staging: comedi: allow buffer wraparound in comedi_read()

`comedi_read()` copies data from the acquisition data buffer, which is
cyclic, to the user buffer using a single call to `copy_to_user()`.  It
currently avoids having to deal with wraparound of the cyclic buffer by
limiting the amount it copies (and the amount returned to the user).
Change it to deal with the wraparound using two calls to
`copy_to_user()` if necessary.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2491,11 +2491,11 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 
 	add_wait_queue(&async->wait_head, &wait);
 	while (nbytes > 0 && !retval) {
+		unsigned int rp, n1, n2;
+
 		set_current_state(TASK_INTERRUPTIBLE);
 
 		m = comedi_buf_read_n_available(s);
-		if (async->buf_read_ptr + m > async->prealloc_bufsz)
-			m = async->prealloc_bufsz - async->buf_read_ptr;
 		n = min_t(size_t, m, nbytes);
 
 		if (n == 0) {
@@ -2532,8 +2532,14 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 			}
 			continue;
 		}
-		m = copy_to_user(buf, async->prealloc_buf +
-				 async->buf_read_ptr, n);
+		rp = async->buf_read_ptr;
+		n1 = min(n, async->prealloc_bufsz - rp);
+		n2 = n - n1;
+		m = copy_to_user(buf, async->prealloc_buf + rp, n1);
+		if (m)
+			m += n2;
+		else if (n2)
+			m = copy_to_user(buf + n1, async->prealloc_buf, n2);
 		if (m) {
 			n -= m;
 			retval = -EFAULT;