diff mbox

seq_file: reset iterator to first record for zero offset

Message ID 20161213143417.GB27207@veci.piliscsaba.szeredi.hu (mailing list archive)
State New, archived
Headers show

Commit Message

Miklos Szeredi Dec. 13, 2016, 2:34 p.m. UTC 
From: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Date: Tue, 29 Nov 2016 15:18:20 +0100

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>
---
 fs/seq_file.c |    7 +++++++
 1 file changed, 7 insertions(+)

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diff mbox

Patch

--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -190,6 +190,13 @@  ssize_t seq_read(struct file *file, char
 	 */
 	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)