[PATCH 4.15 124/163] seq_file: fix incomplete reset on read from zero offset

[Greg Kroah-Hartman]

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Miklos Szeredi <mszeredi@xxxxxxxxxx>

commit cf5eebae2cd28d37581507668605f4d23cd7218d upstream.

When resetting iterator on a zero offset we need to discard any data
already in the buffer (count), and private state of the iterator (version).

For example this bug results in first line being repeated in /proc/mounts
if doing a zero size read before a non-zero size read.

Reported-by: Rich Felker <dalias@xxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Fixes: e522751d605d ("seq_file: reset iterator to first record for zero offset")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.10
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/seq_file.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -181,8 +181,11 @@ ssize_t seq_read(struct file *file, char
 	 * 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)
+	if (*ppos == 0) {
 		m->index = 0;
+		m->version = 0;
+		m->count = 0;
+	}
 
 	/* Don't assume *ppos is where we left it */
 	if (unlikely(*ppos != m->read_pos)) {