[PATCH v2] seq_file: don't set read position for invalid iterator

[Tomasz Majchrzak]

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 bug 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. It doesn't affect any released applications but it
is necessary for upcoming bad block support for external metadata in MD
raid.

Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@xxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 fs/seq_file.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 6dc4296..74197f4 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -235,7 +235,7 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 	p = m->op->start(m, &pos);
 	while (1) {
 		err = PTR_ERR(p);
-		if (!p || IS_ERR(p))
+		if (IS_ERR_OR_NULL(p))
 			break;
 		err = m->op->show(m, p);
 		if (err < 0)
@@ -244,7 +244,8 @@ 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 (!IS_ERR_OR_NULL(p))
+				m->index = pos;
 			continue;
 		}
 		if (m->count < m->size)
-- 
1.8.3.1