staging: comedi: allow buffer wraparound in comedi_write()

`comedi_write()` copies data from the user buffer to the acquisition
data buffer, which is cyclic, using a single call to `copy_from_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_from_user()` if necessary.
Signed-off-by: NIan Abbott <abbotti@mev.co.uk>
Reviewed-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2346,6 +2346,7 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 	add_wait_queue(&async->wait_head, &wait);
 	while (count == 0 && !retval) {
 		unsigned runflags;
+		unsigned int wp, n1, n2;
 
 		set_current_state(TASK_INTERRUPTIBLE);
 
@@ -2360,9 +2361,6 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 		/* Allocate all free buffer space. */
 		comedi_buf_write_alloc(s, async->prealloc_bufsz);
 		m = comedi_buf_write_n_allocated(s);
-		/* Avoid buffer wraparound. */
-		if (async->buf_write_ptr + m > async->prealloc_bufsz)
-			m = async->prealloc_bufsz - async->buf_write_ptr;
 		n = min_t(size_t, m, nbytes);
 
 		if (n == 0) {
@@ -2388,8 +2386,14 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 			continue;
 		}
 
-		m = copy_from_user(async->prealloc_buf + async->buf_write_ptr,
-				   buf, n);
+		wp = async->buf_write_ptr;
+		n1 = min(n, async->prealloc_bufsz - wp);
+		n2 = n - n1;
+		m = copy_from_user(async->prealloc_buf + wp, buf, n1);
+		if (m)
+			m += n2;
+		else if (n2)
+			m = copy_from_user(async->prealloc_buf, buf + n1, n2);
 		if (m) {
 			n -= m;
 			retval = -EFAULT;