Commit 2d4ecb03 authored Jan 30, 2020 by Vasily Averin Committed by Tejun Heo Feb 12, 2020

cgroup: cgroup_procs_next should increase position index

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>

parent db8dd969

kernel/cgroup/cgroup.c

+7 −3

Original line number	Diff line number	Diff line
		@@ -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);
		}

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