[PATCH] seq_file: don't set read position for invalid iterator

[Tomasz Majchrzak <tomasz.majchrzak@intel.com>]

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.

Don't set read position if valid iterator has not been returned.

Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
---
 fs/seq_file.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 19f532e..893db43 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -242,7 +242,8 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 			m->count = 0;
 		if (unlikely(!m->count)) {
 			p = m->op->next(m, p, &pos);
-			m->index = pos;
+			if (p && !IS_ERR(p))
+				m->index = pos;
 			continue;
 		}
 		if (m->count < m->size)
-- 
1.8.3.1