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>

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 368bfb92b115c0e99ce4c654f6fdecb6ec5a2763..a11f271800ef990987b85bc1df1614b2d549650a 100644 (file)
--- a/fs/seq_file.c
+++ b/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)