32 copy, buf_size;
 	void *to;
 
 	do {
@@ -397,6 +397,7 @@ int sk_msg_memcopy_from_iter(struct sock *sk, struct iov_iter *from,
 			goto out;
 		}
 		bytes -= copy;
+		copied += copy;
 		if (!bytes)
 			break;
 		msg->sg.copybreak = 0;
@@ -404,7 +405,7 @@ int sk_msg_memcopy_from_iter(struct sock *sk, struct iov_iter *from,
 	} while (i != msg->sg.end);
 out:
 	msg->sg.curr = i;
-	return ret;
+	return (ret < 0) ? ret : copied;
 }
 EXPORT_SYMBOL_GPL(sk_msg_memcopy_from_iter);