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cgroup: cgroup_procs_next should increase position index

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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>
This commit is contained in:
Vasily Averin 2020-01-30 13:34:59 +03:00 committed by Tejun Heo
parent db8dd96972
commit 2d4ecb030d
1 changed files with 7 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
kernel/cgroup/cgroup.c
View File

@ -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);
}