[PATCH 5.4 054/155] cgroup-v1: cgroup_pidlist_next should update position index

Greg Kroah-Hartman
2020-03-31T09:13:34Z

From: Vasily Averin <vvs@virtuozzo.com>

[ Upstream commit db8dd9697238be70a6b4f9d0284cd89f59c0e070 ]

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

# mount | grep cgroup
# dd if=/mnt/cgroup.procs bs=1 # normal output
...
1294
1295
1296
1304
1382
584+0 records in
584+0 records out
584 bytes copied

dd: /mnt/cgroup.procs: cannot skip to specified offset
83 <<< generates end of last line
1383 <<< ... and whole last line once again
0+1 records in
0+1 records out
8 bytes copied

dd: /mnt/cgroup.procs: cannot skip to specified offset
1386 <<< generates last line anyway
0+1 records in
0+1 records out
5 bytes copied

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/cgroup/cgroup-v1.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 7f83f4121d8d0..2db582706ec5c 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -473,6 +473,7 @@ static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
*/
p++;
if (p >= end) {
+ (*pos)++;
return NULL;
} else {
*pos = *p;
--
2.20.1