From: Vasily Averin <vvs@virtuozzo.com>
Date: Thu, 30 Jan 2020 13:34:59 +0300
Subject: cgroup: cgroup_procs_next should increase position index
Git-commit: 2d4ecb030dcc90fb725ecbfc82ce5d6c37906e0e
Patch-mainline: v5.6-rc6
References: bsc#1169524

If seq_file .next fuction does not change position index,
read after some lseek can generate unexpected output:

1) dd bs=1 skip output of each 2nd elements
$ dd if=/sys/fs/cgroup/cgroup.procs bs=8 count=1
2
3
4
5
1+0 records in
1+0 records out
8 bytes copied, 0,000267297 s, 29,9 kB/s
[test@localhost ~]$ dd if=/sys/fs/cgroup/cgroup.procs bs=1 count=8
2
4 <<< NB! 3 was skipped
6 <<<    ... and 5 too
8 <<<    ... and 7
8+0 records in
8+0 records out
8 bytes copied, 5,2123e-05 s, 153 kB/s

 This happen because __cgroup_procs_start() makes an extra
 extra cgroup_procs_next() call

2) read after lseek beyond end of file generates whole last line.
3) read after lseek into middle of last line generates
expected rest of last line and unexpected whole line once again.

Additionally patch removes an extra position index changes in
__cgroup_procs_start()

Cc: stable@vger.kernel.org
https://bugzilla.kernel.org/show_bug.cgi?id=206283
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/cgroup/cgroup.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 75f687301bbf..927f7b82e5c1 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4595,6 +4595,9 @@ static void *cgroup_procs_next(struct seq_file *s, void *v, loff_t *pos)
 	struct kernfs_open_file *of = s->private;
 	struct css_task_iter *it = of->priv;
 
+	if (pos)
+		(*pos)++;
+
 	return css_task_iter_next(it);
 }
 
@@ -4610,7 +4613,7 @@ static void *__cgroup_procs_start(struct seq_file *s, loff_t *pos,
 	 * from position 0, so we can simply keep iterating on !0 *pos.
 	 */
 	if (!it) {
-		if (WARN_ON_ONCE((*pos)++))
+		if (WARN_ON_ONCE((*pos)))
 			return ERR_PTR(-EINVAL);
 
 		it = kzalloc(sizeof(*it), GFP_KERNEL);
@@ -4618,10 +4621,11 @@ static void *__cgroup_procs_start(struct seq_file *s, loff_t *pos,
 			return ERR_PTR(-ENOMEM);
 		of->priv = it;
 		css_task_iter_start(&cgrp->self, iter_flags, it);
-	} else if (!(*pos)++) {
+	} else if (!(*pos)) {
 		css_task_iter_end(it);
 		css_task_iter_start(&cgrp->self, iter_flags, it);
-	}
+	} else
+		return it->cur_task;
 
 	return cgroup_procs_next(s, NULL, NULL);
 }