[PATCH] sched: debug: use task_pid_vnr in /proc/$pid/sched

[Aleksa Sarai <asarai@suse.com>]

It appears as though the addition of the PID namespace did not update
the output code for /proc/$pid/sched, which made it trivial to figure
out whether a process was inside &init_pid_ns from userspace (making
container detection trivial[1]). This lead to situations such as:

  % unshare -pf head -n1 /proc/self/sched
  head (10047, #threads: 1)

Fix this by just using task_pid_vnr for the output of /proc/$pid/sched.
All of the other uses of task_pid_nr in kernel/sched/debug.c are from a
sysctl context and thus don't need to be namespaced.

[1]: https://github.com/jessfraz/amicontained

Cc: <stable@vger.kernel.org>
Cc: Jess Frazelle <acidburn@google.com>
Signed-off-by: Aleksa Sarai <asarai@suse.com>
---
 kernel/sched/debug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 4fa66de52bd6..a06acbe33e16 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -876,7 +876,7 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
 {
 	unsigned long nr_switches;
 
-	SEQ_printf(m, "%s (%d, #threads: %d)\n", p->comm, task_pid_nr(p),
+	SEQ_printf(m, "%s (%d, #threads: %d)\n", p->comm, task_pid_vnr(p),
 						get_nr_threads(p));
 	SEQ_printf(m,
 		"---------------------------------------------------------"
-- 
2.13.3