cgroup-v1: cgroup_pidlist_next should update position index

[ 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>

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 51063e7a93c28a7c0ad5f8cca34072dffb85e650..c9628b9a41d234626eb994af482fa7cfef173757 100644 (file)
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -501,6 +501,7 @@ static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
         */
        p++;
        if (p >= end) {
+               (*pos)++;
                return NULL;
        } else {
                *pos = *p;