seq_file: reset iterator to first record for zero offset

If kernfs file is empty on a first read, successive read operations
using the same file descriptor will return no data, even when data is
available. Default kernfs 'seq_next' implementation advances iterator
position even when next object is not there. Kernfs 'seq_start' for
following requests will not return iterator as position is already on
the second object.

This defect doesn't allow to monitor badblocks sysfs files from MD raid.
They are initially empty but if data appears at some stage, userspace is
not able to read it.

Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/seq_file.c

@@ -190,6 +190,13 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 	 */
 	m->version = file->f_version;
 
+	/*
+	 * if request is to read from zero offset, reset iterator to first
+	 * record as it might have been already advanced by previous requests
+	 */
+	if (*ppos == 0)
+		m->index = 0;
+
 	/* Don't assume *ppos is where we left it */
 	if (unlikely(*ppos != m->read_pos)) {
 		while ((err = traverse(m, *ppos)) == -EAGAIN)